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removal from the emanation for 20 minutes, radium A has present in the active deposit, but no chemical separation of practically disappeared and the a rays arise entirely from radium B and C has yet been found possible. Hahn has shown that C. Radium C has proved very valuable in radioactive measure- thorium D-a B ray product of period 3 minutes-can easily be ments as providing an intense source of homogeneous a rays. separated by the recoil method. A special interest attaches Twenty-four hours after removal, the activity due to radium to the product thorium X (30), which was first separated by B and C has become exceedingly small. The wire, however, Rutherford and Soddy, since experiments with this substance still shows a very small residual activity, first noted by Mme laid the foundation of the general theory of radioactive transCurie. This residual activity measured by the a rays rapidly formations. A close analysis of thorium has led to the separaincreases with the time and reaches a maximum in about three tion of a number of new products. Hahn (31) found that a years. The active deposit of slow change has been examined very active substance emitting a rays, which gave rise to thorium in detail by Rutherford (23) and by Meyer and Schweidler (24). X, could be separated from thorium minerals. This active It has been shown to consist of three successive products called substance, called radiothorium, has been closely examined by radium D, E and F. Radium D is a rayless substance of slow Hahn and Blanc. Its period of decay was found by Hahn to period of transformation. Its period has been calculated by be about 2 years, and by Blanc to be 737 days. From an Rutherford to be about 40 years, and by Meyer and Schweidler examination of the activity of commercial thorium nitrate of about 12 years. Antonoff (25) fixes the period of about 17 different ages, Hahn showed that another product must be years. Radium D changes into E, a ß ray product of period present, which he called mesothorium. This is separated from about days, and E into F, an a ray product of period thorium with Th X by precipitation with ammonia. Thorium 140 days. It was at first thought that radium E was complex, is first transformed into the rayless product mesothorium, of but no evidence of this has been observed by Antonoff. The period about 5 years. This gives rise to a ß ray product of product radium F is of special interest, for it is identical with quick transformation, which in turn changes into radiothorium. polonium-the first active body separated by Mme Curie. In This changes into thorium X, and so on through a long series a similar way it has been shown that radium D is the primary of changes. When isolated in the pure state, radiothorium source of the activity observed in lead or“ radiolead ” separated would have an activity about a thousand times greater than by Hofmann. It is interesting to note what valuable results radium, but would lose its activity with time with a period of have been obtained from an examination of the minute residual about 2 years. Mesothorium, when first separated, would be activity observed on bodies exposed in the presence of the radium inactive, but in consequence of the production of radiothorium, emanation.

its activity would rapidly increase for several years. After Radium Emanation.-The radium emanation is to be regarded reaching a maximum, it would finally decay with a period as a typical radioactive product or transition element which of five years. Since a large amount of thorium is separated exists in a gaseous form. It is produced from radium at a annually from thorium minerals, it would be of great importance constant rate, and is transformed into radium A and helium. at the same time to separate the radiothorium and mesothorium Its half-period of transformation is 3.86 days. The emanation present. For many purposes active preparations of these from radium has been purified by condensing it in liquid air, substances would be as valuable as radium itself, and the and pumping out the residual gases. The volume (26) of the amount of active matter from this source would be greater emanation at normal pressure and temperature to be derived than that at present available from the separation of radium from from one gram of radium in cquilibrium is about o-6 cubic milli- uranium minerals. metres. This small quantity of gas contains initially more Actinium.-The transformations observed in actinium are than three-quarters of the total activity of the radium before very analogous to those in thorium. Actinium itself is a rayless its separation. In a pure state, the emanation is 100,000 times product which changes into radioactinium, an a ray product as active weight for weight as pure radium. Pure emanation of period 19.5 days, first separated by Hahn (32). This changes in a spectrum tube gives a characteristic spectrum of bright into actinium X, of period 10.2 days, first separated by Godlewski lines (27). The discharge in the gas is bluish in colour. With (33). Actinium X is transformed into the emanation which in continued sparking, the emanation is driven into the walls of turn gives rise to three further products, called actinium A, B the tube and the electrodes. Notwithstanding the minute and C. Although very active preparations of actinium have volume of emanation available, the boiling-point of the emana- been prepared, it has so far not been found possible to separate tion has been determined at various pressures. At atmo- the actinium from the rare earths with which it is mixed. We spheric pressure Rutherford (28) found the boiling-point to be do not in consequence know its atomic weight or spectrum. -67° C., and Gray and Ramsay (29) 71° C. Liquid emanation Origin of Radium.-According to the transformation theory, appears colourless when first condensed; when the temperature radium, like all other radioactive products, must be regarded is lowered, the liquid emanation freezes, and at the temperature as a changing element. Preliminary calculations showed that of liquid air glows with a bright rose colour. The density of radium must have a period of transformation of several thousand liquid emanation has been estimated at 5 or 6.

years. Consequently in order that any radium could exist in Approximate estimates of the molecular weight of the radium old minerals, the supply must be kept up by the transformation emanation were early made by diffusion methods. The mole- of some other substance. Since radium is always found assocular weight in most cases came out about 100. In a com- ciated with uranium minerals, it seemed probable from the parison by Perkins of the rate of diffusion of the emanation beginning that uranium must be the primary element from with that of a monatomic vapour of high molecular weight, viz. which radium is derived. If this were the case, in old minerals mercury, the value deduced was 234. Since the radium atom which have not been altered by the action of percolating waters, in breaking up gives rise to one atom of the emanation and one the ratio of the amount of radium to uranium in a mineral atom of helium, its atomic weight should be 226–4=222. must be a constant. This must evidently be the case, for in a The emanation appears to have no definite chemical properties, state of equilibrium the rate of breaking up of radium must and in this respect belongs to the group of inert monatomic equal the rate of supply of radium from uranium. If P, Q be gases of which helium and argon are the best known examples. the number of atoms of uranium and radium respectively in It is partially soluble in water, and readily absorbed by charcoal. equilibrium, and 1,, d2 their constants of change, then Thorium.—The first product observed in thorium was the

12Q=1, Por Q/P=1,1=T2/T. emanation. This gives rise to the active deposit which has been where T, and T, are the half-periods of transformation of uranium analysed by Rutherford, Miss Brooks and by Hahn, and shown and radium respectively. The work of Bolt wood (34), Strutt (35) to consist of probably four products--thorium A, B, C and D. and McCoy (36) has conclusively shown that the ratio of radium Thorium A is a rayless product of period 10.5 hours; thorium to uranium in old minerals is a constant. Boltwood and Strutt B an a ray product of period about one hour. The presence determined the quantity of radium present in a mineral by the of thorium C has been inferred from the two types of a rays emanation method, and the amount of uranium by analysis In order, however, to obtain a direct proof of the genetic relation appear to be any radioactive connexion between these two between uranium and radium, it is necessary to show that elements. Uranium and thorium are to be regarded as two radium appears after some time in a uranium compound from distinct radioactive elements. With regard to actinium, there which all trace of radium has been initially removed. It can is still no definite information of its place in the scheme of readily be calculated that the growth of radium should be easily transformations. Boltwood has shown that the amount of observed by the emanation method in the course of one week, actinium in uranium minerals is proportional to the content using a kilogram of uranium nitrate. Experiments of this kind of uranium. This indicates that actinium, like radium, is were first made by Soddy (37), but initially no definite evidence in genetic connexion with uranium. On the other hand, the was obtained that radium grew in the solution at all. The rate activity of actinium with its series of a ray products is less than of production of radium, if it took place at all, was certainly that of radium itself or uranium. In order to explain this less than roboth part of the amount to be expected if uranium anomaly, Rutherford has suggested that at a certain stage of were transformed directly into radium. It thus appeared disintegration of the uranium-radium series, the disintegration probable that one or more products of slow period of trans- is complex, and two distinct kinds of matter appear, one in formation existed between uranium and radium. Since uranium much larger quantity than the other. On this view, the smaller must be transformed through these intermediate stages before fraction is actinium, so that the latter is a branch descendant radium appears, it is evident that the initial rate of production of the main uranium-radium series. of radium under these conditions might be extremely small. End Products of Transformation. It is now definitely estab This conclusion has been confirmed by Soddy, who has shown lished that the a particle expelled from any type of radioactive that radium does appear in the solution which has been placed matter is an atom of helium, so that helium is a necessary accomaside for several years.

paniment of radioactive changes involving the expulsion of Since the direct parent of radium must be present in radio a particles. After the radioactive transformations have come active minerals, one of the constituents separated from the to an end, each of the elements uranium and thorium and mineral must grow radium. This was shown to be the case by actinium should give rise to an end or final product, which Boltwood (38), who found that actinium preparations produced may be either a known element or some unknown element of radium at a fairly rapid rate. By the work of Rutherford and very slow period of transformation. Supposing, as seems Boltwood, it was found that the growth of radium was not due probable, that the expulsion of an a particle lowers the atomic to actinium itself, but to a new substance separated in some weight of an element by four units—the atomic .weight of cases with the actinium. This new substance, which emits helium-the atomic weights of cach of the products in the a rays, was separated by Boltwood (38), and called by him uranium and radium series can be simply calculated. Since " Ionium.” It has chemical properties very similar to thorium. uranium expels two a particles, the atomic weight of the next Soddy has shown that the period of ionium is probably not ray product, ionium, is 238.5-8 or 230-5. The atomic weight less than 20,000 years, indicating that ionium must exist in of radium comes out to be 266.5, a number in good agreement uranium minerals in not less than ten times the quantity of with the experimental value. Similarly the atomic weight of radium. It has not yet been directly shown that uranium polonium is 210-5, and that of the final product after the transproduces ionium, but there can be no doubt that it does do so. formation of polonium should be 206.5. This value is very Since ionium produces radium, Boltwood (38) has determined close to the atomic weight of lead, and indicates that this subby direct experiment that radium is half transformed in 2000 stance is the final product of the transformation of radium. years-a number in good agreement with other data on that This suggestion was first put forward by Boltwood (40), who subject. The constant relation between uranium and radium has collected a large amount of evidence bearing on this subject. will only hold for old minerals where there has been no oppor- Since in old minerals the transformations have been in progress tunity for chemical alteration or removal of its constituents for periods of time, in some cases measured by hundreds of by the action of percolating water or other agencies. It is millions of years, it is obvious that the end product, if a stable quite possible that altered minerals of no great age will not element, should be an invariable companion of the radioelement show this constant relation. It seems probable that this is and be present in considerable quantity. Boltwood has shown the explanation of some results of Mlle Gleditsch, where the that lead always occurs in radioactive minerals, and in many relation between uranium and radium has been found not to cases in amount about that to be expected from their uranium be constant for some mineral specimens.

content and age. It is difficult to settle definitely this very Connexion of the Radioelements.-We have already seen that important problem until it can be experimentally shown that a number of slowly transforming radioactive substances, viz. radium is transformed into lead, or, what should prove simpler polonium (radium F), radiolead (radium D) and ionium are in practice, that polonium changes into helium and lead. Unlinked up to the uranium-radium series of transformations. fortunately for a solution of this problem within a reasonable Boltwood (39) has made a systematic examination of the time, a very large quantity of polonium would be necessary. relative activity in the form of very thin films due to each Mme. Curie and Debierne have obtained a very active preof the products present in the uranium-radium family. The paration of polonium containing about oth milligram of pure results are shown in the following table, where the activity of polonium. Rutherford and Boltwood and Curic and Debierne pure uranium itself is taken as unity

have both independently shown that polonium produces helium Uranium

Radium B


-a result to be expected, since it emits a particles. lonium Radium C

Production of Hdium.- In 1902 Rutherford and Soddy sugRadium

Radium F

0.46 gested that the helium which is invariably found in radioactive Emanation 0-62 Actinium and its

minerals was derived from the disintegration of radioactive Radium A

products 0.28 Total activity mineral, 4.64 times uranium.

matter. In 1903 Ramsay and Soddy definitely showed that

helium was produced by radium and also by its emanation. Taking into account the differences in the ionization due From the observed mass of the a particle, it seemed probable to an a particle from the various products, the results indicate from the first that the a particle was an atom of helium. that uranium expels two a particles for one from each of the This conclusion was confirmed by the work of Rutherford and other a ray products in the series of transformations. This Geiger (41), who showed that the à particle was an atom of indicates either that two particles are expelled during the helium carrying two unit charges of electricity. In order to transformation of the atom of uranium, or that another a ray prove definitely this relation, it was necessary to show that the product is present which has so far not been separated from the a particles, quite independently of the active matter from uranium.

which they were expelled, gave rise to helium. This was done Although thorium is nearly always present in old uranium by Rutherford and Royds (42), who allowed the a particles minerals and uranium in thorium minerals, there does not from a large quantity of emanation to be fired through the

1.00 0.34 0.45





very thin glass walls of the containing tube. The collected Rutherford and Geiger (49) have devised an electrical method particle gave the spectrum of helium, showing, without doubt, of counting the a particles expelled from radioactive matter. that the a particle must be a helium atom.

The a particle enters through a small opening into a metal Since the a particle is an atom of helium, all radioactive tube containing a gas at a reduced pressure. The ionization matter which expels a particles must give rise to helium. In produced by the a particle in its passage through the gas is agreement with this, Debierne and Giesel have shown that magnified several thousand times by the movement of the actinium as well as radium produces helium. Observations ions in a strong electric field. In this way, the entrance of an of the production of helium by radium have been made by a particle into the detecting vessel is shown by a sudden and Ramsay and Soudy, Curie and Dewar, Himstedt and others. | large deflection of the measuring instrument. By this method, The rate of production of helium per gram of radium was first they determined that 3.4 X 1060 a particles are ejected per definitely measured by Dewar (43). His preliminary measure- second from one gram of radium itself and from each of its ments gave a value of 134 cubic mms. of helium per year per a ray, products in equilibrium with it. By measuring the gram of radium and its products. Later observations extend-charge on a counted number of a particles, it was found that ing over a larger interval give a rate of production about the a particle carries a positive charge of 9.3 X 10-10 electro168 cubic mms per year. As a result of preliminary measure- static units. From other evidence, it is known that this must ments, Boltwood and Rutherford (44) have found a growth be twice the fundamental unit of charge carried by the hydrogen of 163 cubic mms. per year. It is of interest to note that the atom. It follows that this unit charge is 4.65 X 10-40 units. rate of production of helium by radium is in excellent agreement This value is in good agreement with numerous recent deterwith the value calculated theoretically. From their work of minations of this fundamental quantity by other methods. counting the particles and measuring their charge, Rutherford With this data, it is possible to calculate directly the values and Geiger showed that the rate of production of helium should of some important radioactive data. The calculated and be 158 cubic mms. per year.

observed values are given below: Properties of the a Rays.-We have seen that the rays are

Calculated. Observed. positively charged atoms of helium projected at a high velocity, Volume of the emanation in cubic milliwhich are capable of penetrating through thin metal sheets

metres per gram of radium

-585 .6

Volume of helium in cubic millimetres proand several centimetres of air. Early observations indicated

duced per year per gram of radium.


169 that the ionization due to a layer of radioactive matter decreased Heating effect of radium per gram per hour approximately according to an exponential law with the thick

in gram calories

113 118 ness of the absorbing matter placed over the active matter.

Hall-period of transformation of radium

in year The true nature of the absorption of the a rays was first

1760 shown by Bragg and by Bragg and Kleeman (45).' The active The calculated values are in all cases in good agreement with particles projected from a thin film of active matter of one the experimental numbers. kind have identical velocities, and are able to ionize the air It is well known from the experiments of Sir William Crookes for a definite distance, termed the "range” of the a particle. (50) that the a rays produce visible scintillations when they It was found that the ionization per centimetre of path duc fall on a screen of phosphorescent zinc sulphide. This is shown to a narrow pencil of a rays increases with the distance from in the instrument called the spinthariscope. By means of a the active matter, at first slowly, then more rapidly, near the suitable microscope, the number of these scintillations on a end of the range. After passing through a maximum value given area in a given time can be counted. The number so the ionization falls off rapidly to zero. The range of an a obtained is practically identical with the number of a particles particle in air has a definite value which can be accurately incident on the screen, determined by the electrical method of measured. 'If a uniform screen of matter is placed in the path counting. This shows that each a particle produces à visible of the pencil of rays the range is reduced by a definite amount flash of light when it falls on a suitable zinc sulphide screen. proportional to the thickness of the screen.

All the a par- The scintillations produced by a rays are observed in certain ticles have their velocity reduced by the same amount in their diamonds, and their number has been counted by Regener (51) passage through the screen. The ranges in air of the a rays and the charge on each particle has been deduced. The latter from the various products of the radioelements have been was the first to employ the scintillation method for actual measured. The ranges for the different products vary between counting of a particles. Kinoshita has shown that the number 2.8 cms. and 8.6 cms.

of a particles can also be counted by the photographic method, Bragg has shown that the range of an a particle in different and that each particle must produce a detectable effect. elements is nearly proportional to the square roots of their Absorption of B Rays.-We have seen that the B particles, atomic weights. Using the photographic method, Rutherford which are emitted from a number of radioactive products, carry (46) showed that the velocity V of an a particle of range R cms. a negative charge and have the same small mass as the particles in air is given by V2=K(R+1.25), where K is a constant. In constituting the cathode rays. The velocity of expulsion and his experiments he was unable to detect particles which had a penetrating power of the B rays varies widely for different velocity lower than 8.8X108 cms. per second. Geiger (47), products. For example, the rays from radium B are very easily using the scintillation method, has recently found that a absorbed, while some of the rays from radium C are of a very particles of still lower velocity can be detected under suitable penetrating type. It has been found that for a single B ray conditions by the scintillations produced on a zinc sulphide product, the particles are absorbed according to an exponential screen. He has found that the connexion between velocity law with the thickness of matter traversed, and Hahn has made and range can be closely expressed by Vä=KR, where K is a use of this fact to isolate a number of new products. It has been constant.

generally assumed that the exponential law of absorption is a On account of the great energy of motion of the a particle, criterion that the B rays are all expelled at the same speed. In it was at first thought that it pursued a rectilinear path in the addition, it has been supposed that the B particles do not gas without appreciable deflection due to its encounters with decrease much in velocity in passing through matter. Wilson the molecules. Geiger (48) has, however, shown by the scintil- has recently made experiments upon homogeneous B rays, and lation method that the a particles are scattered to a marked finds that the intensity of the radiation falls off in some cases extent in passing through matter. The scattering increases according to a linear rather than to an exponential law, and that with the atomic weight of the substance traversed, and becomes there is undoubted evidence that the B particles decrease in more marked with decreasing velocity of the a particle. A velocity in traversing matter. Experiments upon the absorption small fraction of the a particles falling on a thick screen are of B rays are greatly complicated by the scattering of the B rays deflected through more than a right angle, and emerge again on in their encounters with the molecules. For example, if a pencil the side of incidence.

of B rays falls on a metal, a large fraction of the rays are scattered sufficiently to emerge on the side of incidence. This scattering REFERENCES.-1. H. Becquerel, Comples Rendus, 1896, pp. 420, of the B rays has been investigated by Eve, McLennan, Schmidt, 501, 559, 689, 762, 1086; 2. Rutherford, Phil. Mag., Jan. 1899: Crowther and others. It has been found that the scattering Curie and G. Bemont, ib., 1898, 127. p. 1215; 4. Mme Curie, ib.,

3. Mme Curie, Comples Rendus, 1898, 126. p. 1101; Mand Mme for different chemical elements is connected with their atomic 1907, 145. p. 422; 5. Thorpe, Proc. Roy. Soc., 1908, 80. p. 298; weight and their position in the periodic table. McCelland and 6. Giesel, Phys. Zeit., 1902, 3. p. 578; 7. Giesel, Annal. d. Phys. Schmidt have given theories to account for the absorption of

1899, 69, p. 91; Ber., 1902, p. 3608; 8. Rutherford and Boltwood,

Amer. Journ. Sci., July 1906; 9. Debicrne, Comptes Rendus, B rays by matter. The whole problem of absorption and scatter

1899, 129. P. 593; 1900, 130. P. 206; 10. Giesel, Ber., 1902, p. 3603; ing of particles by substances is very complicated, and the 1903, p. 342; 11. Marckwald, ib., 1903, p. 2662; 12. Mme Curie question is still under active examination and discussion. The and Debierne, Comptes Rendus, 1910, 150, P: 386; 13. Bolt wood, negative charge carried by the B rays has been measured by Amer. Journ. Sci., May 1908; 14. Rutherford, Phil. Mag., Feb. 1903, a number of observers. It has been shown by Rutherford and Soddy, ib., May 1903; 17. Rutherford and Soddy, ib., Nov. 1902;

Oct. 1906; 15. Rutherford, ib., Jan. 1900; 16. Rutherford and Makower that the number of B particles expelled per second 18. M and Mme Curie, Comples Rendus, 1899, 129. p. 714; 19. from one gram of radium in equilibrium is about that to be ex- Rutherford, Phil. Mag., Jan. and Feb. 1900; 20. Rutherford and pected if each atom of the Bray products in breaking up Phil. Trans., 1904, 204A. p. 169; 21. Russ and Makower, Proc.

Soddy, ib., Sept. and Nov. 1902, April and May 1903; Rutherford, emits one B particle.

Roy. Soc., 1909, 82A. p. 205; 22. Hahn, Phys, Zeil., 1909, 10. P. 81; Heal Emission of Radioactive Maller.-In 1903 it was shown 23. Rutherford, Phil. Mag., Nov. 1904. Sept. 1905; 24. Meyer by Curie and Laborde (52) that a radium compound was always and Schweidler, Wien. Ber, July 1905; 25. Antonoff, Phil Mag., hotter than the surrounding medium, and radiated heat at a June 1910; 26. Cameron and Ramsay, Trans. Chem. Soc., 1907, constant rate of about 100 gram calories per hour per gram of Ramsay Proc. Roy. Soc., 1908, 811. p. 210; 'Rutherford and

p. 1266; Rutherford, Phil. Mag., Aug. 1908; 27. Cameron and radium. The rate of evolution of heat by radium has been Royds, Phil. Mag., 1908, 16. p. 313; Royds, Prot. Roy. Soc., 1909, measured subsequently by a number of observers. The latest 82A. p. 22; Watson, ib., 1910, 83A. p. 50; 28. Rutherford, Phil. and most accurate determination by Schweidler and Hess, using Alag., 1909; 29. Gray, and Ramsay, Irons. Chem. Soc., 1909, about half a gram of radium, gave 118 gram calories per gram Rov.351902; 13i. 'Hahn, Proc. Roy. Soc., Xiarch 1905: "Phil. Alag., per hour (53). There is now no doubt that the evolution of June 1906; Ber., 40. pp. 1462, 3304; Phys. Zeit., 1908, 9. pp. 245, heat by radium and other radioactive matter is mainly a second-346; 32. Hahn, Phil. Mag., Sept. 1906; 35. Godlewski, ib., ary phenomenon, resulting mainly from the expulsion of a

July 1905; 34. Bolt wood, ib., April 1905; 35. Strutt, Trans. Ray. particles. Since the latter have a large kinetic energy and are

Soc., 1905A.; 36. McCoy, Ber., 1904, p. 2641; 37. Soddy, Phil.

Mag., June 1905. Aug. 1907, Oct. 1908, Jan. 1909; 38. Boliwood, easily absorbed by matter, all of these particles are stopped in Amer. Journ. Sci., Dec. 1906, Oct. 1907, May 1908, June 1908; the radium itself or in the envelope surrounding it, and their 39. Bolt wood, ib., April 1908; 40. Boltwood, ib., Oct. 1905. energy of motion is transformed into heat. On this view, the Feb. 1907; 41. Rutherford and Geiger, Proc. Roy. Soc., 1908, 81Ă. evolution of heat from any type of radioactive matter is pro

p. 141; 42. Rutherford and Royds, Phil. Mag., Feb. 1909; 43. Dewar,

Proc. Roy. Soc., 1908, 81A. p. 280; 1910, 83. p. 404; 44. Bolt wood portional to the kinetic energy of the expelled a particles. The and Rutherford, Manch. Lil. and Phil. Soc., 1909, 54. No. 6; view that the heating effect of radium was a measure of the 45. Bragg and Kleeman, Phil. Mag., Dec. 1904, Sept. 1905: 46. kinetic energy of the a particles was strongly confirmed by the Rutherford, ib., Aug. 1906; 47. Geiger, Proc. Roy. Soc., 1910, 834. experiments of Rutherford and Barnes (54). They showed that

P: 505; 48. Geiger, ib., 1910, 834. p. 492; 49. Rutherford and the emanation and its products when removed from radium Geigegenlervorhand... B. ,

51. RegenerVerhandl. d. Phys. Ges., 1908, 10. P. 28; 52. Curie were responsible for about three-quarters of the heating effect and Laborde, Comples Rendus, 1904, 136. p. 673; 53. Schweidler of radium in equilibrium. The heating effect of the radium and Hess,.,Wien. Ber., June 1908, 117; 54. Rutherford and emanation decayed at the same rate as its activity. In addition, Barnes, Phil. Mag., Feb. 1904. it was found that the ray products, viz. the emanation radium Radioactive Transformations, 1906; F. Soddy, Interpretation of

General treatises are: É. Curie, Euvres, 1908; E. Rutherford, A and radium C, each gave a heating effect approximately Radium, 1909; R., J. Strutt, Becquerel Rays and Radium, 1904: proportional to their activity. Measurements have been made W. Makower, Radioactive Substances, 1908; J. Joly, Radioactivity on the heating effect of uranium and thorium and of pitch- and Geology, 1909. See also Annual Reports of the Chemical Society

(E. Ru.) blende and polonium. In each case, the evolution of heat has been shown to be approximately a measure of the kinetic energy RADIOLARIA, so called by E. Haeckel in 1862 (Polycystina, of the a particles.

by C. G. Ehrenberg, 1838), the name given to Marine Sarcodina, Experiments on the evolution of heat from radium and its in which the cytoplasmic body gives off numerous fine radiating emanation have brought to light the enormous amount of pscudopods (rarely anastomosing) from its surface, and is energy accompanying the transformation of radioactive matter provided with a chitinous " central capsule," surrounding where a particles are emitted. For example, the emanation the inner part which encloses the nucleus, the inner and outer from one gram of radium in equilibrium with its products emits cytoplasm communicating through either one or three aperheat initially at the rate of about go gram calories per hour. tures or numerous pores in the capsule. The extracapsular The total heat emitted during its transformation is about cytoplasm is largely transformed into a gelatinous substance 12,000 gram calories. Now the initial volume of the emanation (" calymma "), through which a granular network of plasm from one gram of radium is .6 cubic millimetres. Consequently passes to form a continuous layer bearing the pseudopods at the one cubic centimetre of emanation during its life emits 2 X 10% surface; this gelatingus layer is full of large vacuoles, “ alveoli," gram calories. Taking the atomic weight of the emanation as as in other pelagic Sarcodina (Heliozoa, 9.0.), Globigerinidae, 222, one gram of the emanation emits during its life 2 X 109 &c., among Foraminifera (9.0.). The protoplasm may contain gram calories of heat. This evolution of heat is enormous oil-globules, pigment-grains, reserve-grains and crystals. There compared with that emitted in any known chemical reaction. is frequently a skeleton present, either of silica (pure or containThere is every reason to believe that the total emission of energy ing a certain amount of organic admixture), or of "acanthin ” from any type of radioactive matter during its transformation (possibly a proteid, allied to vitellin, but regarded by W. is of the same order of magnitude as for the emanation. The Schewiakoff as a hydrated silicate of calcium and aluminium); atoms of matter must consequently be regarded as containing never calcareous or arenaceous. The skeleton may consist of enormous stores of energy which are only released by the dis- spicules, isolated or more or less compacted, or form a latticed integration of the atom.

shell, which, in correlation with the greater resistance of its A large amount of work has been done in measuring the substance, is of lighter and more elegant structure than in the amount of the thorium and radium emanation in the atmo- Foraminifera. The alveoli contain a liquid, which, as shown sphere, and in determining the quantity of radium and thorium by Brandt, is rich in carbon dioxide, and in proportion to its distributed on the surface of the earth. The information abundance may become much lighter than sea-water; and already obtained has an important bearing on geology and possibly the gelatinous substance of the calymma is also lighter atmospheric electricity.

than the medium. In Acantharia the protoplasm at the base

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of the projecting spines is often differentiated into a bundle of name of Polycystina (1838), but without more than a very fibres converging on to the spines some way up (distally); these, slight knowledge of a few living forms. T. H. Huxley in 1851 comparable to the myonemes of Infusoria (q.v.), &c., and termed made the first adequate study of the living animal, and was “myophrisks ”, possibly serve to drag outwards the surface followed by Joh. Müller in the same decade. E. Haeckel began and so extend it, with concurrent dilatation of the alveoli, and his publications in 1862, and in two enormous, abundantly lower the specific gravity of the animal. In this group also a illustrated, systematic works, besides minor publications, has thick temporary flagellum“ sarcoflagellum may be formed, dealt exhaustively with the cytology, classification, and distriapparently by the coalescence of a number of pseudopodia. bution of the class. Next in value come the contributions The pigmented mass or "phaeodium " in the ectoplasm of of Richard Hert wig (largely developmental), besides those of Phacodaria appears to be an excretory product, formed within L. Cienkowsky, Karl. Brandt, and A. Borgert, while to F. the central capsule and passing immediately outwards; a similar Dreyer and V. Häcker we owe valuable studies on the physical uniform deposit of pigmented granules occurs in the Colloid relations of the skeleton. species, Thalassicolla nucleata. The wall of the central cap- Our classification is taken from Haeckel. sule is simple in the Spumellaria, but formed of two layers in the Nassellaria and Phaeodaria. In the Nassellaria the perforated with numerous evenly distributed pores.

A. Spumellaria, Haeck. (Peripylaea, Hertwig). Central capsule

Skeleton oscule is simply a perforated area, and a cone of differentiated Siliceous, latticed or of detached spicules, or' absent. Form fibres in the intracapsular cytoplasm has its base on it: it is homaxonic or with at least three planes of symmetry intersecting termed the “porocone,” and the fibres may possibly be muscular at right angles, rarely irregular or spiral, sometimes forming colonies, (myonemes). In Phaeodaria, the inner membrane at each oscule

i.e. with several central capsules in a common external cytoplasm. is prolonged through the outer into a tube (“proboscis "): the outer membrane of the principal oscule forms a large radially

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Fig. 1.Thalassicolla pelagica, Haeckel; CK, central capsule;

EP, extracapsular protoplasm; al, alveoli, liquid-holding vacuoles
in the protoplasm similar to those of Heliozoa, Hastigerina, &c.;
ps, pseudopodia. The minute unlettered dots are the "yellow FIG. II.—Eucyrtidium cranioides, Haeck.; one of the Nassellari.

Entire animal as seen in the living condition. The central capsule

is hidden by the beehive-shaped siliceous shell within which it is striated circular plate, the “astropyle," or "operculum.' lodged. The innermost shell of some with concentric shells may lie within the central capsule, or even within the nucleus; this is

I. Skeleton of detached spicules, or absent.

Fam. I. COLLOIDEA. Skeleton absent. Thalassicolla, Huxl. due to the growth of these organs after the initial shell is formed,

(figs. 1. and 1. 1); Thalassophysa, Haeck.; so that they pass out by lobes through the latticed openings

Collosoum, Haeck. (fig. III. 2-5, 15, 16); of the embryonic shell, which lobes ultimately coalesce outside

Actissa, Haeck.

Fam. 2. BELOIDEA. the embryonic chamber, and so come finally to invest it (fig.

Skeleton spicular. Sphaerozoum,

Haeck.; Raphidozoum, Haeck. III. 17). In some, a symbiosis occurs with Zooxanthella,

II, Skeleton latticed or spongy-reticulate. Brandt, a Flagellate of the group Chrysomadineae, which Fam. 3. SPHAEROIDEA. Skeleton homaxial, sometimes in the resting state inhabits the extracapsular cytoplasm

colonial. Collosphaera, Mull.; Haliomma, growing and dividing freely therein, and only (under study).

Ehrb.; Aclinomma, Haeck. (fig. III. 17),

showing concentric latticed shells, the smallest becoming free and Nagellate on the death of the host (fig. III.

intranuclear, all connected by radial spines; 4, 6-13). The Silicoflagellata or Dictyochidae, also possessing

Spongosphaera, Haeck. (fig. iv. 8); Helioa vegetable colouring matter, but with a skeleton of impure

sphaera, Haeck. (hig. in. 14). silica (like that of Phaeodaria), may pass some of their lives in

Fam. 4. PRUNOIDEA. Skeleton a prolate spheroid or

cylinder of circular section, sometimes consymbiosis with Radiolaria.

stricted like a dice-box. Living Radiolaria were first observed and partially described Fam. 5. DISCOIDEA. Shell Mattened, of circular plan, by W. J. Tilesius in 1803-6 and 1814, by W. Baird in 1830,

rarely becoming spiral. and by C. G. Ehrenberg in 1831, as luminous organisms in the

Fam. 6. LARCOIDEA. Shell with three unequal axes, sea; F. J. F. Meyen in 1834 recognized their animal character

elliptical in the plane of any two, more rarely

becoming irregular or spiral. and the siliceous nature of their spicules. Ehrenberg a little later B. Acantharia, Haeck. (Actipylaea, Hertw.). Skeleton of described a large number of Nassellarian skeletons under the spicules of acanthin radiating from a centre, and usually twenty.

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