green leaves of plants. But the same solar radiation, when | (§ 8), since part of it might be degraded into low-temperature broken up into diffused sky light, which has no definite direction, has fallen into equilibrium with a much lower temperature, through loss of its reversibility. It has been remarked that the temperatures of the planets can be roughly compared by means of this principle, if their coefficients of absorption of the solar radiation are assumed; that of Neptune comes out below 200° C., if we suppose that it is not kept higher by a supply of internal heat.

The previous argument then gives E(X,T) = c1λ-de-c/^Tôλ, | 1.4455 in c.g.s. measure, but not so well when the range is farther a type of formula which was originally suggested by Wien on extended: it appeared that a larger value of c was needed to the basis of the analogy that it assigns the same distribution represent the radiation for high values of TX, that is, for high for the radiant energy, among the various frequencies of vibra-temperature or for very long wave-lengths. Thiesen proposed tion, as for the energy of the molecules in a gas among their the somewhat more general form_c1(TX)*e-TA, and suggested various velocities of translation. But the experimental in- that the value k=agrees better with the experimental numbers adequacy of this formula afterwards suggested a new pro- than Wien's value ko. Lord Rayleigh was led (Phil. Mag., cedure, as infra. June 1900) towards this form with k equal to unity from entirely different theoretical considerations, on the assumption of the Maxwell-Boltzmann distribution of the energy of a system, consisting of an isolated block of aether, among its free periods of vibration, infinite in number; in some cases this form appeared to give as good results as Wien's own.

and

Processes may be theoretically assigned for the direct continuous transformation of radiant into mechanical energy. Thus we can imagine a radiating body at the centre of a wheel, carrying oblique vanes along its circumference, which reflect the radiation on to a ring of parallel fixed vanes, which finally reverse its path and return it to the centre. The pressure of the radiation will drive the wheel, and in case its motion is not resisted, a very great velocity may be theoretically obtained. The thermodynamic compensation in such cases lies in the reduction of the effective temperature of the portion of the radiation not thus used up. We might even do away with the radiating body at the centre of the wheel, and consider a beam of definite radiation reflected backwards and forwards across a diameter. It is easy to see that its path will remain diametral; the work done by it in driving the wheel will be concomitant with increase of the wave-length, and therefore with expansion of the length occupied by the beam. The thermodynamic features are thus analogous to those of the more familiar case of an envelope filled with gas, which can change its thermal energy into mechanical energy by expansion of the envelope against mechanical resistances. In the case of the expanding gas po=3E., where E, is the total translatory energy of the molecules, while in adiabatic expansion p=ku-y.+ 1500° C. of the source of radiation, it has been found that the Thus the work gained in unlimited expansion, fpdv, is E/(y-1). The final temperature being absolute zero, this should by Carnot's principle be equal to the total initial energy of the gas that is in connexion with temperature, constitutive energy of the molecules being excluded; when - is less than there is thus internal thermal energy in the molecules in addition to the translatory energy. In the case of the beam of radiation, of length 1, between n and non reflexions, where in is an integer, its total energy E is ¿E 4 соби by 2 reduced according to the law Also E= (c+v) 7=c+v 20 81 thus E When is small compared with c, this gives C+o T E=2; and is then 2E/1, so that fpdl=E, the temperature of the beam being ultimately reduced to absolute zero by the unlimited expansion. This is in accord with Carnot's principle, in that the whole energy of the beam travelling in a vacuum is mechanically available when reduction to absolute zero of temperature is in our power.

SE

=

12. Experimental Knowledge.-Under the stimulus of Wien's investigation and of improvements in the construction of linear thermopiles and bolometers for the refined measurement of the distribution of energy along a spectrum, the general character of the curve connecting energy and wave-length in the complete radiation at a given temperature has been experimentally ascertained over a wide range. At each temperature there is a wave-length of maximum radiation, which is displaced towards the ultra-violet as the temperature rises, and Wien's law of homology (§ 6) shows that XT should be constant. This deduction, and the law of homology itself, as also the law of Stefan and Boltzmann that the total radiation varies as T', have been closely verified by the experiments of Rubens and Kurlbaum, Lummer and Pringsheim, Paschen and others. They established a steady field of radiation inside a material enclosure by raising the walls to a definite temperature, and measured the radiant intensity emitted from it through an opening or slit in the walls, by means of a bolometer or thermopile, this being the radiation of the so-called perfectly black body. The principle here involved formed one of the foundations of Balfour Stewart's early treatment of the theory, and had already been employed by him and Stokes (1860) in experiments on the polarized emission from tourmaline: cf. Stokes, Math. and Phys. Papers, iv. 136. It has been remarked by Planck and by Thiesen that the coefficient of T in Stefan's law, and the value of AT, are two absolute physical constants independent of any particular kind of matter, which in conjunction with the constant of gravitation would determine an entirely absolute system of physical units. The form of the function (TX) adopted by Wien and in Planck's earlier discussions, namely, Ce, was found to agree fairly with experiment over the range from 100° C. to 1300° C., when c1 = 1·24X10, and c=

Acting on a suggestion advanced by Lord Rayleigh, Rubens and Kurlbaum soon afterwards widely extended the test of the formulae by means of the so-called Reststrahlen. A substance such as an aniline dye, which exhibits selective absorption of any group of rays, also powerfully reflects those rays; and Rubens has been able thus to isolate in considerable purity the rays belonging to absorption bands very far down in the invisible ultra-red, having wave-length of order 10-3 cm., which are intensely absorbed by substances such as sylvine, by means of five or six successive reflexions of the beam of radiation. By experiments ranging between temperatures -200° C. intensity of this definite radiation tends to vary simply as T, with close approximation, thus increasing indefinitely with the temperature, whereas Wien's formula would make it tend to a definite limit. The only existing formula (except the one suggested by Lord Rayleigh) that proved to be in accord with this result was a new one advanced shortly before and supported on theoretical grounds by Planck, namely, E= Cλ="dλ/(c/AT-1), which for small values of XT agrees with Wien's original form, known to be there satisfactory, while for larger values it tends towards C/c.X-T; the new formula is, in fact, the simplest and most likely form that satisfies these two conditions. The point of Lord Rayleigh's argument was that, at any rate at low frequencies, the law of distribution would suggest an equable partition of the energy between temperature heat and radiant vibrations, and that therefore the energy of the latter should ultimately vary as T; and this prediction, which has thus been verified, may be grafted on to any formula that is in other respects appropriate.

Recognizing that his previous hypothesis, restricting the nature of the entropy in addition to its property of continually increasing, had thus to be abandoned, Planck had in fact made a fresh start on the basis of a train of ideas which was introduced by Boltzmann in 1877, in order to obtain a precise physical conception of entropy. According to the latter, for an indefinitely numerous system of molecules, with known properties and in given circumstances, there is a definite probability of the occurrence of each statistical distribution of velocities, or say each "complexion" of the system, that is formally possible when all velocities consistent with given total energy are considered to be equally likely as regards each molecule; the distribution of greatest possible probability is the state of thermal equilibrium of the system, and the probability of any other state is a function of the entropy of that state. This conception can be developed only in very simple cases; the application to an ideal monatomic gas-system led Boltzmann to take the entropy proportional to the logarithm of the probability. This logarithmic law is in fact demanded in advance by the principle that the entropy of a system should be the sum of the entropies of its parts. By means of a priori considerations of this nature, referring to the distribution of internal vibratory energy among a system of linear electric vibrators of given period, and its equilibrium of exchanges with the surrounding radiant energy, Planck has been guided to an expression for the law of dependence of the entropy of that system on the temperature, which corresponds to the form of the law of radiation above stated. The result gains support from the fact that the expressions for the coefficients to which he is led give determinations of the

absolute physical constants of molecular theory, such as the constant of Avogadro, which are in close accord with other But on the other hand these deterrecent determinations. minations are already involved in the earlier formula of Rayleigh, which expresses the distribution for long waves, based merely on the Maxwell-Boltzmann principle of the equable partition of the energy among the high free periods belonging to the enclosure which contains it. It is maintained by Jeans that the reason why this principle is of avail only for very long wavelengths is that a steady state is never reached for the shorter ones, a doctrine which as he admits would entirely remove the foundations of the application of thermodynamic principles to this subject. By an argument based on the theory of dimensions, Lorentz has been led to the conclusion that consistency between temperatures, as measured molecularly, and as measured by the laws of radiation, requires that the ultimate indivisible electric charges or electrons must be the same in all kinds of matter. The abstract statistical theory of entropy, which is here invoked, admits of generalization in a way which is a modification of that of Planck, itself essentially different from the earlier idea of Boltzmann. The molecules of matter, whose interactions control physical phenomena, including radiation, are too numerous to be attended to separately in our knowledge. They, and the phenomena in which they interact, must thus be sorted out into differential groups or classes. Elements of energy of specified types might at first sight constitute such classes: but the identity of a portion of energy cannot be traced during its transformations, while an element of physical disturbance can be definitely followed, though its energy changes by interaction with other elements as it proceeds. The whole disturbance may thus be divided into classes, or groups of similar elements, each with permanent existence: and these may be considered as distributed in series of cells, all equivalent in extent, which constitute and map out the material system or other domain of the phenomena. The test of this equivalence of extent is superposition, in the sense that the same element of disturbance always occupies during its wanderings the same number of cells. This framework being granted, the probability of any assigned statistical distribution of the elements of disturbance now admits of calculation; and it represents, as above, the logarithm of the entropy of that distribution, multiplied however by a coefficient which must depend on the minuteness of scale of the statistics. But in the calculation, all the physical laws which impose restrictions on the migrations of the elements of disturbance must be taken into account; it is only after this is done that the rest of the circumstances can be treated as fortuitous. All these physical laws are, however, required and used up in determining the complex of equivalent cells into which the system which forms the seat of the energy is mapped out. On this basis thermodynamics can be constructed in a priori abstract fashion, and with deeper and more complete implications than the formal Carnot principle of negation of perpetual motions can by itself attain to. But the ratio of the magnitude of the standard element of disturbance to the extent of the standard cell remains inherent in the results, appearing as an absolute physical constant whose value is determined somehow by the other fundamental physical constants of nature. A prescribed ratio of this kind is, however, a different thing from the hypothesis that energy is constituted atomically, which underlies, as Lorentz pointed out, Planck's form of the theory. It has indeed already been remarked that the mere fact of the existence of a wave-length Am of maximum radiation, whether obeying Wien's law AT constant or not, implies by itself some prescribed absolute physical quantity of this kind, whose existence thus cannot be evaded, though we may be at a loss to specify its nature.

13. Modification by a Magnetic Field.-The theory of exchanges of radiation, which makes the equilibrium of radiating bodies depend on temperature alone, requires that, when an element of surface of one body is radiating to an element of surface of another body at the same temperature, the amounts of energy interchanged (when reflexion is counted in along with

radiation) should be equal. This proposition is a genera!
dynamical consequence on the basis of the laws of reciprocity
developed in this connexion (after W. Rowan Hamilton) mainly
by Helmholtz, Kirchhoff, and Rayleigh-of the form of the
equations of propagation of vibrations in the medium. But in
a material medium under the influence of a strong magnetic
field these equations are altered by the addition of extraneous
terms involving differential coefficients of the third order, and
the dynamical consistency of the cardinal principle of the
theory of exchanges is no longer thus directly verified. A
system of this kind has, in fact, been imagined by Wien in which
the principle is imperfectly fulfilled. A beam coming from a
body A, and polarized by passage through a nicol, may have
its plane of vibration rotated through half a right angle by
crossing a magnetically active plate, and may then pass through
another nicol, properly orientated for transmission, so as finally
to fall on another body B. On the other hand, the radiation
from B which gets through this adjacent nicol will have its
plane of vibration rotated through another half right angle by
the magnetically active plate, and so will not get through the
first nicol to the body A. Such possibilities of unequal ex-
change of radiation between A and B are the result of the want
of reversibility of the radiation in the extraneous magnetic
field, which might have been expected to lead to proportionate
inequalities of concentration; in this example, however, though
the defect of reversibility is itself slight, its results appear at
first sight to prevent any equilibrium at all. But a closer
examination removes this discrepancy. In order to make the
system self-contained, reflectors must be added to it, so as to
send back into the sources the polarized constituents that are
turned aside out of the direct line by the nicols. Then, as
Brillouin has pointed out, and as in fact Rayleigh had explained
some years before, the radiation from B does ultimately get
across to A after passage backward and forward to the reflectors
and between the nicols: this, it is true, increases the length of
its path, and therefore diminishes the concentration of a single
narrow beam, but any large change of path would make the
beam too wide for the nicols, and thus require other corrections
which may be supposed to compensate. The explanation of
the slight difference that is to be anticipated on theoretical
grounds might conceivably be that in such a case the magnetic
influence, being operative on the phases, alters the statistical
constitution of the radiation of given wave-length from the
special type that is in equilibrium with a definite temperature,
so that after passage through the magnetic medium it is not in
a condition to be entirely absorbed at that temperature; there
would then be some other element, in addition to temperature,
involved in equilibrium in a magnetic field. If this is not so,
there must be some thermodynamic compensation involving
reaction, extremely small, however, on the magnetizing system.
14. Origin of Spectra.-In addition to the thermal radiations
of material substances, those, namely, which establish tem-
perature-equilibrium of the enclosure in which they are confined,
there are the fluorescent and other radiations excited by ex-
traneous causes, radiant or electric or chemical. Such radia
tions are an indication, by the presence of higher wave-lengths
than belong in any sensible degree to the temperature, that the
steady state has not arrived; they thus fade away, either
immediately on the cessation of the exciting cause, or after an
interval. The radiations, consisting of definite narrow bright
bands in the spectrum, that are characteristic of the gaseous
state in which each molecule can vibrate freely by itself, are
usually excited by electric or chemical agency; thus there is
no ground for assuming that they always constitute true tem-
The absorption of these radiations by
perature radiation.
strata of the same gases at low temperatures seems to prove
that the unaltered molecules themselves possess these free
periods, which do not, therefore, belong specially to dissociated
Although very difficult to excite directly, these free
vibrations are then excited and absorb the energy of the inci-
dent waves, under the influence of resonance, which naturally
becomes extremely powerful when the tuning is exact; this

ions.

The beginning of definite knowledge was the discovery of Balmer in 1885, that the frequencies of vibration (n) of the hydrogen lines could be represented, very closely and within the limits of error of observation, by the formula # ∞ 1-4m2, when for m is substituted the series of natural numbers 3, 4, 5, ... 15. Soon afterwards series of related lines were picked out from the spectra of other elements by Liveing and Dewar. Rydberg conducted a systematic investigation on the basis of a modification of Balmer's law for hydrogen, namely, n=n.-N/(m+μ)2. He found that in the group of alkaline metals three series of lines exist, the so-called principal and two subordinate series, whose frequencies fit approximately into this formula, and that similar statements apply to other natural groups of elements; that the constant N is sensibly the same for all series and all substances, while n, and u have different values for each; and that other approximate numerical relations exist. In each series the lines of high frequency crowd together towards a definite limit on the more refrangible side; near this limit they would, if visible, constitute a band. The principal or strongest series of lines shows reversal very readily. The lines of the first subordinate series are usually nebular, while those of the second subordinate or weakest series are sharp; but with a tendency to broaden towards the less refrangible side. In most series there are, however, not more than six lines visible: helium and hydrogen are exceptions, no fewer than thirty lines of the principal series of the latter having been identified, the higher ones in stellar spectra only. But very remarkable progress has recently been made by R. W. Wood, by exciting fluorescent spectra in a metallic vapour, and also by applying a magnetic field to restore the lines sensitive to the Zeeman effect after the spectrum has been cut off by crossed nicols. The large aggregates of lines thus definitely revealed are also resolved by him into systems in other ways; when the stimulating light is confined to one period, say a single bright line of another substance, the spectrum excited consists of a limited number of lines equidistant in frequency, the interval common to all being presumably the frequency of some intrinsic orbital motion of the molecule. In this way the series belonging to some of the alkali metals have been obtained nearly complete.

indicates, moreover, that the true absorption bands in a gas | progress has been made in recent years. of sufficiently low density must be extremely narrow. There is direct evidence that many of the more permanent gases do not sensibly emit light on being subjected to high temperature alone, when chemical action is excluded, while others give in these circumstances feeble continuous spectra; in fact, looking at the matter from the other side, the more permanent gases are very transparent to most kinds of radiation, and therefore must be very bad radiators as regards those kinds. The dark radiation of flames has been identified with that belonging to the specific radiation of their gaseous products of combustion. There is thus ground for the view that the impacts of the colliding molecules in a gas, or rather their mutual actions as they swing sharply round each other in their orbits during an encounter, may not be sufficiently violent to excite sensibly the free vibrations of the definite periods belonging to the molecules. But they may produce radiation in other ways. While the velocity of an electron or other electric charge is being altered, it necessarily sends out a stream of radiation. Now the orbital motions of the electrons in an actual molecule must be so adjusted, as appears to be theoretically possible, that it does not emit radiation when in a steady state and moving with constant velocity. But in the violent changes of velocity that occur during an encounter this equipoise will be disturbed, and a stream of radiation, without definite periods, but such as might constitute its share of the equilibrium thermal radiation of the substance, may be expected while the encounter lasts. At very high temperatures the energy of this thermal radiation in an enclosure entirely overpowers the kinetic energy of the molecules present, for the former varies as T, while the latter measures T itself when the number of molecules remains the same. The radiation which can be excited in gases, confined as it is to extremely narrow bands in the spectrum, may indeed be expected to possess such intensity as to be thermally in equilibrium with extremely high temperatures. That the same gases absorb such radiations when comparatively cold and dark does not, of course, affect the case, because emissive and absorptive powers are proportional only for incident radiations of the intensity and type corresponding to the temperature of the body. Thus if our adiabatic enclosure of § 3 is prolonged into a tube of unlimited length which is filled with the gas, then when the temperature has become uniform that gas must send back out of the tube as much radiation as has passed down the tube and been absorbed by it; but if the tube is maintained at a lower temperature, it may return much less. The fact that it is now possible by great optical dispersion to make the line-spectra of prominences in the middle of the Sun's disk stand out bright against the background of the continuous solar spectrum, shows that the intensities of the radiations of these prominences correspond to a much higher temperature than that of the general radiating layer underneath them; their luminosity would thus seem to be due to some cause (electric or chemical) other than mere temperature. On the other hand, the general reversing gaseous layer which originates the dark Fraunhofer lines is at a lower temperature than the radiating layer; it is only when the light from the lower layers is eclipsed that its own direct bright-line spectrum flashes out.mination of the formula for the disturbed motions of the system It is not necessary to attribute this selective flash-spectrum to temperature radiation; it can very well be ascribed to fluorescence stimulated by the intense illumination from beneath. When the radiation in a spectrum is constituted of wide bands it may on these principles be expected to be in equilibrium with a lower temperature than when it is constituted of narrow lines, if the total intensity is the same in the cases compared; this is in keeping with the easier excitation of band spectra (cf. the banded absorption spectra), and with the fact that various gases and vapours do appear to emit band spectra more or less related to the temperature.

15. Constitution of Spectra. In the problem of the unravelling of the constitutions of the very complex systems of spectral lines belonging to the various kinds of matter, considerable

Simultaneously with Rydberg, the problem of series was attacked by Kayser and Runge, who, in reducing their extensive standard observations, used the formula n=A+Bm-2+Cm ̄*, higher terms in this descending series being presumed to be negligible. This cannot be reconciled with Rydberg's form, which gives on expansion terms involving m3; but for the higher values of m the discrepancies rapidly diminish, and do not prevent the picking out of the lines, the frequency-differences between successive lines then varying roughly as the inverse squares of the series of natural numbers. For low values of m neither mode of expression is applicable, as was to be expected; and it remains a problem for the future to ascertain if possible the rational formula to which they are approximations. More complex formulas have been suggested by Ritz and others, partly on theoretical grounds.

Considered dynamically, the question is that of the deter

which constitutes the molecule. Although we are still far from any definite line of attack, there are various indications that the quest is a practicable one. The lines of each series, sorted out by aid of the formulae above given, have properties in common: they are usually multiple lines, either all doublets in the case of monad elements, or generally triplets in the case of those of higher chemical valency; in very few cases are the series constituted of single lines. It is found also that the components of all the double or triple lines of a subordinate series are equidistant as regards frequency. In the case of a related group of elements, for example the alkaline metals, it appears that corresponding series are displaced continually towards the less refrangible end as the atomic weight rises; it is found also that the interval in frequency between the double

to the tension of the cord. A single spectral line might thus be transformed into a band of this type, as the effect of disturbance arising from slight elastic connexions established in the molecule between a system of similar vibrators. But the series in line-spectra are of entirely different constitution; thus for the series expressed by the formula p2=po2-Bm-2 the corresponding period-equation might be expressed in some such form as sin k(p3 - p.2) constant, which belongs to no type of vibrator hitherto analysed.

AUTHORITIES.-The experimental memoirs on the constitution of radiation are mostly in the Annalen der Physik; references are given by P. Drude, Lehrbuch der Optik, Leipzig, 1900; cf. also. Physics, Paris, 1900. See also Lord Rayleigh's Scientific Papers, reports in the collection issued by the International Congress of in various connexions; and Larmor, in Brit. Assoc. Reports, 19001902, also the Bakerian Lecture, Roy. Soc. Proc., 1909, for a general discussion of molecular statistical theory in this connexion. Planck's Theorie der Wärmestrahlung, 1906, gives a discussion from his point of view; there is a summary by Wien in Ency. Math. Wiss. v. (3) pp. 282-357; also a lecture of H. A. Lorentz to the Math. Congress at Rome, 1908, and papers by J. H. Jeans, Phil. Kayser's extensive treatise is the standard authority. WinckelMag., 1909, on the partition of energy. In spectrum analysis mann's Handbuch der Physik, vol. ii. (by Kayser, Drude, &c.), may also be consulted. UJ. L.*)

lines of a series diminishes with the atomic weight, and is pro- | such as, for example, a row of masses fixed along a tense cord, portional to its square. These relations suggest that the atomic and each subject to a restoring elastic force of its own in addition weight might here act in part after the manner of a load attached to a fundamental vibrating system, which might conceivably be formed on the same plan for all the metals of the group; such a load would depress all the periods, and at the same time it would split them up in the manner above described, if it introduced dissymmetry into the vibrator. The discovery of Zeeman that a magnetic field triples each spectral line, and produces definite polarizations of the three components, in many cases further subdividing each component into lines placed usually all at equal intervals of frequency, is explained, and was in part predicted, by Lorentz on the basis of the electron theory, which finds the origin of radiation in a system of unitary electric charges describing orbits or executing vibrations in the molecule. Although these facts form substantial sign-posts, it has not yet been found possible to assign any likely structure to a vibrating system which would lead to a frequency formula for its free periods of the types given above. Indeed, the view is open that the group of lines constituting a series form a harmonic analysis of a single fundamental vibration not itself harmonic. If that be so, the intensities and other properties of the lines of a series ought all to vary together; it has in fact been found by Preston, and more fully verified by Runge and others, that the lines are multiplied into the same number of constituents in a magnetic field, with intervals in frequency that are the same for all of them. When the series consists of double or triple lines the separate components of the same compound line are not affected similarly, which shows that they are differently constituted. The view has also found support that the different behaviours of the various groups of lines in a spectrum show that they belong to independent vibrators. The form of the vibration sent out from a molecule into the aether depends on the form of the aggregate hodograph of the electronic orbits, which is in keeping with Rayleigh's remark that the series-laws suggest the kinematic relations of revolving bodies rather than the vibrations of steady dynamical systems.

According to Rydberg, there is ground for the view that a natural group of chemical elements have all the same type of series spectrum, and that the various constants associated with this spectrum change rapidly in the same directions in passing from the elements of one group to the corresponding ones of the following groups, after the manner illustrated in graphical representations of Mendelécff's law by means of a continuous wavy curve in which each group of elements lies along this same ascending or descending branch; the chemical elements thus being built up in a series of types or groups, so that the individuals in successive groups correspond one to one in a regular progression, which may be put in evidence by connecting them by transverse curves. Illustrations have been worked out mathematically by J. J. Thomson of the effect of adding successive outer rings of electrons to stable vibrating collocations.

The frequencies of the series of very close lines which constitute a single band in a banded spectrum are connected by a law of quite different type, namely, in the simpler cases n2= A-Bm2. It may be remarked that this is the kind of relation that would apply to a row of independent similar vibrators in which the neighbours exert slight mutual influence of elastic type. If & denote displacement and x distance along the row, dt dt the equation=-8- would represent the general features of their vibration, the right-hand side arising from the mutual elastic influences. If the ends of the line of vibrators, of length 1, are fixed, or if the vibrators form a ring, the appropriate type of solution is to sin ux sin pt, where ulm and m is integral; further- p2+k2=gu2, hence p2=k2: m2, which is of the type above stated. Dynamical systems of this kind are illustrated by the Lagrangean linear system of connected bodies,

RADICAL (Lat. radix, a root), in English politics, a term applied to politicians who desire to make thorough, or radical, changes in the constitution and in the social order generally. Although it had been used in a somewhat similar way during the reign of Charles II., the term Radical, in its political sense, originated about the end of the 18th century, probably owing its existence to Charles James Fox, who, in 1797, declared that "radical reform " was necessary. The ideas of the first Radicals were borrowed largely from the authors of the French Revolution. The word was more generally employed during the disturbed period between the close of the Napoleonic wars and the passing of the great Reform Bill of 1832, and was applied to agitators like Henry Hunt and William Cobbett. After the Reform Bill had become law, the advocates of violent change were drawn into the Chartist movement, and the Radicals became less revolutionary both in speech and object. Thus in 1842 an observer writes:-" The term Radical, once employed as a name of low reproach, has found its way into high places, and is gone forth as the title of a class who glory in their designation." About this time many members of Parliament were known as Radicals, among these men being George Grote and Joseph Hume. The Radicals never formed Ia distinct party in the House of Commons, and subsequently they formed simply the advanced section of the Liberal party. For a few years in the 19th century the wearing of a white hat was looked upon as the distinguishing mark of a Radical, a hat of this colour having been worn by Hunt when addressing meetings.

See W. Harris, History of the Radical Party in Parliament (1885); S. Bamford, Passages in the Life of a Radical (new ed., 1893); C. B. Roylance Kent, The English Radicals: an Historical Sketch (1899).

RADIOACTIVITY. The subject of radioactivity deals with phenomena exhibited by a special class of bodies of high atomic weight of which uranium, thorium, radium and actinium are the best known examples. These substances possess the property of spontaneously emitting radiations of a special character which are able to penetrate through matter opaque to ordinary light. The beginning of this subject dates from 1896, and was an indirect consequence of the discovery of the X rays made a few months before by Röntgen. It was known that the production of X rays in a vacuum tube was accompanied by a strong phosphorescence of the glass, and it occurred to several investigators that ordinary substances made phosphorescent by visible light might emit a penetrating radiation similar to X rays. Following out this idea, H. Becquerel (1),' a distinguished French physicist, exposed amongst other substances a phosphorescent compound of uranium, uranium1 These numbers refer to papers noted under References (below).