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to the intellect of man; but, after all, their elongation of the bars, during this expanded speculations have produced little that is state of the metal, the nuts were screwed up convincing to others, or even satisfactory to close to the wall again, and, upon the rethemselves. A short abstract of the prin- moval of the lamps, the contraction of the cipal facts which have resulted from the bars actually brought together the walls of experiments tried by philosophers advocating the building. The operation was repeated both theories, will be submitted to the upon alternate bars, till the walls had been reader, and he may judge for himself which restored to their true vertical position. system seems most likely to explain the It is well known, that provision must be various phenomena.

made for the expansion or contraction conIt is a general fact, that all substances sequent upon a change of temperature, in experience an enlargement of their volume the construction of iron bridges, and the by an increase of temperature. The ap- pipes which are placed under ground for parent exceptions to this law, can scarcely the conveyance of water or gas. Where be considered such as to destroy its gene- the lengths of the pipes are very conrality, though they present irregularities in siderable, it is necessary to have some points its application, which it would be injudicious moveable ; so that, by the end of one tube to overlook. Such is the contraction of sliding a little within the other, the acciwater, by an increase of temperature below dental changes, induced by an excess or 40° of Fahrenheit

. The expansion of some diminution of caloric, are provided for. In metals at the instant of congelation after measuring the base of a trigonometrical fusion, is probably owing to a tendency of survey, the utmost care is taken to allow their particles to arrange themselves in a for the contractions and expansions of the crystalline form. In solids this enlarge- metallic rods employed, as a slight change ment of bulk is not so great as in liquids, of temperature has been productive of very but it is the most conspicuous in the aeri- serious mistakes in the admeasurements. form fluids, or gases ; and it appears that The effects of caloric upon the pendulum the greater the absolute dilatation, the less is are well known. the force with which it takes place. For It has been considered as a universal law instance, the dilatation of the gases may be of nature, that, by a sufficient elevation of overcome by the confinement of strong temperature, every substance which is orivessels ; but in the case of liquids, or solids, ginally solid, might be reduced to a fluid the force of this power seems almost irresist- state. It is true, that in practice we fall ible. The strongest rocks are rent by the short of this result, partly because the heat expansive force of water freezing in their which we can produce by artificial means is fissures ; and strong cannon have been burst of very limited extent, and partly from in a similar manner, by water confined other properties in the constitution of bodies, within them.

which interfere with such a consequence. The actual expansive force of solids in It has, however, been found that every the process of heating, has not been made increase which has been made in our powers the subject of any thing like admeasure- for the excitement of intense heat, whether ment; but a very curious application of the by the Voltaic pile, or the gas blow-pipe, force exerted in the converse operation of has placed various substances in succession, cooling, was made a few years ago, in Paris, under the operation of this law, which before by M. Molard. It was discovered, that the formed practical exceptions to it, and an side walls of a large room filled with en- absolutely infusible body now is scarcely gines, at the Conservatoire des Arts and known to exist. It is true, many substances Métiers, were bulging outwards, from the burn rather than melt, but this depends great internal pressure. To remedy this, upon what is called chemical affinity, by strong bars of iron were passed quite which it happens, in some instances, that a through the building; the extremity of each new combination is formed, upon the subbar passing through the main outer wall. version of the original one. The ends of the bars were formed into In some cases, the passage of a solid into screws, and fitted with nuts, which, being the Auid state, upon the application of screwed up closely to the wall, might have caloric, is made immediately, without any served to prevent further mischief. Still, intermediate process of softening ; such is however, the walls had to be brought into the fusion of ice, frozen mercury, and some their original position, to effect a complete of the metals. It has been remarked, that cure; this was accomplished by heating this process chiefly holds good as it regards each bar, a row of lamps being placed those bodies, which, in congelation, affect beneath it, for the purpose; the heat of the a crystalline structure. Other bodies, on lamps having produced a considerable the contrary, seem to change gradually from

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solid to liquid, passing through every inter- at greater distances, this will cease to operate;
mediate degree of cohesion. Arsenic differs any effect from figure must be insensible,
from both processes, this metal, when heated, and the particles will attract equally in ali
passing at once from a solid to a gaseous directions—the circumstance which con-
state. It still seems an established fact, that stitutes the liquid form."
the temperature of Auidity is constant for By the application of heat, a very con-
each individual substance.

siderable number of bodies, both solid and The complete explanation of the state of liquid, may be converted into the form of fluidity, with reference to the general physi- gaseous matter; and as long as a sufficient cal laws of matter, is not so easy a task as elevation of temperature continues, the form might at first be supposed. It has long of an elastic fluid is retained. It is clearly been stated, that fluidity depends upon the proved, that in this state they are liquids in effect of caloric, which first expands bodies, combination with caloric, for, on violent that is to say, removes their integrant condensation, they give out a great degree particles, or atoms, to a greater distance of heat. The only one of the gaseous fluids from each other, until at length, the power which has been liquified by cold alone, is of cohesion being overcome, perfect mobility ammoniacal gas, which assumes the liquid among the particles brings the substance form at 54° of Fahrenheit. Mr. Faraday under the laws and definition of a fluid. has recently shewn, that, by disengaging But this mode of explaining the action of several of the gases from their state of comcaloric, will not apply in all cases, for it bination, under such a pressure that the elastic does not account for the fact, that many form could never be assumed, they might crystalline solids actually contract in the be exhibited as liquids. As by the increase process of fusion, which is quite contrary to of temperature all bodies may be melted, the hypothesis.

and, by a further accession of temperature, Professor Robison remarks, that “to ex- pass to the state of an elastic fluid; so, on plain the mobility of a fluid, or the facility the contrary, there is reason to believe that, with which its parts are separated, it is by a sufficient depression of temperature, ali necessary to suppose only, that the action those bodies which we know now in a of its particles, whatever it may be, is equal gaseous or liquid state, might be reduced in every direction at the same distance; as, to a solid form. Many philosophers supif this equality exist, no force can be required pose that congelation, or solidity, is the to move an adjacent particle from one situa- natural state of all substances. tion to another; nor any force be required, Though great doubt exists relative to the to keep the particle in its new situation substance of caloric, it is evident that there with regard to the rest of the fluid. And are three natural sources from whence it may still the attraction exerted between the be derived, i. e. the sun, electricity, and particles, provided it be equal, may be mechanical action. strong. On the other hand, in a solid, the The sun is the most obvious and unvary. particles must attract more strongly in one ing source from whence heat is communidirection than in another; hence a parti- cated to our earth. The solar rays have cular situation of each particle must be lately been discovered to possess four disassumed, and a force, more or less great, tinct powers, i. e. of heating, illuminating, will be requisite to change its position. It effecting chemical changes, and exciting follows from this view, that fluidity arises magnetism ; and, it is now generally supnot merely from the weakness of cohesion posed, that the rays affording light and between the particles, but also from the heat are entirely different, though so intichange in the mode in which they attract mately blended as to obey the same optical each other. But this, at the same time, laws. The experiments of Sir W. Herschel arises from the expansive energy of caloric, on the heating power of the several priswhich, in separating the particles to certain matic rays, showed satisfactorily, that the distances, gives rise to this change in their more refrangible rays possess very little mutual action; whence they assume diffe- heating power, the calorific effect being at rent positions, and attract with a different its maximum in the extreme red rays. but equal force. It is probable that this It is well known, that when transparent depends on the modification introduced by bodies are exposed to the light of the sun, the figure of the particles of bodies. Within that the heating effect is greatly inferior to a certain distance this must operate on the that which is produced upon opake substrength of the attraction they exert; and stances. Opake bodies, which all equally they will be retained in a certain position resist the passage of light, are not equally rather than in others. But when, by the affected in temperature by its incidence repulsive agency of caloric, they are placed upon them. The power of absorbing caloric

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from the sun's rays, depends in a great is as follows. If a charge from a large measure upon the colour. This was made jar be passed through a small wire, such the subject of experiment by Hooke, Frank- a degree of heat is evolved, as to produce lin, and, more recently, by Sir Humphry the ignition of the wire. In this phenoDavy.

menon the sudden transit of the electric Dr. Franklin placed some square pieces fuid is the primary cause : now, if what of cloth of different colours upon a surface is termed electricity, which is allowed to of snow, and noted those as the best ab- be a modification of caloric, be in reality sorbers of caloric, which sunk most deeply a substantial fluid, it would be consistent by melting the snow. Sir H. Davy exposed with observed analogies, that when a conto the sun, one side of six copper plates, siderable mass of this eminently expansible which had been painted of different colours. fluid, is forced through a very small channel, To the opposite side of each plate, he and thereby greatly condensed, heat and affixed a bit of cerate, which melted at a light should be evolved. temperature of 70° of Fahrenheit. The The effects of gradually increasing the wax melted on the coloured plates in the power of the charge, when wires of the following order ;-first, black; then blue, same length and diameter are employed, green, red, yellow ; and, lastly, on the are very remarkable. If the wire be iron white.

or steel, its colour is first changed to yellow, The same colours, exposed to artificial then (by an increased charge) to blue ; by heat, present the same results. Cavallo a further increase it becomes red-hot, then found that a thermometer with a blackened fused into balls, which disperse in a shower bulb stood - higher than one with its bulb of globules, and, lastly, disappears with a not blackened, whether exposed to the bright flash, producing an apparent smoke, sun's rays, the light of day, or the light of which, if collected, proves to be a very a lamp. M. Pictet also found that two fine powder, weighing more than the metal thermometers, one blackened, the other employed, and consisting of it, and a portion not, manifested the same temperature in of the oxygen of the atmosphere with which the dark. Such are the effects of the beams it has combined. of the sun in their ordinary state, but when The experiments of Mr. Cavallo have condensed by being collected in the focus shewn that the electric fluid differs in its of a concave metallic speculum, their force action upon metals from the common fire, is irresistible, and only to be excelled by and that they are thus distinct substances. the powers of the Voltaic pile : the most He likewise discovered, in fusing the grains refractory substances become fused, while of native platinum by the electric spark, the more volatile dissolve in the fervent that the largest grains were the easiest acted heat. Count Rumford endeavoured to prove, upon, being agglutinated together by a by a course of experiments, that this great moderate shock, while the small greyincrease of power is not the effect of any coloured dust was so refractory as not to change in property, from the altered direction shew any signs of fusion, even when exof the rays, by thus concentrating them amined by the microscope. This remarkable into a focus, but is solely due to the in- fact has been explained by the discoveries tensity of action, arising from the accumu- of Dr. Wollaston, that the small grey parlation of numbers, thus brought to bear on ticles, found mixed with those of the native the same point.

platinum, are in reality different metals, It has long been known that electricity much less capable of fusion, and now is one grand source of caloric, but Dr. known by the names of rhodium, osmium, Franklin was the first who fused metals by and iridium. It has been found by various the electric spark. His method was, to place experiments, that electricity accelerates the small strips of metal between two plates of transmission of caloric in a remarkable glass firmly tied together, and then put them degree; and that metals heated to incanwithin the circuit of the electrical discharge descence by electricity, continue to evolve from a jar or battery. In this experiment light longer than if heated by a common the glass is frequently broken, and a partial fire, other circumstances remaining the fusion of both substances takes place. same. Franklin found that a piece of gold leaf, There are nominally three modes of which he had made use of in this way, re- mechanical action, by which heat is desisted the action of nitro-muriatic acid, from veloped, -percussion, condensation, and the circumstance of its being imbedded in friction.

If a piece of iron be struck with a hamOne strong fact in favour of the mate. mer, the metal acquires a sensible elevation riality of caloric, as derived from electricity, of temperature, and by repeated blows it

the glass.

may be made red-hot. Again, in the com- of the piston, this bit of tinder is in general mon process of striking a light with flint ignited. Desmartiers has shewn that atmoand steel, the heat is evolved by percussion; splieric air, or oxygen gas, are the only fluids and to such an extent, as to determine a with which condensation produces this chemical combination between the minute effect. M. Biot, by condensation, effected fragments of metal separated, and the the combination of oxygen and hydrogen oxygen of the atmosphere. The elevation gases, having compresed them violently by of temperature produced in metals by per- the piston of an air-gun. The heat evolved cussion, is said to be attended by conden- added so greatly to the expansive force of sation, that is, their density is increased. the gases, that, in two out of three ex

The best set of experiments on this subject periments, the barrel was burst by the has been made by Biot, Bertholet, and explosion. Pictet. The experiments were made upon That friction will evolve heat is well pieces of gold, silver, and copper, of the known, even among savage nations, who same size and shape; and care was taken frequently kindle their fires by rubbing two that all the parts of the apparatus had ac- pieces of dry wood smartly together. The quired the same temperature before the wheels of carts and coaches, when not proexperiments began. It may be shortly perly greased, will frequently take fire from stated, that copper evolved the most heat, the friction between the nave and axletree. silver was next in order, and gold evolved In what manner then is caloric evolved or the least. The first blow produced the accumulated in these cases ? Count Rummost heat in every instance, and it dimi- ford instituted a series of most interesting nished gradually, and after the third blow experiments, to ascertain the cause of was hardly perceptible. The heat acquired this phenomenon : in a work like this it was estimated by throwing the piece of would take up too much space to detail metal struck into a quantity of water, and his various experiments, but the results may ascertaining the change of temperature which be briefly mentioned. the water underwent. The change of spe- He found that the heat evolved by friction cific gravity in the metals, was found to be was not produced by increasing the density proportional to the heat thus evolved, thus of the bodies rubbed against each other, as shewing that condensation had accompanied happens in cases of percussion; for heat the action; hence, when they could no is produced by rubbing soft bodies together, longer be condensed, they ceased to evolve as any one may experience by rubbing his heat. In the rolling of metallic plates, and hand smartly against any woollen substance. in the drawing of wires, considerable heat Nor is it owing to the specific beat of the is evolved; and it is worthy of remark, that rubbed bodies decreasing, for the Count after the evolution of caloric from any metal found that there was no sensible decrease ; by mechanical pressure or percussion, the nor, if there were a decrease, would it be metal is rendered more brittle, and will not sufficient to account for the vast quantity of afford any more heat by a repetition of the heat which is sometimes produced by fricprocess, until after it has been again heated tion. Neither is it owing to, or connected in the fire. In this particular the effect of with, the decomposition of oxygen gas, for friction seems to differ-the source of heat precisely the same results ensued when the there appears inexhaustible.

apparatus was enclosed under an The development of caloric by conden- hausted receiver, and also when buried in sation is shewn most evidently, when aeri- water. form fluids are exposed to mechanical ope- In this last experiment, Count Rumford ration. It had been observed, that a slight contrived to enclose a metal cylinder, flash of light accompanied the discharge of bored to admit a blunt steel borer, in a an air-gun in the dark ; this led to the con- wooden box filled with water, so as to effec. struction of what is called the condensing tually exclude all air. The borer was made tinder-box. This instrunent consists of to press upon the bottom of the cylinder a brass tube about six inches long, closed with a weight equal to about 10,000 pounds at one end and open at the other. Into avoirdupois, and the cylinder to revolve at this tube there is fitted a piston, which, by the rate of thirty-two times in a minute. means of a little silk or leather well greased, The quantity of water amounted to 18.77 is made to fit the tube accurately. At the pounds avoirdupois, and, at the beginning end of the piston, a fragment of a particular of the experiment, was at the temperature sort of tinder, called amadou, which is made of 60°. After the cylinder had revolved of a species of fungus well beaten, steeped for an hour, the temperature of the water in a solution of nitre, and then dried, is was 107°; in thirty minutes more it arose fixed ; and by one rapid and violent stroke to 178° ; and in two hours and thirty

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minutes from the commencement of the with safety; but by blackening its surface, experiment, the water actually boiled. the reflecting power was destroyed, and in

M. Haldot repeated the experiments of four minutes it became too hot to hold : Count Rumford, and obtained the same thus shewing, that the calorific rays proceedresults. He found that different metalsing from a common fire, follow, in some gave different degrees of heat; zinc evolved measure, the same law with those proceeding the greatest degree of heat, then brass and from the sun. lead, and afterwards tin, which only pro- From the experiments of Herschell, it duced of the heat evolved during the fric- would, however, appear, that the calorific tion of lead. By quadrupling the pressure, rays which accompany the solar light, and the heat evolved was seven times greater those which issue from heated bodies, than before. When the rubber was rough, though similar in some points, have some it produced but half as much heat as when dissimilarity; for the former pass through smooth. When the apparatus was sur- transparent media with much greater ease rounded by bad conductors of heat, or by than the latter. On exposing two thermonon-conductors of electricity, the quantity meters of equal sensibility, the one covered of heat evolved was diminished. Mr. Wedg- with glass, the other uncovered, first to the wood found, that by pressing a piece of solar rays, and afterwards to those of a window-glass against a revolving wheel of candle, he found that a greater proportion grit, the glass became red-hot at the point of calorific rays were intercepted in the of friction, and gave off sparks capable of latter case than in the former. igniting gunpowder. Sir H. Davy con- The experiments of Saussure and Pictet trived to melt ice by friction, within an seem to prove that the calorific rays exist atmosphere which was not suffered to rise independently of the luminous ones; that above the temperature of 32° of Fah- they proceed in right lines from heated renheit.

bodies; and that they are capable of reThe most obvious properties of caloric flection from polished metallic substances. are radiation, reflection, absorption. These gentlemen placed two concave inirrors

Radiation may be defined to be the free of polished tin, each a foot in diameter, at motion of heat ; that is, those philosophers a distance of twelve feet apart; the focal who suppose caloric to be an existent ma- length of the mirrors was four and a half terial fluid, suppose that the particles of inches each. In the focus of one was the which this Auid is composed, are endued bulb of a thermometer, and in the focus of with a vast idio-repulsive force, and that the other they placed a ball of iron two inches they move in right lines with unmeasured in diameter, which was first heated red, and velocity in appropriate media, wherein no then suffered to cool until it ceased to be resistance is opposed to them.

visible in the dark. Another thermometer The earliest experiments usually cited was placed at the same distance from the upon this subject, are those of Mariotte. heated ball as the former one, but without He states that “ the heat of a fire reflected the focus of the reflecting mirror. Upon by a burning mirror is sensible in its focus ; the introduction of the heated ball to its but if a glass screen is interposed between place, the thermometer instantly rose, and the mirror and the focus, the heat is no in six minutes indicated an increase of temlonger sensible.” Scheele is the first who perature of 100.5 of Reaumur, while that made use of the term radiant heat, and not in the focus advanced only 20.5. Here shewed that it did not communicate warmth the two thermometers, being at equal disto the air through which it was made to tances, may be supposed to bave been pass. He proved also, that its passage equally affected by the direct rays from the through a space filled with air, was not hot ball; but the one in the focus of the changed in direction by a current in that mirror received in addition the reflected air, and that its intensity was not diminished rays, and of course rose much higher. by violent agitation taking place in the air. To prove that the calorific rays exist inBy the interposition of a pane of glass dependently of the luminous ones, and that between the fire and his hand, he found the they can only proceed in right lines, M. heat was intercepted, though the light was Pictet made use of a lighted candle in place transmitted, and might afterwards be con- of the heated ball. The candle was put in centrated to a focus by a lens. He states one focus; and when it had raised the therthat a glass mirror reflects the light of a mometer in the opposite focus from 2° to fire, but not the heat; that a polished me- 12°, a plate of glass was interposed, and tallic surface reflects both the light and the in nine minutes the thermometer fell to heat. The metallic reflector, therefore, may 5o.7, and rose again on the removal of the be opposed to the fire, and held in the hand glass. llere was an instance of a trans

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