symbol of the unseen Hooke illuminated to a marvellous extent every subject touched by his genius. He refers to his own observations on the shining sparks of steel produced by a quick and violent motion, proving them to be perfectly similar to the sparks produced by heating the steel particles in a flame. Like Boyle, he also refers to the heating of iron by filing. He compares the vibrations of heat with sonorous vibrations, and adds the following remark: " Now, that the parts of all bodies, though never so solid, do yet vibrate, I think we need go no farther for proof than that all bodies have some degrees of heat in them, and that there has not yet been found anything perfectly cold." Despite these utterances and arguments, so clear, and, it might be added, so conclusive, in favor of the mechanical or dynamical theory of heat, its rival, the material theory, found a firm lodgment in many scientific minds. Within certain limits this theory involved conceptions of a very simple kind, and this simplicity secured for it a century ago universal acceptance. It was then assailed by Benjamin Thompson, better known as Count Rumford; but, despite his assault, it held its ground until quite recently among the chemists of our own day. The laborious. Gmelin, for example, in his great Handbook of Chemistry, defines heat to be that substance whose entrance into our bodies causes the sensation of warmth and its egress the sensation of cold. He also speaks of heat combining with bodies as They combining with bodies as one ponderable substance does with another, and I have frequently heard other eminent chemists treat the subject from the same point of view. These notions regarding the nature of heat were generally prevalent among the scientific writers of the seventeenth century. They were also shared by philosophical writers. In his Essay on the Human Understanding, Locke frequently refers to heat as being kind of motion. But the very remarkable utterance which of late years has been most widely circulated is the following. "Heat," says Locke, "is a very brisk agitation of the insensible parts of the object, which produces in us that sensation from whence we denominate the object hot; so what in our sensation is heat, in the object is nothing but motion. This appears by the way heat is produced; for we see that the rubbing of a brass nail upon a board will make it very hot, and the axletrees of carts and coaches are often hot, and sometimes to a degree that it sets them on fire, by the rubbing of the naves of the wheels upon them. On the other side, the utmost degree of cold is the cessation of that motion of the insensible particles which to our touch is heat." RUMFORD ON THE FRICTION OF IRON.-DAVY With Rumford, however, a new and powerful factor appeared on the scene. He began by proving the hypothetical matter of heat to be imponderable, but the main drift of his experiments was to prove friction to be an inexhaustible source of heat, while the whole force of his logic went to show that an inexhaustible emission is irreconcilable with the notion that heat is a kind of matter. By those who held the material theory the matter of heat was supposed to hide itself in the inter-atomic spaces of bodies, out of which it could be squeezed by compression or percussion, as water is squeezed from a sponge. They were acquainted with the fact that different bodies possess different powers of holding heat, if such a term may be employed. Take, for example, the two liquids water and mercury, and warm a pound of each of them, say from fifty degrees to sixty. The absolute quantity of heat required by the water to raise its temperature ten degrees is fully thirty times the quantity required by the mercury. Technically speaking, water was said to have a greater capacity for heat than mercury, and this term "capacity" suggests the views of those who invented it. Water was supposed to possess an enormous power of storing up caloric or the matter of heat-of hiding heat, in fact to such an extent that it required thirty measures of this caloric to produce the same sensible effect on it that one measure could produce upon the same weight of mer cury. All substances possess, in a greater or lesser degree, this apparent power of storing up heat. Lead, for example, possesses it; and our experiment with the lead bullet, in which heat was generated by compression, was explained by those who held the material theory in the following way: The uncompressed lead, they said (without, however, proving what they said), has a higher capacity for heat than the compressed substance; the size of its atomic storehouse is diminished by compression, and hence, when the lead is squeezed, a portion of that heat which previous to compression was hidden must make its appearance, for the compressed substance can no longer hold it all. In some similar way the experiments on friction and percussion were accounted for, the idea of calling new heat into existence being rejected by the believers in the material theory. According to their views, the quantity of heat in the universe is as constant as the quantity of ordinary matter, and the utmost we can do by mechanical and chemical means is to store up this heat or to drive it from its lurking-places into the open day. Such views were rudely shaken by the experiments and arguments of Rumford. Surprised by the degree of heat which a brass gun acquires in a short time on being bored, and the still more intense heat, "much greater than boiling water, of the metallic chips separated from it by the borer," he proposed to himself the following questions: "Whence comes the heat actually produced in the mechanical operation above mentioned? "Is it furnished by the metallic chips which are separated from the metal?" If this were the case, then the capacity for heat of the parts of the metal so reduced to chips ought not only to be changed, but the change undergone by them should be sufficiently great to account for all the heat produced. No such change, however, had taken place; for the chips were found to have the same capacity as slices of the same metal cut by a fine saw where heating was avoided. Hence, it is evident that the heat produced could not possibly have been furnished at the expense of the latent heat of the metallic chips. Rumford describes these experiments at length, and they are conclusive. He then designed a gun-metal cylinder for the express purpose of generating heat by friction. A blunt rectangular piece of hardened steel, called by Rumford a borer, was forced edgeways against the solid bottom of the cylinder while the latter was turned round its axis by the force of horses. To measure the heat developed, a small round hole was bored in the cylinder, into which was introduced a small mercurial thermometer. The weight of the cylinder was 113.13 lbs. avoirdupois. The borer was 0.63 of an inch thick, 4 inches long, and nearly as wide as the cavity of the bore of the cylinder namely, 3 inches. The area of the surface by which its end was in contact with the bottom of the bore was therefore nearly 23 inches. At the beginning of the experiment the temperature of the air in the shade, and also that of the cylinder, was 60° F. At the end of thirty minutes, after the cylinder had made nine hundred and sixty revolutions round its axis, the temperature was found to be 130°. Having taken away the borer, he now removed the metallic dust, or scaly matter, which had been detached from the bottom of the cylinder, and found its weight to be 837 grains troy. "Is it possible," he exclaims, "that the very considerable quantity of heat produced in this experiment—a quantity which actually raised the temperature of above 113 pounds of gun-metal at least 70° of Fahrenheit's thermometer-could have been furnished by so inconsiderable a quantity of metallic dust, and this merely in consequence of a change in its capacity for heat? But, without insisting on the improbability of this supposition, we have only to recollect that, from the results of actual and decisive experiments made for the express purpose of ascertaining that fact, the capacity for heat of the metal of which great guns are cast is not sensibly changed by being reduced to the form of metallic chips, and there does not seem to be any reason to think that it can be much changed, if it be changed at all, in being reduced to much smaller pieces by a borer which is less sharp." Rumford next surrounded his cylinder by an oblong deal box, so that the cylinder could turn water-tight in the centre of the box, while the borer was pressed against the bottom of the cylinder. The box was filled with water until the entire cylinder was covered, and then the apparatus was set in action. The temperature of the water on commencing was 60° Fahrenheit. 66 'The result of this beautiful experiment,' writes Rumford, "was very striking, and the pleasure it afforded me amply repaid me for all the trouble I had had in contriving and arranging the complicated machinery used in making it. The cylinder had been in motion but a short time when I perceived, by putting my hand into the water and touching the outside of the cylinder, that heat was generated. "At the end of one hour the fluid, which weighed 18.77 lbs., or 2 gallons, had its temperature raised 47°, being now 107°. "In thirty minutes more, or one hour and thirty minutes after the machinery had been set in motion, the heat of the water was 142°. "At the end of two hours from the beginning, the temperature was 178°. "At two hours and twenty minutes it was 200°, and at two hours and thirty minutes it ACTUALLY BOILED. "It would be difficult," says Rumford, "to describe the surprise and astonishment. expressed in the countenances of the bystanders on seeing so large a quantity of water heated and actually made to boil without any fire. Though there was noth ing that could be considered very surprising in this matter, yet I acknowledge fairly that it afforded me a degree of childish pleasure which, were I ambitious of the reputation of a grave philosopher, I ought most certainly rather to hide than to discover." I am sure we can dispense with the application of any philosophy which would stifle such emotion as Rumford here avowed. This is an extremely significant passage, intimating, as it does, that Rumford saw clearly that the force of animals was derived from the food, no creation of force taking place in the animal's body. 66 By meditating on the results of all these experiments we are naturally," he says, "brought to the great question which has so often been the subject of speculation. among philosophers—namely. What is heat? Is there any such thing as an igneous fluid? Is there anything that with propriety can be called caloric? 66 'We have seen that a very considerable quantity of heat may be excited by the friction of two metallic surfaces, and given off in a constant stream or flux in all Rumford carefully estimated the quantity of heat possessed by each portion of his apparatus at the conclusion of his experiment, and, adding all together, found a total sufficient to raise 26.58 lbs. of ice-cold water to its boiling-point, or through 180° Fahrenheit. By careful calculation he found this heat equal to that given out by the combustion of 2303.8 grains (=4% oz. troy) of wax. He then deter-directions without interruption or intermismined the "celerity" with which the heat was generated, summing up thus: "From the results of these computations, it appears that the quantity of heat produced equably-or in a continuous stream, if I may use the expressionby the friction of the blunt steel borer against the bottom of the hollow metallic cylinder was greater than that produced in the combustion of nine wax candles, each threequarters of an inch in diameter, all burning together with clear, bright flames. "One horse," he continues, "would have been equal to the work performed, though two were actually employed. Heat may thus be produced merely by the strength of a horse, and in a case of necessity this heat might be used in cooking victuals. But no circumstances could be imagined in which this method of procuring heat would be advantageous, for more heat might be obtained by using the fodder necessary for the support of a horse as fuel." sion, and without any signs of diminution or exhaustion. In reasoning on this subject we must not forget that most remarkable circumstance that the source of the heat generated by friction in these experiments appeared evidently to be inexhaustible. [The italics are Rumford's.] It is hardly necessary to add that anything which any insulated body or system of bodies can continue to furnish without limitation cannot possibly be a material substance; and it appears to me to be extremely difficult, if not quite impossible, to form any distinct idea of anything capable of being excited and communicated in those experiments, except it be MOTION." With regard to the illustration which compared heat to water contained in a sponge, Rumford replied thus: "A sponge filled with water and hung by a thread in the middle of a room filled with dry air communicates its moisture to the air, it is true, but soon the water evaporates and the sponge can no 66 longer give out moisture." The case, he contended, is not at all similar to heat, for here, by renewed mechanical action, we can cause the heat to flow out at will. "A bell," 'A bell," he says, sounds without intermission when it is struck, and gives out its sound as often as we please, without any perceptible loss. Moisture is a substance; sound is not." Heat, he contended, was typified by the vibrating bell, and not by the evaporating sponge. The conclusion drawn from these experiments by Rumford was contested by Bertholet, who stood forth as the champion "of the received theory of caloric." His arguments were fully set forth by Rumford, and totally overthrown. When the history of the dynamical theory of heat is completely written, the man who, in opposition to the scientific belief of his time, could experiment, and reason upon experiment, as Rumford did in the investigation here referred to, may count upon a foremost place. Hardly any thing more powerful against the materiality of heat has been since adduced, hardly anything more conclusive in the way of establishing that heat is what Boyle, Hooke and Locke considered it to be-motion. And here we may refer to an observation of Rumford's which indicates at once his penetration and the limit of his knowledge. In 1778 he was engaged in experiments on the force of gunpowder, employing a musketbarrel, which he sometimes fired without any bullet and sometimes with one, two, three, or even four, bullets. Immediately after each discharge it was his practice to seize the barrel in his hand while it was wiped out, and he was astonished to notice that the barrel was always hotter when the charge consisted of powder alone than when loaded with one or more bullets. Rumford rejected the notion that the gun was heated by the flame of the gunpowder, which he considered far too transitory to produce the heating effect observed. He referred that effect to mechanical concussion. Assuming heat to be " a more or less rapid vibratory motion among the particles of solid bodies," he concluded that when the powder alone was fired, the shock was "more vibrating or heavier" than when the combustion was obliged "to push slowly before it one or two balls which were anything but light." Had Rumford been aware of the entire bearing of the mechanical theory of heat, he would not, I think, have omitted to mention, in connection with this experiment, that the gunpowder urging the ball could not possibly generate the same amount of heat as when urging no ball. Rumford omitted all allusion to this, and Mayer was the first to discern the meaning of his observation. Stimulated probably by Rumford, with whom he was personally connected at the Royal Institution, Davy took up this subject and enriched it by a beautiful and conclusive experiment. Ice is solid water, and the solid has only one-half the capacity for heat that liquid water possesses. A quantity of heat which would raise a pound of ice ten degrees in temperature would raise a pound of water only five degrees. Further, simply to liquefy a mass of ice an enormous amount of heat is necessary, this heat being so utterly absorbed or rendered "latent" as to make no impression upon the thermometer. What I am desirous of impressing on you at present is that, taking the materialists on their own ground, liquid water, at its freezing |