There are few subjects connected with heat which are more inexplicable, than the manner in which it is generat ed by those means which appear to act mechanically, as friction, percussion, and condensation. The latter of these operations, condensation, has already come under our no tice, in treating of the change of capacity which gases ex perience, according as their particles are compressed to gether, or allowed to expand themselves; and the same effect, at. least to a certain extent, seems to follow from the condensation of solids. A very familiar instance of this occurs in the heating of a bar of iron merely by repeated hammering, where, by a few smart blows on the anvil, the smith is able to produce a sufficient extrication of heat to kindle a match. It is, however, known that iron does not possess this property of producing heat by being hammer ed, unless it has been previously heated for some time in the forge, and suffered to cool slowly in the air ; and that, after it has been hammered, and lost part of its heat, its texture seems to become harder, and it is rendered less flexible than before the operation. The rolling out of me tals into thin sheets or leaves, and the drawing of fine wires, likewise excite heat, which is to be referred to the compres sion the substance undergoes during these various opera tions. In all these cases there is seine reason to suspect, that the bodies have had their capacity diminished, and that the heat, which becomes sensible, is to be assigned to this change. The effect of percussion may perhaps be explained on the same principle, as we may conceive, that, whenever percussion takes place, it must cause a propor tionate condensation. It is to be remarked, that liquids seem to be incapable of extricating heat, either by any at tempts at condensation, or by any kind of percussion ; and as they are supposed to be nearly incompressible, we may conceive that their capacity is not liable to be affected by these means. Gases, however, which are readily com pressible, are capableof having heat extricated from them, to a considerable amount;by condensation; and here we suppose that their capacities are changed, and in this way we account for the effect.
This mode of reasoning will not, however, apply to the extrication of heat which is caused by friction. The pow er of friction in producing heat, in various mechanical ope rations, is the subject of daily observation, and, if not pre vented by different expedients, produces the most fatal consequences. The motion of wheels, and pulleys upon their axes, excites a degree of heat, which would quickly set fire to the machinery, was it not prevented by the ap plication of different lubricating substances, which dimin ish the friction, by interposing between the hard surfaces a body which readily yields to the impression. The quan tity of heat generated in these cases is so considerable, and seems so disproportioned to any effect that can be attribut ed to a change either in the capacity or chemical nature of the body employed, as to have been always regarded a serious objection to the hypothesis, which regards heat as a substance capable of being transferred trout one body to another, like other material agents. The objection was
reduced to a more palpable form by Rumford, who estima ted the exact amount of the heat which was produced by a certain degree of friction, where all other extraneous sources for its admission were carefully guarded against. A piece of brass was fixed in a machine used for boring cannon, and a steel cylinder was pressed against the brass, with a weight equal to 1000 lbs. and then made to revolve On its axis with a given velocity. After some preparatory experiments, the apparatus was all inclosed in a vessel of water ; and after the friction had been kept up for some time, the water was actually brought to the boiling heat. Here a very considerable quantity of heat was liberated, and the only mechanical change effected upon the materi als was, that a quantity of brass turnings were formed; but neither these, nor the cylinder itself, appeared to have experienced any change, except a slight degree of com pression. Rumford found by experiment, that the capacity of these turnings would not be affected by the operation ; and the effect of the compression, which the metal had experienced must have been very inconsiderable. Yet the power of the substance to extricate heat was apparently unlimited ; for there is no reason to suppose that any thing Iike exhaustion was produced, or that the apparatus would not have continued to evolve heat, until its texture had been destroyed, by tne brass being all reduced into small frag ments. (Essays, vol. ii. p. 469, et. seg.) Upon this ex periment we shall offer some remarks in the next Section, where we treat upon the nature of heat.
The heat excited by the electric or galvanic shock has been commonly referred to a mechanical cause, although upon this point, a considerable diversity of opinions has pre vailed. The effect, however, is well known to be very pow erful, perhaps even More so than that produced by the con vex lens ; but it is still more confined as to the extent of its operation. If the two wires, in the interrupted galvanic circuit, be brought nearly into contact, and any substance, in very minute quantity, be placed upon them, it may be subjected to a temperature more intense than can be pro duced in any other manner ; and by this means bodies have been burned or fused that had been before completely in tractable. Whether in this operation the heat is, as it were, merely forced out of the wire by the commotion which its particles experience from the passage of the galvanic in fluence ; or whether, as has been supposed, under certain circumstances, heat and electricity can be converted into each other, or may be separated by a kind of decomposi tion, are intricate questions of theory, upon which it seems at present beyond our power to decide, and which must depend very much upon the opinion that we entertain re specting the nature of heat. The simple facts, however, independent of hypothesis, seem to indicate, that heat and electricity are distinct from each other, whether they are to be regarded as species of subtile fluids, or only as pro perties of matter.