THE

ACTION OF HEAT UPON MATTER.

WHAT is Heat? This question vexed the infant philosophy of antiquity, it went further than even the penetrating intellect of Newton and of Bacon could follow, and, with all the variety and subtlety of modern physical inquiry, it still remains unanswered. We can conceive of nothing with which primitive Man could have been more conversant than with the ordinary effects of Heat; and we can conceive of nothing in all the works of Creative Power, of which the first cause is further beyond the utmost range of human apprehension. We may discover further relations of Heat to matter, but our knowledge of the absolute nature of Heat itself can never be other than conjectural and inferential.

In the absence, however, of positive knowledge, an hypothesis, if nowhere in discordance with known physical facts, may serve a most useful purpose in explaining the action of Heat; especially if it embrace in an intelligible sequence of cause and effect, the phenomena of freezing, thawing, boiling, vaporisation, devaporation, rusting, welding, &c. Whatever Heat may be, in itself,

B

F

we shall have surmised to some purpose, if we can thus distinguish, with admissible probability, the mode in which it acts upon the particles of matter; not referring by this to the mere separation of material particles, for that is an effect, while we are seeking, if not the inscrutable cause, at least for the earliest physical circumstances which attend (or rather precede by an interval of inconceivable brevity), any given manifestation of Heat. The detection of these circumstances must be the ultimate achievement of physical research in this direction, whatever expanse for metaphysical speculation may still be left open to the mind.

In the language of science, Heat has now no reciprocal term. Bacon, it is true, regarded Heat as "the right hand of Nature, and cold as her left," and Boyle also believed in the "principle of cold." But the notion of spicular frigorific particles was generally rejected in the last century, especially by those who embraced the Galilean hypothesis, that Heat consists in the motions of the particles of matter. Rumford, who adopted this view, observed that Heat and cold were relative terms like fast and slow; but, that as there is no relation between motion and a state of rest, neither could there be between Heat and absolute cold, and that all attempts to refer the latter to the thermometric scale must be nugatory. This conclusion, so far as subsequent inquiry has shown, is correct, although the reasoning by which Rumford approached it does not place it upon the firm ground upon which it really rests. Fahrenheit adopted a zero which was supposed to correspond to an absolute privation of Heat. It was found, however, that thermal phenomena manifested themselves in the thawing of frozen mercury (or, to put it in other phrase, the liquefaction of a solid), far below his datum. But even frozen

mercury may be lowered in temperature, and hence certain physicists have adopted a zero 461 deg. Fahrenheit below the zero of that scale, or 493 deg. below the freezing point of water; it being assumed from the known rate of the expansion of gases by Heat, that at this temperature, a "perfect gas" would lose its elasticity and become either a liquid or a solid. The liquefaction or even the solidification of air, would not, however, prove that its Heat had been completely extracted from it, and we might suppose the possibility of a still further abasement of the temperature, and the further contraction of the liquid or solid air in its dimensions, just as a bar of iron or a column of mercury conforms to a cooling atmospheric current. Dalton considered that Heat might be reasonably estimated to many thousands of our thermometric degrees below the freezing point of water; and for all purposes of argument we may conclude that the range of temperature, if not infinite, is limited only by our knowledge of the means whereby it may be increased or diminished. Thus, at the lowest attainable, if not the lowest possible temperature, each particle of matter must retain a modicum of Heat, and no hypothesis respecting Heat need, therefore, be encumbered with any independent consideration of cold.

Reasoning from the effects of Heat, philosophers have framed many and various hypotheses to explain its nature. Of these, three have respectively been adopted, each by a more or less numerous class among the first authorities in science.

First following the natural inclination of the mind to materialise the unseen, we have the assumption of a substance to which the name of caloric has been applied, and to which all the phenomena of Heat are ascribed. This substantial Heat, or caloric, if it exist, is known to

elude the utmost delicacy of libration, and it is accordingly imagined to be a fluid of exceeding tenuity, its density bearing, possibly, a relation to that of hydrogen like that existing between equal volumes of hydrogen and gold. This fluid is further supposed to consist of particles highly repulsive of each other, but having a strong attraction for the particles of all other kinds of

matter.

Second: arguing from a supposed inextinguishability of motion, it has been further supposed that Heat consists in molecular vibrations or revolutions; the ultimate particles or atoms of bodies, of whatever nature, being imagined to be constantly whirling or vibrating with velocities inconceivable, but corresponding to their respective temperatures.

Third to escape from grave inconsistencies involved in the hypotheses just described, it has been supposed that the particles of bodies are surrounded by "elastic atmospheres" (by which would appear to be meant substantial Heat or caloric), the particles of which "atmospheres" are imagined to whirl or vibrate around or about their nuclei, and with a velocity depending upon their temperature. To meet the cases of radiation and reception, the motion of the "atmospheres" is further imagined to be at times communicated to, and at others withdrawn from their nuclei. This is the "hypothesis of molecular vortices."

All these hypotheses proceed upon the common assumption of the substantiality of matter, an assumption which, indeed, would appear to be inevitable. Matter, we are all disposed to believe, is indefinitely, if not infinitely divisible into solid atoms, and in order to reconcile their existence with the phenomena of Heat these atoms are supposed never to be in absolute contact with