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“ the nameless form of speech
Which lovers' eyes alone can reach.” Some craniums were exhibited in illustration, and the lecture concluded. — Perhaps the very kind services of Dr. Allen, for which of course he receives the thanks of the committee of the institution, will be less appreciated than they deserve to be, on account of the highly-wrought beauty of his style, and the perfect novelty of his subjects to the bulk of the audience. We anticipate the time when topics of the highest intellectual character will be valued and understood by every man.
We are not unmindful of the difficulties which present themselves when we attempt to treat of the nature of Heat in an elementary style. It is a subject on which Philosophers have long held different opinions, and upon which even at the present day, (we think) no cheory has been offered that is satisfactory on all points, or sufficient to explain all its effects. In this paper the corpuscular, and undulatory theories, will be stated, and the reader left to choose or reject for himself.
The term Heat is used to express two different things; we sometimes use the word to express the sensation excited in our organs, and sometimes to express the state of bodies whose temperature is elevated ;-thus we say, we feel heat, and, there is heat in a burning coal.
When in 1787 the French chemists contrived the new nomenclature, they thought it would be advantageous to possess a distinct term for the cause, and the sensation or effect; accordingly chemists use the term Heat to denote the sensation, and Caloric to denote the cause of this sensation, or the condition of bodies by which the sensation is produced. This innovation was not necessary, and it had the effect of fixing down the opinions of chemists to the hypothesis respecting Heat adopted by the contrivers of the new nomenclature ; namely, that Heat is not a property of matter, but a peculiar substance; altbough no evidence sufficiently decisive to settle this disputed point, has hitherto been advanced.
Some philosophers consider heat as a peculiar substance which by entering into bodies, combining with them, or leaving them, produces the different phenomena of expansion, ignition, combustion, fluidity, vaporisation, &c. which we shall have to consider hereafter. Others think it a property of matter; a motion of a particular kind, the nature of which has never been explained in a satisfactory manner.
During the 17th century, from the time of Bacon to that of Newton, the latter of the two above opinions prevailed ; (that is to say, the undulatory theory.) During the 18th century, the former opinion was equally prevalent, probably from the popularity of Cullen, and Black, both of whom taught it in their lectures. Since the beginning of the present century, some of the most popular writers on Che. mistry in this country have reverted to the old opinion; among these were Count Rumford, Sir H. Davy, and Dr. Thomas Young, who doubt the separate entity of a calorific matter, and rather refer the phenomena to a vibratory or intestine motion of the particles of common matter-(which constitutes the undulatory theory.)
· From the most careful experiments that it has been possible to make, it may be concluded that increase in the temperature of bodies, produces no increase in their weight whatever. It cannot therefore be shown that heat, if it be a peculiar substance, possesses gravity ; but if the matter of heat, being a body, was lighter than hydrogen gas, in the same proportion that hydrogen gas is lighter than platinum, perhaps none of the experiments yet conducted would discover its presence.
Expansion, specific heat, and radiation, seem to admit of the readiest explanation by supposing heat a substance ; thus we can conceive if heat be a peculiar substance, how by entering bodies it would cause them to expand, and increase their bulk;—and how the bulk would diminish when this substance was withdrawn. There is no difficulty in conceiving how more heat may be required to produce a given effect on one body than upon another, or why the specific heat of different bodies is different on the supposition that heat is a body. The radiation of heat admits of an equally simple explanation, if heat be a body, and so do fluidity and evaporation. If heat were mere motion, neither the property which it has to expand bodies, nor the different capacity of bodies for heat, could be explained in a satisfactory manner.
But ignition and combustion do not admit of so easy an explanation if we admit heat to be a body. No satisfactory explanation of the evolution of heat and light during every case of combustion, has been yet given, flowing from the opinion that heat is a peculiar substance. It would be much easier to explain it, if we consider heat rather as a property of matter than as a substance. We might then assign a reason why it should make its appearance in all cases of ra. pid combustion, and never in any other case. The expla nation that fire evolved during combustion is merely a union of the plus and minus (or positive and negative) electricity, with which the bodies combining together, were charged, appears at first sight plausible ; but it will not bear a rigid examination. For it is incompatible with the very hypothesis from which it professes to flow
Or Caloric Sir H. Davy writes, “It seems possible to account for all the phenomena of heat, if it be supposed that in solids the particles are in a constant state of vibratory motion, the particles of the hottest body moving with the greatest velocity, and through the greatest space; that in liquids and elastic fluids, besides the vibratory motion, which must be conceived greatest in the last, the particles have a motion round their own axis, with different velocities, the particles of elastic fluids moving with the greatest quickness; and that in ethereal substances the particles move round their own axis and separate from each other, penetrating in right lines through space. Temperature may be conceived to depend upon the velocities of the vibrations, increase of capacity, on the motion being performed in greater space; and the diminution of temperature during the conversion of solids into fluids, or gases, may be explained on the idea of the loss of vibratory motion in consequence of the revolution of particles round their axis, at the moment when the body becomes liquid or aeriform; or from the loss of rapidity of vibration, in consequence of the motion of the particles through greater space.
“If a specific fluid of heat be admitted, it must be supposed liable to most of the affections which the particles of common matter are assumed te possess, to account for the phenomena; such as losing its motion when combining with bodies, producing motion when transmitted from one body to another, and gaining projectile motion when passing into free space; so that many hypotheses must be adopted to account for its agency, which renders this view of the subject less simple than the other.”
(To be continued.)
A MORNING IN JUNE.
[From Time's Telescope.]
It is the hour before the labouring bee has left his golden hive; not yet the blooming day buds in the blushing East; not yet has the victorious Lucifer chased from the early sky the fainting splendour of the stars of night. All is silent, save the light breath of morn waking the slumbering leaves. Even now a golden streak breaks over the grey mountains. Hark! to shrill chanticleer! As the cock crows, the owl ceases. Hark! to shrill chanticleer's feathered rival! the mounting lark springs from the sullen earth, and welcomes, with his hymn, the coming day. The golden streak has expanded into a crimson crescent, and rays of living fire flame over the rose-enamelled east. Man rises sooner than the sun; and already sound the whistle of the ploughman, the song of the mower, and the forge of the smith,—and hark! to the bugle of the hunter, and the baying of his deep-mouthed hounds. The sun is up, the generating sun ! and temple, and tower, and tree - the massy wood, and the broad field, and the distant hill, burst into sudden lightquickly upcurled is the dusky mist from the shining riverquickly is the cold dew drunk from the raised heads of the drooping flowers !
Higgins, Printer, Dunstable.