CAL

CAL

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The fubftitution of the term caloric to that ofheat, has in a great meafure arifen out of theprevailing idea of the materiality of heat, whenceit was thought neceffary to guard againft a con-fulion of ideas that might arife from giving thefame term to the fenfation of warmth in theanimal body, and to the caufe of that fenfation.But though the term caloric is well devifed, itdoes not happen in fa<ft that when its nature isunderftood, any material inconveience arifesfrom this mixture of terms; and therefore incompliance with common language we {hall ufeindifcriminately the appellations of heat or caloricto imply that fuppofed material fubftance whichpofleffes the power of exciting the feeling ofheat in the animal body, together with a vaftnumber of other properties, that render it thenioft important chemical agent in nature.

The general properties of caloric are the fol-lowing : if material (and the phenomena whichit exhibits are on the whole more readily under-ftood by fuppofing it fo) it is a fubftance ofextreme fubtlety and tenuity, entirely impon-derable by human inftruments, derived imme-diately from the fun together with light, and,like light, obeying certain and fimilar laws ofreflection and refraction, and alfo elicited bycombuftion, by friftion, and fome other fources:when uncombined, moving with immenfe ve-locity in every direction, and conftantly tendingto an equilibrium, penetrating all bodies innature with more or lefs facility, and therebyenlarging their dimenfions without encreafingtheir abfolute weight: when combined, probablyentering into chemical union with other bodies,and thereby conftituting the different ftates ofliquid, vapour, or gas: and laftly, when appliedfuddenly to the animal body occafioning thefenfations of warmth, heat, or in greater inten-fity, burning.

The fimpleft ftate of caloric that we are ac-quainted with takes place when a heated bodyis placed in air, or any elaftic medium muchcolder than itfelf. The great fource of heat,the fun, is probably in this fituation, as likewileare all heated bodies, whether in combuftion ornot, fnnply expofed to the atmofphere. Thus{jtuated, it is found that fenfible heat is con-ftantly radiating from them in every directionwith immenfe velocity, and with an intenfityregulated by the nature of the heated furface,and its proportional excefs of heat over the fur-rounding medium.

Again, when a body has been long enoughexpofed to the radiations of heat to have itfelfreceived thereby an excefs of caloric above that

of the ambient air, it becomes a fecondary radi-ating fource, and immediately begins to fendforth rays of caloric with apparently equal velo-city with the primary fource, and precifelyobeying fimilar laws of tranfmiffion. Thus aveffel of water Handing near a hot fire firftreceives from it conftant rays of heat, the wateritfelf becomes hot, and it then begins to fendoff its acquired heat through the air to the fur-rounding objects. Hence the conftant tendencyto an equilibrium which radiant or fenfible heatpofleffes, fo that no heated body can remain aninftant in contact with a colder one withoutlofing a portion of its caloric; and as the heat-evolving proceffes are going on with unequalintenfity through every part of the materialworld, an eternal interchanging motion of caloricis every where perpetually kept up, which ap-pears to be a grand agent in all the phenomenaof nature.

But on.the other hand there are two circum-ftances which prevent or impede this conftanttendency to equilibrium which caloric pofleffes,the one, the extreme retardation which heat ex-periences in its paffage through folids and liquids,and the confequent change in the mode of itspropagation; and the other, the intimate andprobably chemical union which takes place be-tween part of the caloric and the body that,receives it. Hence it is proper to have an appro-priate term to diftinguiih the paffage of caloricthrough folids and liquids, from that of radi-ation, which expreffes its free and inftantaneouspaffage through air or an attenuated medium.If a rod of iron is held with one extremity clofeto a fire, the heat firft radiates inftantly to theneareft end of the iron, and enters its fubftance,after which it paffes (not immediately as beforebut flowly) through the whole length of the rodto the molt diftant extremity, fo that feveralfeconds elapfe before the hand holding the fur-ther end of the rod will be fenfible of the paffageof the heat. Now, as we {hall prefently men-tion, it is probable that heat radiates with thefame velocity as light, that is about 200,000miles in a fecond, whence may be conceivedthe immenfe retardation of its motion when ittakes feveral feconds to pafs through an ironrod of a few inches only. Therefore it is pro-bable that the caloric enters into a kind of com-bination with each particle of iron fucceflivelyin its paffage through, and in this mode of pro-pagation it is faid to be conducted. This facultyalfo varies very confiderably, according to the -nature of the fubftance heated; thus fome metalsconduct heat eafier than others, thefe again