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CAL (21 9 ) CAL

example) relatively with the, mutual capacities;and hence we are compelled to incline to thehypothefis (in itfelf by no means {trained orimprobable) that caloric may truly and chemi-cally combine with bodies and lofe all its dif-tniguilhing characters, till it is expelled byftronger affinity, as any other body lofes itsfpecific characters by chemical union, and re-gains them as foon as that union is broken.Thus, in the above example of lime, it is certainthat a part of the heat arifes from the folidi-fication of the water, or its change of Hate froma fluid to a folid; and another part perhapsarifes from the capacity of flacked lime beinglefs than the mean capacities of the lime and ofwater, but a third portion may be afcribed withhigh probability to the expulfion from the limeof a quantity of caloric, owing to the electiveaffinity of water being the Itrongeft of the two.This caloric would readily be furnifhed by thefuel during the previous burning of the lime,and hence this earth in its quick or caujlic {latemight perhaps be confidered as a compound oflime and caloric, exclufive of that quantity ofcaloric which is inherent in all natural bodies,and which is alone confidered in the eftimationof fpecific caloric.

As an additional argument for this chemicalcombination of an excefs of caloric in quick-lime, it may be added that calcined Plafter ofParis will abiorb and render folid to the full asmuch water as lime will, but with fcarcely anychange of temperature, and yet the fubftancewhofe change in capacity is the moll reducedby the combination (that is, the water), is thefame in both inftances.

Many other examples of violent heat on mix-ture might be adduced, which feem to demandthe hypothefis of a chemical union of caloric inone or other of the fubftances: but fome ofthem, fuch as the heat and inflammation pro-duced by the nitric and oxymuriatic acids, belongto the fubject of combuftion.

Of Fridlion and Percuffion.

The heat which is elicited by (apparently)Ample mechanical means, by rubbing, hammer-ing, and the like, cannot be fatisfaCtorily re-ferred to change of capacity, or to combuftion,and fo little is yet known on this head that we®uft be content with the mention of the faffs.The heat from percuffion is very well fhewn bythe method often ufed by blackfmiths, of ham-tnering a piece of cold foft iron till it is quitehot enough to kindle a fulph ur match. This isperformed in a few minutes by vigorous ham-

mering. It is to be obferved that iron thustreated becomes much harder and denfer, andwhen cold cannot again be brought to the lameheat by the fame means, unlefs it is firft annealed,that is, heated red hot in the fire, and allowedto cool fpontaneoufly in the air, when it againreturns to the ftate of foft iron, The rudenelsof the experiment renders it impracticable toeftimate the abfolute quantity of caloric given-out during the hammering, but as it is obviouflyconnected with the alteration of the texture ofthe metal, fome have fuppofed that the heat isforced or fqueezed out by a fimple mechanicaladtion, fomewhat in the way that water isfqueezed out of a moift fponge ; and hence thatthe annealing caufes a quantity of heat to adhereto the metal by a very loofe combination. Inall the cafes of the evolution of caloric by per-cuffion, the fubftance decreafes in capacity forcaloric, in proportion as its denfity encreafes.Thus, metals become heated by lamination, andwhen denfer, their fpecific caloric is lefs thanbefore. No heat is produced in denfe fluids bythe mutual friction or percuffion of their par-ticles in any way, and fluids are nearly incom-preffible, fo that their capacity for caloric cannotbe altered by mechanical means. On the otherhand the compreffion of gaffes produces veryconfiderable heat, as in the air-gun experimentsand others already related. Thefe fa£ts wouldindicate that the caufe of the heat from per-cuffion and mechanical compreffion was limplythe change in the capacity for caloric; butthough this is certainly true to a confiderableextent, it does not appear that the evolution ofcaloric is uniformly proportional to the changein capacity, and the phenomena of friCtion pointout a diftincl and hitherto inexplicable fourceof caloric which may combine its aftion, inmany cafes of percuffion, with the proper effeftsof change in fpecific caloric.

The examples of the production of heat byfriClion are innumerable. Wherever machineryrequiring rapid motion is ufed, every one knowsthe heat which is produced, and when themachinery is combuftible, the great precautionsrequifite to avoid its deftruCtion by the heatwhich its motion excites. The ufe of lubri-cation is not more to leflen the mechanicalobftacles to free motion, than to prevent thegreat accumulation of heat by friction. Un-greafed carriage wheels taking fire, gunpowderor cotton mills deftroyed by overheating themachinery, are familiar examples of this fact :and the method {till employed by man in the

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