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lamp that bums fteadily and equally. If a ther-mometer be immerfed in the ice it will graduallyrife from 20° to 32 0 . The ice -will now meltflov/ly, and during the whole time of melting,the thermometer touching it will remain fta-tionary, notwithftanding the acceflicn of heatcontinues as before. Therefore, during themelting of ice, a quantity of heat infenfible tothe thermometer is abforbed; and if the heat iscommunicated from the bottom, the water al-ready produced is prevented fro i acquiring ahigher temperature till every particle of the iceis thawed. Hence it follows that a quantity ofheat is neceflary to be added to ice before it canbe converted into water, and which does notraife the thermometric temperature. Let theheating continue as before, and the water pro-duced by the thawing of the ice now again/hews an encreafe of fenlible heat, and the ther-mometer continues to rife gradually through thewhole fcale up to 212°, in the latter part ofwhich time the water begins to fend forth fteam,and at laid to boil violently. Now again a fecondand permanent flop is put to the rife of thethermometer, for though the addition of heatreceived from the lamp continues as before, thethermometer never rifes higher than 212°, butinftead, a large quantity of aqueous vapour fliesoft', alfo at the heat of 212°, till every drop ofthe water is evaporated. Now if in the lirftcafe all the heat received during the thawing ofthe ice (at which time the thermometer wasftationary) was abforbed by the water in orderto enable it to affume the liquid form, it follows,by parity of reafoning, that all the heat re-ceived during the boiling (the thermometer being' now as before ftationary) is employed in theconverfion of water into aqueous vapour. In' both cafes heat is received, and as it is the com-mon efFerit of heat to raife the thermometric' temperature, the abfence of this effect can only’ be accounted for by fuppofing it neceflary forthe exiftence of aqueous vapour that it fhouldbe compofed of a greater quantity of heat thanwater, and water more than ice.

Upon thefe fundamental experiments, Dr.Black-formed that beautiful fyftem of the natureof heat which carried with it irrefiftible con-viction wherever it fpread, which is now (withfome modifications) univerfally adopted, andhas been extended to the full explanation ofan infinite number of the moft importantphenomena in natural philofophy. The par-ticular experiments and inferences from themmuft be examined more minutely. The leading

circumftanees or heads of inquiry fuggefted andilluftrated by Dr. Black’s difcovery, are thefollowing:

1. The abforption of caloric from the fur-rounding bodies by liquids in the procefs ofliquefaction, and by vapours in the procefs ofevaporation, being neceflary to the conftitutionof liquids and vapours. Caloric in this ftate istermed by Dr. Black Latent Heat, becaufe it is asit were concealed in the fubftance of the liquidor vapour; and is oppofed to Free or Senftblc Heat.Latent heat has been alfo termed Combined caloric,which is perhaps a better term, as it exprefi’es(what is probably the fact) that a chemical com-bination takes-place; as (for example) betweenice and caloric to conftitute the fluid, water.

2. The expulfion of caloric from liquids duringcongelation, and from vapours during theircondenfation into liquids. In this cafe the caloricwhich was before in a latent ftate now becomesfree, and therefore fenfible to the thermometer.This expulfion of caloric is of courfe a neceflaryinference from the fact of its abforption inoppolite circumftanees, and the proofs of itstaking place are equally decided.

3. The proportional quantity of caloric necef-fary to be added to any fubftance to raife it to agiven thermometrical temperature. Ihis com-parifon refers both to the different ftates of thefame body (as ice, water, and vapour) or to"different bodies indifcriminately, the quantitiesof the fubftances being always equal. As thefeexperiments are proportional to each other, theyare for convenience all referred to a commonftandard, which in this cafe, as in that of fpecific

•gravities, is water. This is called the SpecificCaloric; of bodies, or as Dr. Black terms it theCapacity of Bodies for Heat . Thus if it takesthree times as much heat to raife a certain quan-tity of water 10 degrees, as the fame quantityof mercury, the fpecific caloric of water, or itscapacity for heat, is faid to be three times asgreat: or if that of water be called 1. that ofmercury will be -j.

4. The abfolute quantity of caloric containedin any fubftance. We have no reafon to thinkthat we have ever by natural or artificial colddeprived any fubftance of all the caloric whichit contains; but many ingenious attempts havebeen made to eftimate the point of aftual pri-vation of heat, or the real zero in differentbodies, and thence to compute the abfolutequantity which is contained in fubftances atgiven temperatures. As heat cannot be weighedor meafured, fome meafure of effect muft be