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a tub, or any other veffel that contains the coldwater, which is to receive the heat given outby the fteam during its condenfation. Here thevapour does not indeed actually come in contactwith the water, but its latent caloric is givenout and paffes through the metallic pipe im-merfed in the water. In perfect accuracy thequantity of caloric required to raife the tem-perature of the metallic pipe fhould alfo beeftimated, but here (as in the experiments oncongelation) this minutenefs of obfervation isnot in the leaf! neceffary to eftablifh the generallaw of latent caloric, the thermometrical dif-ference on which it is founded being fo greatand palpable that very confiderable errors arenot felt. The experiments of Dr. Black and ofMr. Watt on the condenfation of a given weightof fteam in a known quantity of water, giveabout 950° for the latent heat of fteam oraqueous vapour at the boiling point: that is tofay, that boiling water renders latent during itstotal converfion into fteam a quantity of caloric,which otherwife would have raffed it to 950degrees. This enormous quantity of heat givenout by fteam was long obferved during thecommon procefs of diftillation, and remainedunaccounted for till the theory of latent heatoccurred to Dr. Black. Of late the ufe of fteamhas been molt advantageoufly extended to theheating of large maffes of water in breweries,dye-houfes, bleacheries, &c. In thefe the fteamis produced in a ftrong iron boiler, with noother outlet than the pipe or pipes which con-vey off the fteam when generated. One pipepaffes to each ciftern of cold water, and heats it,not as in diftillation by merely pafling through,but by difcharging the fteam into the body of thewater, being generally admitted from below. Afew minutes of a ftrong jet of fteam will bringa large ciftern of cold water to a fcalding heat,and a further quantity will foon raife it toboiling.

The facts that have been advanced concern-ing the latent heat of fteam (and the fameapplies to all condenfible vapours) are finelyilluftrated by the inftrument called from theinventor Papin’s digefter. Its intention is for-cibly to confine heated water by mechanicalpreffure, and thus to prevent the formation offteam, whereby the water is compelled to afliimea much higher temperature than 212°. Itconfifts merely of a very ftrong iron or copperoiler, whofe lid fits on perfectly tight, and iskept down by thick fcrews. This is half filledWith water, the lid clofed, and fet on a fire.The water foon rifes to the boiling point, and

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fo much fteam is formed as will fill the vacantfpace above. By continuing the heat, the waterbeing prevented by the confinement from form-ing more than a limited quantity of fteam, nowrapidly acquires a much greater heat than 212°,and by this is enabled to diffolve bone, horn,mufcle, and a great many other fubftances onwhich unconfined boiling water has but verytrifling adtion. To prevent the great hazard ofa dangerous explofion from the prodigiouflyaccumulating expanfive force of the fteam with-in, the digefter is furnffhed with a very tightvalve, kept down by a confiderable preffure, butwhich will fly open before the veflel is in anydanger of burfting. The circumftance thatftrongly illuftrates the theory of latent heat isthe following. When the water within thedigefter has rifen to an exceffive heat (fuppofe400°, which is 188 degrees above the naturalboiling point) if the valve is now opened,a quantity of fteam rallies out with extremeviolence, and the heat of the remaining wa-ter within almoft immediately finks to about212°. Thus we find that the formation of acertain quantity of fteam from over-heatedwater, carries off with it a great proportion ofthe fenfible heat of the water, and both fteamand water fall to a much lower temperaturethan before, which can only be explained fatis-fadtorily by the hypothefis of the large propor-tion of heat rendered latent in the fteam at themoment of its formation.

The converfion of water into aqueous vapouris therefore a cooling procefs with regard to all thecontiguous bodies, for the fame reafon that theliquefaction of ice or other folid bodies pro-duces the fame effedt: and on the contrary, thecondenfation of a vapor into a liquid, and of aliquid into a folid, are for the fame reafon heatingproceffes. This general law applies not only towater, but to all known fubftances whofe form,alters by different dofes of caloric.

The cold produced by the evaporation offluids at low temperatures furnffhes many in-terefting and very illuftrative experiments. Al-cohol and ether are fluids that boll at a muchlower temperature than water, and they, as wellas water and many other fluids, readily evaporateat a much lower heat than their boiling point,even at the cpmmon heat of the atmofphere,and thus any fubftance moiltened with anyevaporable fluid is rendered much colder, or isrobbed of much caloric, by the fpontaneousevaporation of the fluid. This is much affiftedby a brifk wind, or'any artificial renewal of theair on the moiftened furface. Thus if a wet2 9