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following experiment of Fahrenheit {hews it.If water (arid better juft after having boiled fora quarter of an hour) be put into a deep veflelimmerfed in fait and fnow, or any other power-ful freezing mixture, and allowed to remainperfectly at reft and without any wind playingover its furface, it may readily be cooled toabout B or io degrees below 32 0 , or even morewithout freezing. If now the veflel be {lightlyfhaken or jarred, or efpecially if an atom of icebe dropped in, almoft the whole water fuddenlyfhoots into fine cryftals of ice, and the tempera-ture {lowly rifes to 3 2°, the freezing point. Thereafon of this considerable rife of the actualtemperature is the fuddennefs of the congelation.In common circumftances the formation of icein freezing water is fo gradual, that the latentheat extricated from the water by congelationis immediately diffufed through the furroundingmedium, and does not fenfibly affect the ther-mometer : but when the whole is cooled fo lowas 2o° or 22 0 , the whole of the latent heat isready to quit-the water at the inftant that con-gelation begins in any one point, and then be-comes fenfible to the thermometer.

The cryftallization of thofe falls that are morefoluble in hot than in cold water is a true con-gelation, and the extrication of caloric fromthem in the moment of cryftalfization equallytakes place. None of them {hews this phe-nomenon more beautifully than fulphat of foda(Glauber’s fait); but fome little management isrequifite to enfure its cooling down below itscongealing point, and remaining fluid. The fol-lowing method feldom fails. Take any quantityof this fait, a pound for example, and heat it ina clean iron ladle on a clear briflt fire. The faitfpeedily melts in its own water of cryftalfization,and boils violently. Continue the heat till allthe water is expelled, and a white dry falinemafs remains. Then add fufficient water to it(without removing it from the ladle) to rediflolveby boiling almoft, but not quite, the whole ofthe fait. This liquor, which is a perfectly fatu-rated folution, Should be poured, while ftill quitehot, into a thin glafs phial, and immediatelycorked quite clofe, or better, covered with wetbladder, fo as to exclude all the external airfrom the folution. Set this by in a quiet placewithout Shaking, and it will now cool to thetemperature of the air, remaining perfectly fluid.On opening the cork and letting in the externalmr, the liquor immediately begins to congeal onits upper furface, and the congelation fpreadsdownwards in a few feconds, like a denfe whitecloud; after which it is fo entirely folidified

that on inverting the bottle not a drop ofliquor falls out. If it fhould not immediatelycongeal on opening the cork, a flight {hake, ordropping in an atom of the fame fait will beginthe procefs, after which it never fails to fpreadthrough the whole in a few feconds. It maybe added that the fame folution wifi anfwerrepeatedly any number of times, and after thefirft time there is no occafion previoufly to cal-cine the fait, but merely to heat the phial con-taining it till it is totally liquefied. During thefudden congelation, a quantity of heat is givenout fufiicient to make the outfide of the phialfenfibly warm to the hands, and to raife thethermometer 20 or 25 degrees. The caufe ofthis encreafe of temperature can be no otherthan the efcape of the latent heat of the folutioncaufed by its change from a fluid to a folidftate.

Numberlefs fa£is might be adduced in illus-tration of what has been mentioned concerningthe evolution of fenfible heat during congelation,and its abforption during liquefaction. As ageneral rule it may be remarked that whatevermelts the moft rapidly is likely to produce thegreateft cold, and whatever congeals the moftrapidly wifi (ceteris paribus) produce the moftheat. However this is alfo regulated by therelative affinity for caloric which different fub-llances may have. The mode in which artificialcold is produced by faline folutions partly be-longs to this part of the fubjedt.

In all thefe cafes a fudden liquefaction of thefait employed takes place, which therefore re-quires a proportional fupply of caloric from allthe adjacent bodies to furnifh it. Thus if faitand fnow (one of the moft powerful of thefreezing compounds) are mixed together, theymelt with great rapidity (the fnow fimply bythe abforption of heat, the fait by the unitedpower of caloric and water), and every fubftanceimmerfed in the mixture is robbed of a largeportion of its caloric to fupply the demand. Thequantity of heat abforbed, or the meafure of thefrigorific power, always bears a proportion tothe cold required to congeal the mixture. Thusthe freezing point of the brine is at leaft 32degrees lower than that of water, or the zeroof the common fcale, and hence we may inferthat a much larger quantity of latent caloric iscontained in brine than in water at equal tem-peratures, fince fo much more powerful a coldis required to congeal the former than the latter.The utmoft limit of the frigorific power of thefemixtures, therefore, is their point of conge-lation ; for the fame reafon that prevents melting