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iee from producing a greater- cold- than 3 2°during its liquefaftiou, will prevent brine fromproducing 3 lower temperature than about o°,thefe being refpeclively the freezing points ofthe two iubftances. In like manner the coldoccafioned by fait and fnow continues nearly atthe fame intenfity during the whole time ofmelting; as alfo takes place with melting ice: andif brine be flowly cooled a few degrees below o°,its freezing point, as foon as any portion ofthe fait feparates in a folid form, the tempera-ture rifes to that point; juft as the temperatureof over-cooled water rifes to 32 0 on fudden con-gelation. k

The particular freezing mixtures, with thebell mode of their application, will be de&ribedunder the article Freezing.

Of Vipour and Evaporation.

When water is heated in any open veffel, thethermometrical temperature rifes gradually, atthe fame time that aqueous vapour foon beginsvifibly to form at the furface. This encreafestill the water has rilen to its boiling point, whichis about 212 0 of the common thermometer; atwhich time the whole is thrown into violentagitation, and the formation of fteam is rapidand copious. The temperature now no longerrifes, but remains, both in the water and in thefteam, at the fame point, till the whole is eva-porated or boiled away.

The two principal circumftances to be at-tended to in this procefs, which is every daybefore our eyes, are, the expanfion which thewater undergoes by being converted into fteam;and the uniformity of temperature maintainednotwithftanding the conftant acceflion of heatduring the boiling.

The invariable relation between the quantityof heated vapour formed, and the quantity ofheat carried off in a ftate infenfible to the ther-mometer, led Dr. Black to apply his beautifultheory of latent heat to the vaporization ofliquids as well as the liquefaction of folids, andhas furnifhed the moft irrefragable argumentsfor its truth.

The expanfion of water when converted intofteam was long known to be very great, andmany experiments were made, before Dr.Black’sinquiries, to afcertain the exa£t amount. Moreaccurate trials, by Dr. Black, and his eminentpupil and friend, Mr. James Watt , fix the ex-panfion at 1800 times the bulk of the water.As vapour, while foch, is a rare, invifible andhighly elaftic fluid, the temperature and pref-fure mult be taken into account in this eftimate.

The former is 212°, or the higheft heat thatunconfined watery, vapour can affume, and thelatter is the common preffure of the atmofphere.Hence we may reckon generally that water keptfully boiling in open veffels is converted into1800 times its bulk of boiling-hot vapour.

The combination of a great quantity of caloricwith water in the formation of fteam, wasdetermined' by Dr. Black in a fimilar manner asthe combination of caloric with ice, in order toproduce liquefaction. Water was heated asuniformly as poftible in a tin veffel to boiling,and the time required for this was noted.The fame heat was then continued till thewhole was evaporated, and the time taken upby the procefs was noted. Now fince on theone hand the acceflion of heat was conftant, itwas readily computed how high the temperaturewould have been, fuppofing the rife to havegone on above 212 0 in the fame ratio as belowit; and, on the other, as tire temperature ofthe fteam was not raifed, it was inferred thatall the acceflion of caloric from 212° was efien-tial to the very ftate and conftitution of vapourat that temperature. This quantity Dr. Blackeftimates from the average of many of his firftexperiments to be about 810 degrees: that is tofay, that the fame quantity of caloric which isrequired totally to evaporate boiling water at212°, wouhl be fufficient to raife the water 810degrees above tire boiling point, or to 1022 if ithad remained in its liquid ftate.

On the other hand, it will follow that whenfteam is condenfed again into water, it will giveout all the latent caloric which was neceflary toits vaporous ftate, which being now fet freefrom its combination, will readily be diffufedamong all the adjacent bodies, and raife theirtemperature as much more than an equal weightof boiling water will do, as the latent caloric offteam exceeds that of water. This furnilhesanother method of eftimating the latent caloricof watery vapour, and fomewhat more accuratethan the former, fince in the experiment ofboiling water fome of it evaporates before itarrives at 212°> and it is neceflary to affume asa datum that the rate of thermometrical heat inthis fluid would be the fame above the boilingpoint (were there no evaporation) as below it.

The moft convenient method of heating waterby means of fteam, is to imitate the procefs ofcommon diftillation; that is, to heat water ina clofe veffel in order to generate the aqueousvapour, and to caufe the vapour to be condenfedin a metallic pipe palling in a fpiral form through

* Black.