THE EFFECTS OF HEAT. 245 



the heat which is added serving only to convert the water at 212° from a 

 liquid condition into steam, or vapor, at 212°. 



564r. If we immerse a thermometer in boiling water, it 

 know that Stands at 212° ; if we place it in steam iramediately above it, 



steam at 2;-2° jj incjicates the same temperature. "We know, however, that 

 IS hotter than ,,,.,. , .j. 



water at the Steam contains more heat than bouing water, because u we 

 same tempera- ^^ ^^ ^^^^3 q|- ^^^^^j. at 212° with five and a half ounces of 

 ture .' 



water at 32°, we obtain six and a half ounces of water at a 



temperature of about 60° ; but if we mix an ounce of steam at 212° with five 

 and a half ounces of water at 32°, we obtain six and a half ounces of water at 

 212°. The steam, from which the increased heat is all derived, contains as 

 much more heat than the ounce of water at the same temperature, as would 

 be necessary to raise six and a half ounces of water from the temperature of 

 60° to 212°, or six and a half times as much heat as would be requisite to 

 raise one ounce of water through about 152° of temperature. This quantity 

 of heat will, therefore, be found by multiplying 152° by six and a half, 

 which will give a product of 983° — the excess of heat contained in an 

 ounce of steam at 212° over that contained in an ounce of boUing water at 

 the same temperature. 



y^ . 565. In the conversion of solids into hquids, and liquids into 



of the heat vapors by heat, we may suppose the heat, the sohd, aud the 

 pea'rs in li'nue- ^^^d to have respectiveh" combined together ; — forming a 

 faction and va- liquid in the one case, and a vapor in the other. A liquid, 

 poriza loa . therefore, may be regarded as a compound of a solid and 



heat, and a vapor as a compound of heat and the liquid from wliich it was 

 formed. The heat wliich disappears in these combinations is called Latent, 

 or CoiiPoujTD Heat. 



What are '^'^^ absorption of heat consequent on the conversion of 



freezing mix- solids into liquids, has been taken advantage of in the arts for 

 ^'^^' the production of artificial cold; and the compounds of dif- 



ferent substances which are made for this purpose, are called f'-eezing mix- 

 tures. 



Wh does th '^^ most simple freezing mixture is snow and salt. Salt 



mixture of dissolved in water would occasion a reduction of temperature, 

 producTint^M ^^^ when the chemical relations of two solids are such, that 

 cold? on mixing, both are rendered liquid, a still greater degree of 



cold is produced. Such a relation exists between salt and snow, or ice, and 

 therefore the latter substances are used in preference to water. "When the 

 two are mixed, the salt causes the snow to melt by reason of its attraction 

 for water, and the water formed dissolves the salt : so that both pass from 

 the solid to the liquid condition. If the operation is so conducted that no 

 heat is supplied from any external source, it follows that the heat absorbed 

 in liquefaction must be obtained from the salt and snow which comprise the 

 mixture, and they must therefore suffer a depression of temperature propor- 

 tional to the heat which is rendered latent. 



In this way a degree of cold equal to 40° below the freezing point of 



