84 HEAT OU CALORIC. 



A saturated solution of sulphate of potash precipitated hy alcohol 

 evolves considerable heat, when the salt congeals. Henry. 



!/.) Were there no absolution of heat to become latent during the 

 ting of ice, countries covered with snow might be instantaneously 

 devastated. The torrents are even now, very destructive ; then, they 

 would be ruinous. Snow and ice would instantly melt, as soon as 

 the temperature rose above 32, but as the absorption of 140 of 

 heat is indispensable, the process is necessarily a slow one. 



(/.) The heat absorbed in liquefaction, is given out again in 

 freezing. Thus one cause tends to correct the effect of the other, 

 and both causes conspire to regulate the temperature ; for thawing is 

 a cooling, and freezing is a warming process. 



IV. VAPORIZATION AND GASIFICATION OR THE FORMATION OF 



AERIFORM BODIES. 



Introductory Remarks. 



Weight and pressure of the atmosphere. This subject belongs to 

 mechanical philosophy,* but it is impossible to make any progress in 

 investigating the nature of aerial agents, without taking into view the 

 pressure of the atmosphere. Its existence is fully demonstrated, by 

 the rise of water in a pump, and by the stationary condition of the 

 column of mercury in a barometer tube, as w r ell as by many com- 

 mon occurrences. f The pressure, in any given place, varies at dif- 

 ferent times, but the mediuni is about fifteen pounds on the square 

 inch, corresponding to a column of thirty inches in the barometer ; to 

 about thirty three feet of water, and to columns of other fluids varying 

 in height according to their specific gravity. 



Taking the doctrine of atmospheric pressure for granted, we pro- 

 ceed to aeriform bodies. 



(a) Jin atriform body is one having the mechanical properties of air ; J 

 a vapor is a transient aeriform body, condensible by cold, or pressure, or 

 both united ; a gas is supposed to be permanently aeriform under eve- 

 ry degree of pressure and cold. Some latitude is allowed in the use 

 of these terms, and a few bodies continue to be called gases, which 

 have been condensed ; e. g. ammonia, euchlorine, sulphurous acid. 



* Consult EnfielcPs Philosophy, and any other treatise on Natural Philosophy. 

 This subject and that of statical pressure in general, is ably illustrated by Dr. Hare, 

 in his Compendium, p. 25. 



t It is now said that flies and other insects walk on the ceiling of a room with 

 iheir backs downwards, in consequence of the peculiar webbed structure of their 

 feet, which enables them to press the wall so closely, that little or no air intervenes, 

 and thus the pressure of the atmosphere keeps them in their places. L,. u. K. 



$ Atmospheric air has, by pressure, been reduced to T l^ part of its volume, with- 

 out losing its clastic form. 



