On Evaporation. 575 
Many philosophers concur in the theory of 
. chemical solution: atmospheric air, it is said, 
has an affinity for water; it is a menstruum 
in which water is soluble to a certain degree, 
It is allowed notwithstanding by all, that each 
liquid is convertible into an elastic vapour in 
vacuo, which can subsist independently in’ any 
temperature; but as the utmost forces of these 
vapours are inferior to.the pressure of the at~ 
mosphere in ordinary temperatures, they are 
supposed to be incapable of existing in it in the 
~ same way as they do in a. torricellian. vacuum : 
hence the notion of \affinity is induced.—Ac- 
cording to this theory of evaporation, atmos- 
pheric air (and every other species of air for 
aught that appears) dissolves water, alkohol, 
ether, acids, and even metals. Water below 
212° is chemically combined with the gases; 
above 212° it assumes a new form, and becomes 
a distinct elastic fluid, called steam: .whether 
water first chemically combined with air, and 
then heated above 212°, is detached from the 
air Or remains with it, the advocates of the the- 
ory have not determined.—This theory has al- 
ways been considered as complex and attended 
with difficulties; so much that M.  Pictet 
VOL, Vv. KK 
