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former weight : so that it is evident, that the water 
united to it was derived from the air. And that it 
existed in the air in an invisible and elastic form, is 
proved by the circumstance, that if a given quan- 
tity of air be exposed to the salt, its volume and 
weight will diminish, provided the experiment be 
correctly made. 
The quantity of water which exists in air, as 
vapour, varies with the temperature. In propor- 
tion as the weather is hotter, the quantity is greater. 
At 50° of Fahrenheit air contains about ^ of its 
volume of vapour ; and as the specific gravity of 
vapour is to that of air nearly as 10 to 15, this is 
about -J 3 . of its weight. 
At 100 °, supposing that there is a free commu- 
nication with water, it contains about parts in 
volume, or in weight. It is the condensation 
of vapour by diminution of the temperature of the 
atmosphere, which is probably the principal cause 
of the formation of clouds, and of the deposition 
of dew, mist, snow, or hail. 
The power of different substances to absorb 
aqueous vapour from the atmosphere by cohesive 
attraction was discussed in the last Lecture. The 
leaves of living plants appear to act upon the va- 
pour likewise in its elastic form, and to absorb it. 
Some vegetables increase in weight from this cause, 
when suspended in the atmosphere and uncon- 
nected with the soil ; such are the houseleek, and 
different species of the aloe. In very intense heats, 
and when the soil is dry, the life of plants seems 
to be preserved by the absorbent power of their 
leaves : and it is a beautiful circumstance in the 
