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from the air. And that it existed in the air in an invis- 

 ible and elastic form, is proved by the circumstance, 

 that if a given quantity 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 va- 

 pour, varies with the temperature. In proportion as 

 the weather is hotter, the quantity is greater. At 50 

 of Fahrenheit air contains about ^V 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 T V of its weight. 



At 100, supposing that there is a free commu- 

 nication with water, it contains about /j parts in vol- 

 ume, or TT in weight. It is the condensation of va- 

 pour by diminution of the temperature of the atmos- 

 phere, 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 at- 

 traction was discussed in the last Lecture. The leaves 

 of living plants appear to act upon the vapour likewise 

 in its elastic form, and to absorb it. Some vegetables 

 increase in weight from this cause, when suspended in 

 the atmosphere and unconnected 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 ceconomy of nature, that aqueous vapour is 

 most abundant in the atmosphere when it is most need- 



