LAWS OF HEAT. WATER. 87 



in this shape. We remark how suitable and 

 indispensable to the well-being of the creation it 

 is, that the fluid should possess the property of 

 assuming such a form under such circumstances. 



The moisture which floats in the atmosphere is 

 of most essential use to vegetable life.* " The 

 leaves of living plants appear to act upon this 

 vapour 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, as the house-leek and 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." It follows 

 from what has already been said, that, with an 

 increasing heat of the atmosphere, an increasing 

 quantity of vapour will rise into it, if supplied 

 from any quarter. Hence it appears that aqueous 

 vapour is most abundant in the atmosphere when 

 it is most needed for the purposes of life ; and 

 that when other sources of moisture are cut off, 

 this is most copious. 



4. Clouds are produced by aqueous vapour 

 when it returns to the state of water. This pro- 

 cess is condensation, the reverse of evaporation. 

 When vapour exists in the atmosphere, if in any 

 manner the temperature becomes lower than the 

 constituent temperature, requisite for the mainte- 



* London, 1219, 



