86 EFFECTS OF SUPPRESSED EVAPORATION. 



a clear and distinct object become immediately 

 very dim and tremulous. Nor was there any dry 

 gravelly vein in the ground, along the course of this 

 scorch. It was therefore probably owing to the 

 much greater quantity of scorching vapours in the 

 middle than outsides of the ground, and that being 

 a denser medium, it was much hotter than a more 

 rare medium." 



" This is an effect which the gardeners about 

 London have too often found to their cost, when 

 they have incautiously put bell-glasses over their 

 cauliflowers early in a frosty morning, before the 

 dew was evaporated off them ; which dew being 

 raised by the sun's warmth, and confined within 

 the glass, did there form a dense transparent scald- 

 ing vapour which burnt and killed the plants." 

 The mflu- When these observations are translated into our 



ence of 



evapora- present language, we perceive with what acuteness 



tion on the 



life of and accuracy HALES recognized the influence of 



plants re- 



cognized evaporation on the life of plants. 



Hb views. According to him, the development and growth 

 of the plant depends on the supply of nourishment 

 and moisture from the soil, which is determined by 

 a certain temperature and dryness of the atmo- 

 sphere. The absorbent power of plants the motion 

 of their sap, depends on evaporation ; the amount of 

 food necessary for their nutrition, which is absorbed, 

 is proportional to the amount of moisture given out 

 (evaporated) in a given time. When the plant has 



