64 PERSPIRATION. 



For the same reason common green glass is 

 less fitted for glazing forcing-houses than white 

 crown glass. 



Poisonous gases in very minute quantities 

 act upon vegetation with great energy. A ten- 

 thousandth part of sulphurous acid gas is quickly 

 fatal to the life of plants ; and hence the danger 

 of flues heated by coal fires, and the impossi- 

 bility of making many species grow in the 

 vicinity of houses heated by coal fires, or in 

 large towns. 



Heating by hot water is now so well under- 

 stood, and so simplified by the method of first 

 heating the air in a large reservoir, or air cham- 

 ber, from which it circulates to any required 

 part, either of a green house or dwelling house, 

 that no one erecting a glass structure for plants 

 would now think of heating on the old princi- 

 ple of the fire flue. 



Perspiration. 



It is not, however, exclusively by the action 

 of light and air that the nature of sap is al- 

 tered. Evaporation from the leaves is con- 

 stantly going on during the growth of a plant, 

 and sometimes is so copious, that an individual 



