ON THE CLIMATE OF HOTHOUSES. 271 



ARTICLE V. 



ON THE CLIMATE OF HOTHOUSES. 



(Continued from page 256.) 



Some idea may be formed of the prodigiously increased drain 

 upon the functions of a plant, arising from an increase of dryness 

 in the air, from the following consideration. If we suppose the 

 amount of its perspiration, in a given time, to be 57 grains, the 

 temperature of the air being 75<>, and the dew-point 70, or the 

 saturation of the air being 849, the amount would be increased 

 to 120 grains in the same time, if the dew-point were to re- 

 main stationary, and the temperature were to rise to 80o; or, 

 in other words, if the saturation of the air were to fall to 726. 



Besides this power of transpiration, the leaves of vegetables 

 exercise also an absorbent function, which must be no less dis- 

 arranged by any deficiency of moisture. Some plants derive the 

 greatest portion of their nutriment from the vaporous atmosphere, 

 and all are more or less dependant upon the same source. The 

 Nepenthes Distillatoria lays up a store of water in the cup formed 

 at the end of its leaves, which is probably secreted from the air, 

 and applied to the exigencies of the plant when exposed to 

 drought ; and the quantity which is known to vary in the hot- 

 house, is no doubt connected with the state of moisture of the 

 atmosphere. 



These considerations must be sufficient, I imagine, to place in 

 a strong light the necessity of a strict attention to the atmos- 

 phere of vapour in our artificial climates, and to enforce as 

 absolute an imitation as possible of the example of nature. The 

 means of effecting this is the next object of our inquiry. 



Tropical plants require to be watered at the root with great 

 caution, and it is impossible that a sufficient supply of moisture 

 can be kept up from this source alone. There can, however, be 

 no difficulty in keeping the floor of the house and flues constantly 

 wet, and an atmosphere of great elasticity may thus be main- 

 tained in a way perfectly analogous to natural process. Where 

 steam is employed as the means of communicating heat, an occa- 

 sional injection of it into the air may also be had recourse to ; 

 but this method would require much attention on the part of the 



