SECTION IV. 



i-dANAGEMENT OF THE ATMOSPHERE 



1. Notwithstanding all the discussion which has taken 

 place upon the abstract question of atmospheric motion, — and 

 which, under certain temperatures, as we have already seen, 

 cannot be disputed, — the tnie principles of ventilation still 

 remain unsettled ; and the mechanical operation of admitting 

 the air in larger or smaller quantities with facility does not, 

 in the slightest degree, remove the general objections that have 

 been urged against its effects on the internal atmosphere. In 

 considering, therefore, the question, how far the admission of 

 external air into forcing-houses is practicable and proper, it is 

 necessary to ask, in the first place. For what purpose is the 

 admission of external air resorted to under certain circum- 

 stances ? and, secondly, How does it act upon the atmosphere 

 when admitted ? 



The first of these questions is of comparatively easy solution : 

 the latter requires more deep consideration, and more close 

 investigation, before we can find a satisfactory reply. 



First. The necessity for ventilation arises from two prime 

 causes, which are briefly these : to regulate and reduce the 

 internal temperature ; and to allow the escape of impure air, or 

 that portion from which some of the .essential constituents have 

 been abstracted by the plants, or in which the natural equiva- 

 lents have been changed in their proportions, and consequently 

 the health-imbuing balance destroyed, — an effect which may 

 arise from various causes. The first of these points is a distinct 

 consideration, forming an important branch in vegetable physi- 

 ology : the others constitute a different branch of scientific 

 research ; but in relation to our present subject, they both merge 

 into one. 



