272 EFFECTS OF VENTILATION. 



accurate calculation, it would certainly reach, yet the accuracy 

 of these calculations will appear sufficiently obvious to any one 

 who has paid studious attention to the subject. I say studious 

 attention, because a person may be tolerably observant of atmos- 

 pheric phenomena, and yet not form anything like an accurate 

 idea of this extraordinary process going on in his presence, and 

 the effect thereby produced on the vegetable system. When 

 we enter a hot-house, on a cold, frosty morning, after a strong 

 fire has been kept up during the night, we are very apt to regard 

 the moisture condensed upon the lower surface of the glass 

 as an evidence of a healthy atmosphere and luxuriant veg- 

 etation ; and often have I heard it stoutly asserted, that it 

 was merely the effect of an excess of moisture in the atmos- 

 phere of the house. This may be partly true, but the conclusions 

 which are drawn from the fact are founded on misconception, 

 that the moisture thus deposited on the glass has already per- 

 formed its purpose of benefit to the plants. 



