SECTION V. 



CHEBIICAL COMBINATIONS OF THE ATMOSPHERE. 



1. With respect to the chemical effects of ventilation, upon 

 an artificial atmosphere, there are two important things to be 

 kept in view, in providing an artificial atmosphere for plants in 

 a glazed structure; namely, the nourishment they ought to 

 receive from it, and how to maintain it in this nutrient state. 



It is needless, in this place, to enter upon the minute detail 

 of the various substances which enter into the composition of 

 plants, or of the various elements which combine to form the 

 different bodies of which they are composed, — bodies, in them- 

 selves so different in their qualities, yet so identical in their for- 

 mulae, and consisting of the same elements, united together in 

 the same proportions. This is one of those facts in chemical 

 science which appear so very remarkable to those who have not 

 directed their attention to chemistry", but are scarcely capable of 

 being clearly comprehended and explained, even by those who 

 have profoundly studied this branch of natural science. Starch 

 and sugar — how different their properties ! — how unlike their 

 uses! — how unequal their importance to the human race! 

 Yet they consist of the same weights, of the same substances 

 differently conjoined. The skilful architect can put together 

 the same proportions of the same stone and cement; and 

 the painter can combine the same colors, to produce a thou- 

 sand varied impressions on the sense of sight. But in the hand 

 of the Deity matter is infinitely more plastic. In his hands, 

 and at his bidding, the same particles can unite in the same 

 quantities, so as to produce the most dissimilar impressions, and 

 on all our senses at once. 



A knowledge of the above close relations, in composition 

 among a class of substances occurring so abundantly in plants, 

 imparts a degree of simplicity to our ideas of this otherwise so 

 very complicated subject. It does not appear so mysterious that 



