CHEMICAL COMBINATIONS. 297 



we should have woody fibre, and starch, and gum, and sugar, 

 occurring together in variable quantities, when we know that 

 they all are made up of the same materials, in the same pro- 

 portions ; or that one of these should occasionally disappear 

 from a plant, to be replaced in whole or in part by another. 



A further question arises in our minds, in connection : Are 

 these elements formed in an artificial atmosphere, such as that 

 of a hot-house, from the same combinations of matter as in 

 the natural atmosphere ? A reply, though probably not a satis- 

 factory one, may be drawn from the following considerations : 



During the day plants assimilate carbonic acid, and evolve 

 oxygen ; and during the night this system is reversed, although 

 we have no accurate data from which to conclude that the rela- 

 tive proportions of these gases are, at all times and under all 

 circumstances, the same. From the latest experiments, we are 

 induced to suppose that, in an artificial atmosphere, oxygen is 

 the most important element to be attended to, in the regulation 

 of its elements; and from the ^ fact .that its presence, to the 

 amount of 21 per cent, in common atmospheric air, is essential 

 to the existence of animals and plants, there can be little 

 doubt that it is more frequently in deficiency, than in excess, 

 in an artificial atmosphere, and that hot-house plants are more 

 frequently injured by the want of a proper supply, than by an 

 excess of it in the atmosphere, when we consider the quantity 

 of this substance which nature has stored up for the use of 

 plants and animals. Nearly one half of the solid rocks which 

 compose the crust of our globe, of every solid substance we 

 see around us, of the houses in which we live, and of the 

 stones on which we tread, of the soils which we daily culti- 

 vate, and much more than one half by weight of the bodies of 

 all living animals and plants, consist of this elementary body, 

 oxygen, known to us only in the state of a gas. It may appear 

 surprising that any one elementary substance should have been 

 formed, by the Creator, in such abundance as to constitute nearly 

 one half by weight of the entire crust of our globe. But this 

 is not so surprising, when we consider that it is on the presence 

 of this element that all animal and vegetable life depends ! Nor 

 is it less wonderful that a substance, which we know only in a 



