392 THE POPULAR SCIENCE MONTHLY, 



that agency is introduces us to the second branch of Priestley's dis- 

 covery. 



He had put some mice in a glass containing atmospheric air, close- 

 ly stopped, and found, as usual, that they died of suffocation as soon 

 as the air became sufficiently impure by their breathing ; an abso- 

 lutely poisonous quality being gradually assumed. But, if a few 

 vegetable leaves, or a small plant, were placed in the glass, and ex- 

 posed to the sun, in a very short time the poisonous quality disap- 

 peared, and the power of supporting animal life was regained. Here, 

 then, was an unexpected result — a discovery that gave a solution to all 

 the difficulty, and which has been verified in its minutest details by 

 more modern experiments. It has revealed the great and interesting 

 fact that plants and animals stand in a relation of antagonism to one 

 another ; that whatever changes the one tends to impress on the air, 

 the other undoes ; and that, while animals discharge their duty in con- 

 sequence of their being living and moving things, plants perform 

 theirs under the influence of the light of the sun ; for these changes 

 do not go on in the dark. 



Let us look at these facts by the aid of modern chemistry, premis- 

 ing that oxygen is an invisible substance, existing in the air, and that 

 carbonic acid arises from its union with carbon. When carbon burns, 

 it is merely uniting with atmospheric oxygen, and the resulting car- 

 bonic acid escapes away under an invisible form. So, too, when a 

 man breathes, he draws in oxygen from the air ; it is distributed to 

 all parts of his system, and, combining therein with carbon, turns into 

 carbonic acid, which is expelled when he throws out his breath. 

 Every animal, therefore, to use the language of chemistry, is an oxi- 

 dizing machine, the physical end of its existence being to rob the air 

 of oxygen, and put back, in its stead, carbonic-acid gas. 



With plants it is just the reverse. As long as the sun is shining 

 upon them, they take carbonic acid from the air, and, decomposing it 

 by their leaves, they set free its oxygen, which escapes away ; its car- 

 bon they appropriate. With it they form their various parts, their 

 stems, roots, flowers, seeds ; but they do this only so long as the sun 

 shines, and when night or winter comes the process stops. 



The animal, therefore, takes from the air oxygen, and turns it into 

 carbonic acid ; the plant takes that carbonic acid, and turns it back 

 into oxygen, which has thus discharged the great office of carrying 

 carbon from the bodies of animals, and transferring it to the systems 

 of plants. In what an interesting relation do the two kingdoms, the 

 animal and the vegetable, thus stand to one another, not alone as 

 respects the air in maintaining its constitutimi uniform by a mutual 

 antagonization, but also as respects their own structures ! The ele- 

 ments of which plants are formed have all been derived from the pre- 

 existing parts of animals ; and the elements of which animals consist, 

 from the preexisting parts of plants. To the classical scholar, what a 



