668 THE POPULAR SCIENCE MONTHLY. 



comes to an end. This gas is a necessary part of the animal dietary. 

 It supplies the tinder which kindles life's fuel into a vital blaze, and 

 in other ways it assists not only the building-up but the physiological 

 "breakdown" of the animal frame. Part of this "breakdown" or 

 natural waste accompanying all work, like the inevitable shadow, con- 

 sists of carbonic acid gas. This latter compound is made up of so 

 much carbon and so much oxygen. It arises from the union of these 

 two elements within the body, and is a result of the production of 

 heat, representing, in this way, part of the ashes of the bodily fire. 

 Viewed as an excretion, as a something to be got rid of, and as a 

 deadly enough element in the animal domain, this carbonic acid is a 

 thorough enemy of animal life. It is not only useless in, but hurtful 

 to, the animal processes. Ventilation is intended as a practical war- 

 fare against the carbonic acid we have exhaled from lungs and skin ; 

 and " the breath, rebreathed," is know T n to be a source of danger and 

 disease to the animal populations of our globe. Here, however, the 

 system of natural economics appears to step in and to solve in an ade- 

 quate fashion this question of carbonic acid and its uses. Just as the 

 chemist elaborates his coal-tar colors from the refuse and formerly de- 

 spised waste products of the gas-works, so Dame Nature contrives a use 

 for the waste carbonic acid of the animal world. She introduces the 

 green plants on the scene as her helpmates and allies in the economical 

 work. Every green leaf we see is essentially a devourer of carbonic- 

 acid gas from the atmosphere. That which the animal gives out, the 

 green plant takes in. Not so your mushrooms and other grovelers of 

 the vegetable kingdom, which, having no green about them, refuse to 

 accept the cast-off products of the animal series, and despise the car- 

 bonic acid as a poor but proud relation discards the gift of our old 

 garments. The green plant is the recipient of the animal waste. The 

 leaves drink in the carbonic acid which has been exhaled into the at- 

 mosphere by the tribes of animals. They receive it into their micro- 

 scopic cells, each of which, w T ith its living protoplasm and its chloro- 

 phyl or green granules, is really a little chemical laboratory devoted 

 to the utilization of waste products. Therein, the carbonic-acid gas 

 is received ; therein, it is dexterously split up, " decomposed," as 

 chemists would have it, into its original elements, carbon and oxy- 

 gen ; and therein is the carbon retained as part of the food of the 

 plant, while the oxygen, liberated from its carbon bonds, is allowed 

 to escape back into the atmosphere, to become once again useful for 

 the purposes of animal life. 



There would thus appear to be a continual interchange taking 

 place between the animal and plant worlds a perpetual utilization by 

 the latter of the waste products of the former. It is immaterial to 

 this main point in natural economics that the reception of carbonic 

 acid by green plants can only proceed in the presence of light. It is 

 equally immaterial that by night these green plants become like ani- 



