162 CHEMICAL ACTION OF 



words, purify the air again. This beautiful discovery, 

 for the main principles of which we are indebted to 

 the celebrated Dr. Priestley, shows a mutual depend- 

 ance of the animal and vegetable kingdoms on each 

 other, which had never been suspected before his time. 

 Comparative experiments upon the lower tribes of 

 these kingdoms have not yet been made, but they 

 "would probably afford us a new test for distinguishing 

 them. The air so copiously purified by a Conferva^ 

 one of the most inferior in the scale of plants, may be 

 very extensively useful to the innumerable tribes of 

 animated beings which inhabit the same waters. The 

 abundant air-bubbles which have long ago given even 

 a botanical name to one supposed species, Cojiferva 

 bullosa, are probably a source of life and health to 

 whole nations of aquatic insects, worms and polypes, 

 whenever the sun shines. 



In the dark, plants give out carbon and absorb 

 oxygen : but the proportion of the latter is small, com- 

 pared to what they exhale by day, as must likewise be 

 the proportion of carbon given out ; else the quantity 

 of the latter added to their substance would be but 

 trifling, especially in those climates where the propor- 

 tion of day to night is nearly equal, and which, not- 

 withstanding, we know to be excessively luxuriant in 

 vegetation. Plants also give out azotic gas : but M. de 

 Saussure is of opinion that this proceeds from their 

 internal substance ; and it appears by his experiments 

 to be rather a sign of disease or approaching decay, 



