MICROSCOPIC ANIMALCULES. 59 



again decomposed by the plants. The fact of 

 oxygen gas being so produced was carefully 

 tested by Liebig. In order to satisfy himself of 

 its accuracy, he placed several kinds of Infusoria 

 (Protophytes) in a jar of water, and found, as 

 the result, that oxygen gas was rapidly manu- 

 factured, and accumulated in the upper part of 

 the jar. At the end of fourteen days, one-third 

 of the water had been forced out, and the gas, 

 which had taken its place, was capable of ignit- 

 ing a glowing mass of wood. Thus these mi- 

 croscopic plants and animalcules perform the 

 office of scavengers to the important element of 

 water, removing all dead and putrifying organic 

 matter, and rendering the water bright and 

 inodorous. And so exquisite is the balance 

 between animals and plants, that the streams of 

 a thousand rivers, and the waters of the great 

 ocean itself, are thus kept in clearness and purity 

 throughout their wide extent. Professor Owen 

 has well and happily styled these minute crea- 

 tures " Nature's invisible police." 1 



1 Owen's Compar. Anat. vol. i. p. 27. 



