24 VIEWS OF THE MICROSCOPIC WORLD. 



bly ensues whenever the cold is sufficiently intense to congeal this enclosing film 

 of water. 



AIR. Air is as necessary to existence of Infusoria as to any other class of 

 animated nature ; for when they are denied access to the atmosphere, and are 

 thus prevented from receiving constant supplies of pure air, life becomes extinct 

 within a short time. If oil is poured upon the water containing animalcules, 

 and the surface of the fluid is entirely covered with the oil, the air is necessarily 

 excluded and the creatures speedily die. Or should the naturalist fill a phial 

 with water in which animalcules reside, and leave it corked tightly for any length 

 of time, he will have the mortification of finding, on examination, that the fluid, 

 once so full of life and activity, has become entirely inert, and that millions of 

 existences have passed away. The fact that air is necessary to the existence of 

 Infusoria has been particularly noticed in regard to the larger kinds of wheel- 

 animals ; for when experiments have been made by placing these creatures under 

 the receiver of an air-pump, they have always ceased to live soon after the air 

 has been withdrawn from the vessel in which they were contained. 



Dr. Ehrenberg affirms, that if animalcules are placed in nitrogen gas they 

 exist for a longer time than if they are immersed in carbonic acid or hydrogen 

 gas. In the fumes of sulphur they quickly perish. 



POISONS. The most powerful poisons, which mingle simply in a mechanical 

 manner with water, like earth, do not effect the lives of animalcules placed in 

 the mixture ; but those which unite chemically, and are dissolved in the fluid, 

 soon deprive them of their existence. One kind of Infusoria has been known to 

 live so long in water with which calomel and corrosive sublimate had been mixed, 

 that it was doubtful whether their death was to be attributed to the effect of 

 these ingredients or not. 



Many species of animalcules can adapt themselves to a gradual change in the 

 nature of the element in which they live, but a sudden transition kills them. 

 For instance, similar kinds are found at the heads of rivers where the water is 

 fresh, and at their mouths, where the streams mingle with the briny ocean. 



If sea-water, abounding with marine animalcules, is mixed by slow degrees 

 with the fluid in which fresh-water species reside, the latter survive ; but if the 

 mixture is suddenly made, they perish immediately. 



PHOSPHORESCENCE OF THE SEA. Various opinions have at times been enter- 

 tained in respect to the cause of this beautiful phenomenon ; but it is now cor- 

 rectly attributed chiefly to the presence of animalcules, which crowd the waters 

 in vast multitudes. This appearance, although confined to no particular 

 part of the ocean, attains its greatest splendor in the tropical climes, where the 

 spectacle is often exceedingly grand and beautiful. 



This brilliant phenomenon is thus graphically described by Darwin in his 

 " Voyage of a Naturalist " : " While sailing a little south of the River La Plata, 



