POLYGASTRIC ANIMALCULES. 547 



\vhich the water they inhabit is in contact, is necessary to the 

 continued life of Animalcules. But this may be in very small 

 amount. Many species will live in water placed under the 

 receiver of an air-pump, from which the greater part of the air 

 has been removed. Others are not killed for a long time, by the 

 interposition of a stratum of oil between the water and the air 

 above. The microscopic observer well knows that he can keep 

 most kinds of Animalcules for several days in a drop of water, 

 flattened between two pieces of glass, so that it is only in con- 

 tact with the air at its edges. But if some other gas be substi- 

 tuted for atmospheric air, a positively injurious effect results ; 

 when a vessel was three-parts filled with water containing 

 Animalcules, and the remaining fourth was filled with hydrogen, 

 they died within seventeen hours. 



1 1 24. Animalcules are very susceptible of the influence of 

 Electricity. A spark drawn through water inhabited by them, 

 usually destroys all those that happen to be in its current ; and 

 this is the case whether the Electricity be generated by an 

 electric machine, by a galvanic pile, or by a magnetic appara- 

 tus, provided that the current be of sufficient intensity to decom- 

 pose water. These beings are also readily acted upon by various 

 substances, which are soluble in water, or which can otherwise 

 act upon them in a fluid form ; but the susceptibility to such 

 influences varies with the species. Thus Animalcules inhabiting 

 fresh-water are killed by a single drop of sea- water, which may 

 nevertheless be full of other species peculiar to it. Alcohol 

 usually destroys the inhabitants of fluids to which it is added even 

 in small quantities ; sugar has the same effect on many species ; 

 and strychnine on all. On the other hand, substances which are 

 only mechanically suspended, or but slightly soluble in water, 

 such as arsenic, appear to have little or no influence, even when 

 their particles are swallowed ; and calomel, corrosive sublimate, 

 and camphor, are not fatal until some hours have elapsed. 



1125. The universal presence of Animalcules in fluids con- 

 taining organic matter, and exposed to the air, joined to the 

 suddenness of their appearance under particular circumstances, 

 has led to the belief that they are generated by the decomposition 



