INFUSOBIAL ANIMALCULES. 531 



isms. The eggs of the infusoria, accordingly, may be easily raised 

 and held suspended in the atmosphere, under the form of minute 

 dust-like particles, ready to germinate and become developed when- 

 ever they are caught by the surface of a stagnant pool, or of any 

 artificially prepared infusion. In point of fact, the atmosphere 

 does really contain an abundance of such dust-like particles, even 

 when it appears to be most transparent and free from impurities. 

 This may be readily demonstrated by admitting a single beam of 

 sunshine into a darkened apartment, when the shining particles sus- 

 pended in the atmosphere become immediately visible in the track 

 of the sunbeam. Again, if a perfectly clean and polished mirror 

 be placed with its face upward in a securely closed room, and left 

 undisturbed for several days, its surface at the end of that time will 

 be found to be dimmed by the settling upon it of minute dust, 

 deposited from the atmosphere. There is no reason, therefore, for 

 disbelieving that the air may always contain a sufficient number of 

 organic germs for the production of infusorial animalcules. 



There is some difficulty, however, in obtaining direct proof that it 

 is through the medium of the atmosphere that organic germs pene- 

 trate into the watery infusions. It is true that if such an infusion 

 be prepared from baked meat or vegetables and distilled water, and 

 afterward hermetically sealed, no infusoria are developed in it ; but 

 this only shows, as we have already intimated, that the free access 

 of air is necessary to the development of all organic life, just as it is 

 to the support of animals and plants under ordinary conditions of 

 growth and reproduction. Such a result, therefore, proves nothing 

 with regard to the external origin of the infusoria. In order to be 

 conclusive, such an experiment should be so contrived that the 

 watery infusion, previously freed from all foreign contamination, 

 should be supplied with a free access of atmospheric air, while the 

 introduction of living germs by this channel should at the same time 

 be rendered impossible. An experiment of this kind has in reality 

 been contrived and successfully carried out by Schultze, of Berlin. 1 



This observer prepared an infusion containing organic substances 

 in solution, and inclosed it in a glass flask (Fig. 169, a) of such a 

 size, that the infusion filled about one-half the entire capacity of the 

 vessel. The mouth of the flask was fitted with an air-tight stopper 

 provided with two holes, through which were passed narrow glass 

 tubes bent at right angles. To each of these tubes was attached a 



1 Edinburgh New Philosophical Journal, Oct. 1637. 



