ON THE BACTEKIA. 149 



powers. While experimenting on the decomposition of vapours 

 by light, Tyndall was under the necessity of obtaining air that was 

 absolutely free from any impurity. He found that if the air was 

 passed over the tip of the flame of a spirit-lamp, the floating 

 matter no longer appeared, having been burnt up by the flame ; it 

 was therefore of orgmiic origin. By further experiments he 

 discovered that this organic matter could be removed by filtering 

 the air through cotton-wool, calcining it by passing it through a 

 platinum tube containing a roll of platinum gauze heated to vivid 

 redness, or allowing it to deposit its impurities by subsidence in a 

 closed chamber : the proof of its purity being, that the electric 

 beam in passing through it, leaves no trace of its path. When 

 air thus purified was admitted to a sterilised infusion no develop- 

 ment of life took place ; but if, on the other hand, ordinary 

 atmospheric air was admitted, in a few days the infusion was 

 found to be swarming with living organisms ; the deduction being 

 that these organisms were developed from the minute particles of 

 organic dust which are always floating in the air, which particles 

 are actually spores; and these, on obtaining admission to the 

 putrescible infusions, gave rise to the Bacteria. 



The advocates of Spontaneous Generation declare that there 

 is absolutely no evidence whatever of these invisible spores ; but 

 Pasteur first, and after him several other observers, claim to have 

 demonstrated their existence microscopically. By causing a 

 current of air to pass for some time through a glass tube, in which 

 a pledget of gun-cotton is placed, the germs and other particles of 

 atmospheric dust are intercepted^ and if the gun-cotton is then 

 dissolved in ether, and the sediment examined with the micro- 

 scope, a certain number of germs are always visible. 



It is, however, quite within the bounds of possibility, that the 

 germs of many of the Bacteria may be invisible, even to our 

 highest microscopic powers, until they have commenced to grow, 

 and are somewhat developed beyond their original size. Some of 

 the beautiful observations of Messrs. Dallinger and Drysdale 

 throw light upon this suggestion. In watching some of the 

 larger Monads, a process of conjugation was seen to take place 

 between two individuals, after which a delicate glossy sac opened 

 gently at one place, and from it streamed out a glairy fluid 



N 



