40 



Lectures on Bacteria. Qi v. 



in which they find the conditions of vegetation, which are them- 

 selves of very common occurrence. We shall make acquaint- 

 ance with other illustrative instances in subsequent special 

 discussions. Dispensing with an exact determination of the 

 species in every case, we shall be perfectly safe in declaring, 

 as the result of direct observations, that the vital germs of 

 Bacteria are scattered abroad with such profusion in earth, air, 

 dust, and water, that their appearance at all spots where they 

 find the conditions' necessary for vegetation is more than 

 sufficiently explained. 



The way to prove this, and at the same time to determine 

 approximatively the number of germs within a given space, is ob- 

 viously the same in the case of the germs of Bacteria as in that 

 of other lower organisms, Fungi and others ; both necessarily 

 come under our observation at the same time, when they are 

 present. It consists first of all and simply in microscopical 

 examination. But in this method we encounter considerable 

 difiiculties. Sometimes the germs are not present in every 

 smallest spot ; they must be sought for, and this is at all times a 

 troublesome process, especially when it is intended to count them. 

 Various devices may it is true be applied to lighten this labour. 

 Pasteur (13), for example, employed an ingenious contrivance 

 for finding germs in the air in the form of a suction-apparatus, 

 an aspirator, which drew in the air through a tube stopped with 

 a dense plug of gun-cotton. The plug allows the air to pass, 

 while the solid substances in suspension in the air and the 

 germs therefore with them are caught on or in the plug. The 

 quantity of air passing through the apparatus within a given 

 time can be easily determined. The gun-cotton is soluble in 

 ether, and by taking advantage of this property the germs which 

 have been intercepted in the plug may be obtained suspended 

 in a clear solution, and collected within a narrow space for ex- 

 amination and even for counting. 



But in this process the germs are very liable to be killed by 

 the ether, and even in ordinary microscopical examination it is 



