264 A MANUAL OF BACTERIOLOGY 



and De Freudenreich attempted the quantitative 

 estimation of aerial microbes. Their method con- 

 sists in aspirating a known volume of air through a 

 tube containing previously sterilised plugs of glass- 

 wool (Fig. 51). The solid particles, including any 

 microbes, are arrested ; and the plugs of glass-wool 

 are then thoroughly mixed with a known volume of 

 sterilised water. The mixture is now sub-divided 

 into such a number of equal parts that each part 

 shall contain not more than one microbe. Each of 

 these sub-divisions is then introduced into a cultiva- 

 tion tube or flask (see Fig. 17) containing sterilised 

 bouillon. These tubes or flasks are placed in an 

 incubator, and any that have received a living 

 microbe will, in a short time, exhibit the fact by 

 suffering visible alteration. As an example, sup- 

 posing the plug through which twenty litres of air 

 were drawn, by the aspirator (Fig. 51 5), was mixed 

 with 25 cc. of sterilised water, and twenty-five 

 tubes of bouillon were then each inoculated with 

 1 cc. of this mixture, and if, after a suitable incuba- 

 tion, it was found that only sixteen of them suffered 

 alteration, it would be concluded that only sixteen 

 microbes were present in the 25 cc. of water distri- 

 buted among the twenty-five tubes, or, in other 

 words, that the twenty litres of air contained sixteen 

 living microbes. 



Miquel and De Freudenreich have since substi- 

 tuted soluble media (powdered sugar or de-hydrated 

 sodium sulphate) for the insoluble glass-wool. By 

 the use of soluble filtering media, there is no chance 

 of any microbes becoming imprisoned, as is the case 



