THE MICROBES OF HUMAN DISEASES. 163 



are capable of resisting a prolonged boiling at a 

 temperature of 100 ; they still live and are capable 

 of germinating and multiplying when the liquid is 

 cooled. 



Culture liquids may also be sterilized without 

 the use of heat, which to some extent affects their 

 nature, by filtering them through a porous substance 

 biscuit- ware, or a mixture of plaster and amianthus, 

 etc. A more perfect apparatus is employed by Miquel, 

 consisting of a filter of very thick paper, through 

 which the liquid is forced by the simultaneous action 

 of a vacuum on one side, and of strong pressure on 

 the other. 



For the artificial culture of microbes, solid or 

 partially solid substances are by preference often used, 

 such as gelatine, or slices of potatoes, carrots, hard 

 eggs, etc., prepared in different ways and sterilized 

 before use. We cannot here describe in detail all the 

 processes employed and the precautions necessary in 

 order to avoid error. We must content ourselves with 

 giving the results obtained by Miquel. 



There are on an average 80 bacteria in a cubic 

 metre of Montsouris air. A hundred of these bacteria 

 includes 66 Micrococci, 21 Bacteria and 13 Bacilli 

 In rain water there is a different proportion : 28 

 Micrococci, 9 Bacteria, 63 Bacilli. At the beginning 

 of a thunderstorm, the rain-water includes a consider- 

 able number, about 15 to the cubic centimetre; then 

 the number diminishes, but Miquel states that " after 



