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 Bacillt. 
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 
