166 MICROBES, FERMENTS, AND MOULDS. 
These numbers are the minima. The putrefaction 
of stagnant sewer-water produces germs from which, 
in a few days, microbes are multiplied by thousands. 
Certes, in France, and Maggi, in Italy, have lately 
been occupied with the micrographic study of drink- 
ing-water. These observers reveal the presence of 
microbes in the water under examination by means 
of staining reagents. The reagent most in use is a 
15 per cent. solution of osmic acid (Certes). Osmic 
acid kills the microbes without changing their form, 
and precipitates them to the bottom of the glass 
vessel, whence it is easy to collect them. A cubic 
centimetre of the solution suffices for 30 or 40 cubic 
centimetres of water. It is allowed to settle, then 
the liquid is poured off, and the thick, dark-coloured 
deposit which remains consists of all the organisms 
previously diffused in the- liquid, and may be examined 
under the microscope. The only drawback to the 
use of this reagent is the high price of osmic acid, 
a matter worth consideration in the extensive and 
comparative researches necessary in these cases. 
Maggi obtained analogous results with chloride of 
palladium, and Certes with iodide of glycerine, and 
alcoholic solutions of cyanine, gentian, etc.; but none 
‘of these reagents are as efficient. as osmic acid, of 
which the effect is more precise, constant, and durable. 
Microbes of the Soil—The presence of microbes in 
the soil has been proved by Pasteur and his fellow- 
workers, Chamberland and Roux, in the researches into 
