THE MICROBES OF THE AIR 265 



when glass-wool is used. Drs. Miquel, 1 Fol, 2 

 Gautier, 3 and other French bacteriologists use 

 soluble filtering media ; and bouillon as the medium 

 for the growth of microbes. 



In England and Germany solid cultivation media 

 have been substituted for the liquid bouillon ; and 

 when the microbian mixture is introduced into 

 melted nutrient gelatine, it ' can be evenly dis- 

 persed throughout the medium by gentle agitation, 

 and by subsequently allowing it to solidify, the 

 microbes are not only isolated, but rigidly confined 

 to one spot. Thus each individual microbe becomes 

 a centre round which extensive multiplication takes 

 place, and in a few days definite points of growth 

 are visible to the naked eye, which are appropriately 

 described as colonies. Although each colony con- 

 sists of many thousands, or even millions of in- 

 dividual microbes, yet, as in the first instance, they 

 owe their origin to a single organism or indivisible 

 group of organisms, it is correct to regard the number 

 of colonies as representing the number of microbes/ 



One of the best methods for estimating the num- 

 ber of microbes in a known volume of air, is that 

 devised by Dr. W. Hesse. 4 Hesse's method con- 

 sists in slowly aspirating a known volume of air 

 through a glass tube (28 X If in.) which has pre- 

 viously been coated internally with a film of sterilised 

 nutrient gelatine. The microbes suspended in the 



1 Annuaire, de VObservatcrire de Afontsourix, 1880-92. 



2 La Nature, 1885. 



3 Rente Scientijique, 1886. 



4 Mittheilungen am dem Icaiserliche.n Oesundheitnamte, vol. ii. 



