oo SOLID CULTIVATION MEDIA. 



duration, and by comparing this with the number of foci of growth 

 which appear subsequently in the jelly, one can calculate ap- 

 proximately the extent to which germs are present in the atmo- 

 sphere that, is tested. The result of such an experiment is shown 

 in Fig. 46, which is taken from a photograph of a capsule 

 exposed to the atmosphere of Saint Giles' Cathedral, Edinburgh, 

 for a period of fifteen minutes. During that period sixteen germs 

 fell into the capsule and produced the sixteen foci of growth, which 

 could be counted in the capsule eight days after its exposure. The 

 individual species of organisms present in an atmosphere may be 

 recognised by the naked eye appearances of the resulting growths, 

 or by putting the capsule upon the stage of the microscope, and 

 examining its contents with a low power. In the case cited the 

 organisms present were chiefly Aspergilli, Penicillia, and various micro- 

 cocci; Bacterium termo did not appear, but a colony of pink Torula was 

 at once recognised by its bright colour. In a subsequent series of 

 experiments performed in the hot rooms of the Turkish Baths in 

 Princes Street, Edinburgh, it was found that Penicillium glaucum 

 greatly preponderated ; Bacterium termo was also largely present, but 

 the various forms of micrococci did not appear in the capsules. x 



Another method, based on Cohn and Miflet's plan, by which the 

 number of germs in any given bulk of atmosphere can be ascer- 

 tained, is that of Hesse. His apparatus (Fig. 47) consists of a 

 hollow glass cylinder, 18 inches in length and 2}4 in diameter, 

 over one of its ends is stretched a thin indiarubber sheeting, in 

 the centre of which is a small pore made with a fine needle. 

 The other end of the cylinder is occupied by a perforated india- 

 rubber stopper pierced by a piece of glass tubing. To the tubing 

 is attached a litre flask, which has another tube running from it to a 

 second litre flask, the interior of the two flasks being thus in com- 

 munication. The cylinder, indiarubber sheeting, and stopper are 

 purified by washing first with corrosive sublimate solution (1 in 1000), 

 and then with absolute alcohol. A quantity of liquefied nutrient jelly 



1 The success of these experiments depends on the facts, demonstrated by 

 Tyndall, that the germs present in any atmosphere constantly gravitate to its lower 

 strata, and that by allowing subsidence to occur for a sufficient length of time, in a 

 closed space, a perfectly pure germless atmosphere is produced. 



