32 MICRO-ORGANISMS AND DISEASE. [CHAP. 



methyl blue, or magenta, washing with water, then spirit, then 

 distilled water, then drying, and finally mounting it in Canada- 

 balsam solution, (c) Test-tubes containing sterile nourishing 

 material (broth, Agar-Agar mixture, gelatine mixture, Cohn's 

 or Pasteur's fluid) are inoculated in the manner described 

 previously, i.e. by piercing the cotton-wool plug with the 

 pointed end of the capillary pipette. These test-tubes are 

 then exposed in the incubator, and after one or two days or 

 more, a sample is withdrawn with a capillary pipette, and used 

 for microscopic examination. As a rule, after a day or two of 

 incubation we can already distinguish with the unaided eye 

 whether there are any organisms present, the nourishing 

 fluid either being uniformly turbid this is generally the 

 case or there being a growth at the bottom of the fluid. But 

 of course the microscopic examination only shows what kind 

 of organisms are present. New cultivations are made from 

 this one, if any are required, (d] A good plan of recognising 

 easily that there are present various kinds of organisms in 

 such cultures is one similar to that recommended by Professor 

 Angus Smith. 1 Sterile gelatine broth or gelatine only, 

 contained in sterile test-tubes plugged with sterile cotton-wool, 

 is liquefied, but of course not heated to more than about 35 

 40 C., then inoculated with the water (to be tested), by 

 means of the capillary pipette ; after inoculation the gelatine 

 is mixed by shaking the test-tube slightly. In this way the - 

 organisms present in the water are distributed in the gelatine. 

 Then the -gelatine is allowed to set and is kept in this solid 

 state. The organisms being distributed in the gelatine, after 

 some days' growth are noticeable as clusters which gradually 

 increase in extent and are distributed in various parts of the 

 medium. The various species, owing to difference of growth, 

 form clusters differing in aspect, size, and arrangement. 



4. Examination of Air. The simplest plan to test for the 

 presence of organisms in the air is to draw out the cotton- wool 

 plug of several test-tubes or flasks containing the sterile 

 nourishing material, or, if this be boiled, potato, paste, or gela- 

 tine (see p. 14), to expose their surface, and to leave it thus for 

 variable periods, from a few seconds to several minutes. Then 

 replace everything and expose the material to incubation, or 

 keep it only at the ordinary temperature of the room. An- 

 other method is to collect the particles present in the air on 

 glasses moistened with pure glycerine (Maddox), and then 

 to make microscopic specimens or inoculate tubes with this 

 glycerine. 



1 Sanitary Record, p. 344, 1883. 



