STAINING or CHOLERA BACILLI 141 



flame to fix the micro-organisms, and then laid on the 

 surface of the staining solution with the infected side down- 

 wards. The solution is allowed to act for ten minutes on 

 the preparations, with, or even without, slight warming, and 

 it need hardly be said that care must be taken to exclude all 

 substances liable to decompose the stain. The cover-glasses 

 are next taken from the solution with the forceps, washed 

 in water, and dried with the prepared surface uppermost, 

 after which a drop of Canada balsam is laid upon the 

 prepared face of the glass, which is in this way cemented 

 to the slide. Microscopic examination is carried out with 

 the help of the oil-immersion lens and Abbe's condenser. 



When a sample of the material to be examined for 

 cholera bacilli is diffused through gelatine, and a plate- 

 culture made, colonies are soon obtained possessing in- 



Non-liquefied marginal portion 



Fib. 49.— Islet of B.«'Ili.us Choleu^ Asiatics ox a Gelatine Plate, 

 IX Process ok Liquefactiox. 



dented and bowed margins, and presenting, when magnified 

 100 diameters, an appearance as if strewn with bits of 

 glass. After a time, small round excavations appear, 

 corresponding to the colonies, and soon give the entire 

 gelatine the aspect of pock-marked skin, the funnel-shaped 

 cavities being due to evaporation of water from the slowly- 

 liquefying gelatine. The liquefaction does not extend very 

 far at first (fig. 49) . 



In thrust-cultures also the gelatine liquefies very slowly, 

 the liquefaction being chiefly seen on the surface, and in 

 this case too evaporation of the fluid occurs, so that a 

 bubble communicating with the external air appears in the 

 upper part of the funnel-shaped excavation. From this 

 bubble a thin prolongation runs down along the track of 



