KOCH'S METHOD OF PLATE CULTURES 53 



the third and so on. There is thus effected a twenty-fold 

 dilution in each successive tube. After these steps have been 

 carried out a definite amount, say, '05 c.c. is transferred from 

 each tube 'to a tube of melted gelatine, the gelatine being after- 

 wards plated and the colonies counted when growth occurs. 

 The number of tubes required will vary according to the 

 number of bacteria in the original mixture, but usually four or 

 five will be sufficient. It is quite evident that this method not 

 only enables us to separate bacteria, but if necessary gives us a 

 means of estimating exactly the number in the original mixture. 

 The colonies appear as minute rounded points, whitish or 

 variously coloured. Their characters can be more minutely 

 studied by means of a hand-lens or by inverting the capsule on 

 the stage of a microscope and examining with a low power 

 through the bottom. From their characters, colour, shape, 

 contour, appearance of surface, liquefaction or non-liquefaction 

 of the gelatin, etc., the colonies can be classified into groups. 

 Further aid in the grouping of the varieties is obtained by 

 making film preparations and examining them microscopically. 

 Gelatin or agar tubes may then be inoculated from a colony of 

 each variety, and the growths obtained are then examined both 

 as to their purity and as to their special characters, with a view 

 to their identification (p. 115). 



2. Glass Plates (Koch). When plates of glass are to be used, an 

 apparatus on which they may be kept level while the medium is solidi- 

 fying is, as has been said, necessary. An apparatus devised by Koch is 

 used (Figs. 18, 19). This consists of a circular plate of glass (with the 

 upper surface ground, the lower polished) on which the plate used for 

 pouring out the medium is placed. The latter is protected from the air 

 during solidification by a bell jar. The circular plate and bell jar rest 

 on the flat rim of a circular glass trough, which is filled quite full with 

 a mixture of ice and water, to facilitate the lowering of the temperature 

 of whatever is placed beneath the bell jar. The glass trough rests on 

 corks on the bottom of a large circular trough, which catches any water 

 that may be spilled. This trough in turn rests on a wooden triangle 

 with a foot at each corner, the height of which can be adjusted, and 

 which 'thus constitutes the levelling apparatus. A spirit level is placed 

 where the plate is to go, and the level of the ground glass plate thus 

 assured. There is also prepared a "damp chamber," in which the 

 plates are to be stored after being made. This consists of a circular 

 glass trough with a similar cover. It is sterilised by being washed out- 

 side and inside with perchloride of mercury 1-1000, and a circle of filter- 

 paper moistened with the same is laid on its bottom. Glass benches on 

 which the plates may be laid are similarly purified. 



To separate organisms by this method three tubes, a, b, c, are inocu- 

 lated as in using Petri's capsules (p. 52). The hands having been 

 washed in perchloride of mercury 1-1000 and dried, the plate box is 

 opened, and a plate lifted by its opposite edges and transferred to the 



