30 MICROORGANISMS IN WATER 



in 1882, using the hcematimeter to approximately esti- 

 mate the number of yeast-cells contained in the liquid 

 which was to be diluted. 



Gelatine-plate cultures. — Considering, then, what 

 great difficulties attach to the preparation of pure cul- 

 tures by means of liquid media, it may be imagined 

 how welcome was the introduction by Koch of the new 

 methods of culture on solid media, which greatly facili- 

 tated the process of purification. 



Ordinary photographic glass plates (quarter-plate 

 size) serve admirably for plate-cultivations. If new, 

 they should be soaked in caustic soda, then washed 

 with water, dilute hydrochloric acid, with water again, 

 and finally rinsed with distilled water. They are then 

 put in a metal box and placed in a hot-air oven and 

 exposed to a temperature of from 150° to 160° C. for 

 two hours. The gas. is then turned out and they are 

 allowed to cool, the door of the oven being kept closed 

 from the beginning until the moment when the plates 

 are required for use. 



A cylindrical glass dish is filled with ice and water, 

 care being taken that it is quite full, as otherwise in 

 placing the thick glass plate over it bubbles of air 

 become enclosed and thus prevent the uniform cooHng 

 of the plate. 



The glass plate covering the dish is then carefully 

 levelled by means of a 3-screw leveUing-stand (fig. 6) 

 and spirit level. When this is done the condensed 

 moisture which has formed on the surface of the now 

 horizontal glass covering plate is wiped off* and a glass 

 bell jar placed upon it. It is convenient to have two 

 such arrangements in use if a large number of plate- 

 cultivations are to be made, a great economy of time 

 being thereby secured. 



The steriHsed glass plate which is to receive the 



