126 BACTERIOLOGY 



volume of water used for observation must be diluted with 

 sterilised distilled water (generally in equal parts or in the 

 proportion of one to nine), as otherwise the colonies lie so 

 close together that they cannot be counted, or else they 

 liquefy the gelatine too speedily. 



The counting apparatus can be rendered complete by 

 cutting a piece of some size from the upper part of a square 

 box, and placing a plane mirror obliquely in the interior. 

 If the gelatine plate be now laid upon the box, the number 

 of islets on each square can easily be ascertained by the 

 transmitted light. 



Pfuhl's method. — If the examination can be carried out 

 immediately at the spring, the water to be analysed is poured 

 into sterilised vessels, which are at once closed with a 

 sterilised plug of cotton-wool. To obtain the water without 

 catching extraneous germs, 'Pfuhl uses flat-bottomed 

 glass tubes partially emptied of air, and having the 

 ends drawn out into capillaries, bent at a right angle, and 

 sealed. The points are broken off actually at the spring, 

 and the tubes filled with water and again sealed. For the 

 purpose of transport small cylindrical glass bottles, provided 

 with ground-glass stoppers, are used, which have been 

 sterilised and covered with india-rubber caps. To collect 

 the water from a delivery -pipe the cap and glass stopper 

 are removed, the bottle completely filled, and carefully closed 

 again. The water which first flows away, however, must 

 not be used for examination. To obtain water from a spring 

 the rubber cap is taken off, but the stopper is only removed 

 under the surface of the water, which is allowed to flow into 

 the flask for about a minute, and then the bottle is closed 

 again and lifted out and the rubber cap drawn over the 

 glass stopper. 



Kirchner's method, — About 36 cm. length of glass tube, 

 of the diameter commonly used for making connections 



