40 BACTERIA 



deposit may indicate organic or inorganic pollution. If 

 abundant or conspicuous, a microscopic examination of the 

 sediment may be made. The reaction, whether acid, neutral, 

 or alkaline, must be tested, and the exact temperature taken. 

 Any and every fact will help us, perhaps not so much to 

 determine the contents of the water as to interpret rightly 

 the facts we deduce from the entire examination. 



At the beginning of the bacteriological work the water 

 should be examined by means of the gelatine plate method. 

 This consists in drawing up into a fine sterilised pipette a 

 small quantity of the water and introducing it thereby into 

 a test-tube of melted gelatine at a temperature below 40^ C.^ 



Levelling Apparatus for Koch's Plate 



It will depend upon the apparent quality of the water as to 

 the exact quantity introduced into the gelatine ; about .5 or 

 .1 of a cubic centimetre is a common figure. The stopper is 

 then quickly replaced in the test-tube, and the contents gently 

 mixed more or less equally to distribute the one-tenth cubic 

 centimetre throughout the melted gelatine. A sterilised sheet 

 of glass (4 inches by 3) designated a KocJt s plate is now 

 taken and placed upon the stage of a levelling apparatus, 

 which holds iced water in a glass jar under the stage. 

 The gelatine is now poured out over the glass plate, and by 



^ The gelatine is reduced to liquid form by heating in a water-bath. Before 

 inserting the suspected water it is essential that the gelatine be under 40" C. or 

 thereabouts, in order not to approach the thermal death-point of any bacteria. 



