96 BACTERIOLOGICAL ANALYSIS LESSON 



(4) Place the gelatine tube, containing the water to be 

 tested, in this groove, keeping it horizontal; and roll it 

 rapidly round and round, until the gelatine sets on the 

 inner surface of the tube in a uniform layer. 



(5) Place these roll tubes in the cool incubator at 22 C. 



Examine the tubes from day to day, and count the 

 colonies, before liquefaction sets in. 



The counting must be done by means of Esmarch's 

 apparatus. 



The quantitative examination of water is not of great 

 value, .except for the purpose of testing niters, or of 

 comparing different kinds of water, as for instance tap 

 water and distilled water, or of studying the effect of 

 physical or chemical processes on a given sample of 

 water. 



B. DISTILLED WATER. 



Prepare gelatine tubes and work them up in exactly the 

 same manner as described above, and compare the result of 

 this examination with that of the previous one. 



Fresh distilled water contains fewer organisms than tap 

 water. 



C. TANK WATER. 



Remove two samples : (a) from the surface j (b) from a 

 given depth (a foot below the surface). 



Work these samples up in the manner already described, 

 using, however, not more than -1 or -12 cc. 



Compare the results : the surface water will contain 

 more organisms than the water removed from the depth. 



