EXAMINATION OF WATER FOR MICRO-ORGANISMS 77 



the gelatine-plates, whilst most convenient of all are the 

 covered shallow dishes (commonly called Petri-dishes) 

 already referred to (see p. 34). After the ice-plate has 

 been arranged, and all the necessary preparations made 

 for the pouring of plates, the sample of water is taken 

 and violently shaken for several minutes, in order to 

 disintegrate any aggregations of microbes, as well as to 

 secure even distribution of the bacteria throughout the 

 liquid. A definite quantity of the water is now removed 

 by means of a sterilised and graduated pipette, and 

 introduced into the gelatine-tube, which, during the 

 operation, should be held in a slanting position. The 

 cotton-wool stopper is then replaced and the contents 

 gently agitated, and the stopper removed with all the 

 precautions already described, and the plate poured in 

 the usual manner. Supposing the water to be fairly 

 pure as regards microbes, 1 c.c. may be taken for one 

 plate, and '5 c.c. for a duplicate. In all cases at least 

 two plates must be poured of each sample of water. If 

 the water is suspected of containing a large number of 

 bacteria, then it will be necessary to dilute, say, 1 c.c. 

 of it- 50 or 100 or 500 times, as the case may be, before 

 pouring the plates. For this purpose a small sterile 

 stoppered bottle containing, say, 50 c.c. of sterilised 

 natural water (not distilled water) may receive 1 c.c. 

 of the original water. After thorough shaking, 1 c.c. 

 from this bottle may be introduced into another similar 

 bottle, and so on, until the attenuation is considered 

 sufficient ; plates may then be poured from the two 

 last attenuations. As in all such manipulations success 

 can only be attained after practice and much expe- 

 rience. The plates are incubated in the usual manner, 

 and the counting of the colonies is conveniently carried 

 out with the assistance of a counting apparatus (fig. 13). 

 This consists of a wooden stand A on which is sup- 



