EXAMINATION OF WATER 299 



between a pair of sterile forceps or tongs. Wholly immerse 

 the bottle under the surface of the water until filled ; the 

 bottle is then tightly stoppered, wiped with a clean duster, 

 and returned to its case. In some cases it will be found more 

 convenient to draw up a sample with a sterile pipette 

 plugged with cotton wool, from which the sample bottle is 

 filled.. When collecting from a tap, the water should be 

 allowed to run for at least ten minutes before taking a sample. 

 In the case of well-waters it is sometimes necessary to draw 

 off the water for some hours or more before proceeding to 

 take a sample. If it is thought desirable to take a sample 

 from definite depths, this can be done by the employment 

 of small vacuum bulbs similar to those described above, 

 which are let down to the necessary depth by means of a 

 weighted wire or string ; the drawn-out point of the bulb 

 is then broken by a suitable mechanical arrangement. 



All samples intended for bacteriological examination 

 should be examined at once. If this is impossible, they 

 should be kept in an ice- chamber, in order not to have the 

 results vitiated by the multiplication of the contained 

 organisms. 



Determination of the Number of Bacteria. — From 0"02 to 

 0*5 c.c. of the sample, dependent upon its purity, is with- 

 drawn by means of a small sterile graduated pipette, and 

 added to a tube containing melted sterile nutrient gelatine 

 at a temperature of about 27° C. The cotton-wool plug 

 of the tube is then replaced, and the contents of the 

 tube gently agitated, so as to thoroughly mix the contents. 

 The plug is again withdrawn, and the contents of the tube 

 poured on to a sterile glass plate, or into a Petri dish, as 

 described on p. 70. 



It is even more satisfactory to drop the proper quantity 

 of water with due precautions into the molten gelatine 

 (which should not have a temperature above 27° C.) con- 



