BACTERIOLOGICAL EXAMINATION OF SOIL. 



WATER. 



133 



In the bacteriological examination of water three lines of in- 

 quiry may have to be followed. First, the number of bacteria 

 per cubic centimetre may be estimated. Second, the kinds of 

 bacteria present may be investigated. Third, it may be neces- 

 sary to ask if a particular organism is present, and, if so, in 

 what number per cubic centimetre it occurs. 



Methods. All samples of water taken for analysis should be, if possible, 

 treated at once by one or other of the methods indicated below. This is espe- 

 cially necessary in warm weather, where the delay even of an hour or two in 

 transportation of the sample to the laboratory will greatly increase the num- 

 ber of bacteria present, and lead to erroneous ideas of the actual character of 

 the water under examination. When transportation is 

 unavoidable, the samples should be packed in ice, and 

 sent forward with the shortest possible delay to the 

 laboratory. The collection of water is usually made in 

 four-ounce, wide-mouthed, stoppered bottles, which are to 

 be sterilised by dry heat (the stopper must be sterilised 

 separately from the bottle, and not inserted in the latter 

 until both are cold ; otherwise it will be so tightly held 

 as to make removal very difficult). In using such a 

 bottle for collecting water from rivers, ponds, or lakes 

 at various depths it is advisable to make use of one 

 of the several mechanical devices commonly used for 

 such work (see Fig. 60), in which the bottle is secured 

 in a metal frame at one end of a graduated pole, and 

 lowered into the water; the stopper then is extracted 

 half its distance up the neck of the bottle by pulling upon 

 a spring-wire attachment which clutches the stopper, and 

 when the bottle is full the wire being released from the 

 finger carries back the stopper into place. The operator 

 is warned not to touch the water-bed in making a col- 

 lection, nor surface scum, for such contain large numbers 

 of bacteria, and in the collection of water from a house- 

 tap it is advisable that the water be let run for at least 

 an hour or two previous to taking the sample. 



Upon arrival at the laboratory, if the samples had 

 to be transported, the bottles are shaken vigorously and 

 by means of sterile pipettes of i c.c. capacity, divided into 

 tenths, quantities of water varying from ^ to I c.c. are 

 transferred to melted tubes of gelatin or agar, whose FIG. 60. Appara- 

 reaction is 1.5+ (acid), and plated, and incubated at a tus for collecting water 

 temperature not over 22 C. nor under 16 C. Where a s 

 water is suspected to contain large numbers of bacteria it is well to transfer 

 i c.c. to a small flask containing 99 c.c. of sterile water, and, shaking thoroughly, 



