BACTERIA IN WATER 135 



of one or other certain soil bacteria, it is found that sometimes the 

 typhoid bacillus, sometimes the soil bacterium in the course of a 

 few weeks, or even in a few days, disappears. Further, the char- 

 acter of the soil exercises an important effect on what happens ; 

 for instance, the typhoid bacillus soon dies out in a virgin sandy 

 soil, even when it is the only organism present. In experiments 

 made by sowing cultures of cholera and diphtheria in plots in a 

 field it was found that after, at the longest, forty days they were 

 no longer recognisable. Further, it is a question whether 

 ordinary disease organisms, even if they remain alive, can 

 multiply to any great extent in soil under natural conditions. 

 If we are dealing with a sporing organism such as the b. 

 anthracis, the capacity for remaining in a quiescent condition of 

 potential pathogenicity is, of course, much greater. The most 

 important principle to be deduced from these experiments is that 

 the ordinary conditions of soil rather tend to be unfavourable 

 to the continued existence of pathogenic bacteria, so that by 

 natural processes soil tends to purify itself. It must, however, 

 be noted that such an organism as the typhoid bacillus can exist 

 long enough in soil to be a serious source of danger. 



WATER. 



In the bacteriological jexamination of water three lines of 

 inquiry 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 necessary 

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

 number per c.c. it occurs. 



Methods. In the two first cases a small quantity ("5-1 c.c.) is taken 

 in a sterile pipette and added to a tube of gelatin, which is then plated 

 and incubated at the room temperature. In the case of water taken 

 from a house tap the water should be allowed to run for several hours 

 before the sample is taken, as water standing in pipes in a house is under 

 very favourable conditions for multiplication of bacteria taking place, 

 and if this precaution be not adopted an altogether erroneous idea of the 

 number present may be obtained. In the case of the examination of 

 river water the gelatin plates ought to be prepared on the spot ; at any 

 rate, the time clasping between the sample being taken and the plates 

 being prepared must be as short as possible, otherwise the bacteria will 

 multiply, and again an erroneous idea of their number be obtained. 

 When samples have to be taken for transport to the laboratory, these 

 are best collected 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 till both are cold, otherwise 

 it will be so tightly held as to make removal very difficult). In using 

 such a bottle it is best to immerse it in the water and then remove the 



