INTERPRETATION OF RESULTS. 295 



not produced a few drops of ammonia ; let the precipitate settle, 

 and pour off the clear solution as completely as possible, and 

 collect the precipitate by centrifuging. Add this to a tube of 

 milk, pour a layer of melted vaseline on the surface, and heat to 

 80 C. (176 F.) for twenty minutes. Incubate at 40 for forty- 

 eight hours ; if B. sporogenes enteritidis be present, the milk is 

 curdled, and the dry-looking curd is nearly blown out of the tube. 



Interpretation of Results. The presence of B. coli communis in 

 1 c.c. or less of the water, especially if accompanied by the presence 

 of B. sporogenes enteritidis, is a very unfavourable sign, and points 

 to sewage contamination. B. coli communis in 10 c.c. is very 

 suspicious, whilst if this organism is present in 100 c.c., but not 

 in less water, an investigation of the supply should be under- 

 taken, but the water should not be condemned on this ground 

 alone. 



The presence of glucose fermenting organisms which do not 

 prove to be B. coli communis is not a very favourable sign, but 

 the other evidence should be considered carefully before con- 

 demning ; they are often present in surface waters. 



The colonies liquefying gelatine are usually those of putre- 

 factive organisms, and any great proportion is undesirable. The 

 number of organisms growing on gelatine varies greatly with 

 the source of the water. Water from deep wells should be 

 almost sterile and certainly should not give more than 100 colonies 

 per c.c. ; any number exceeding this may be taken as indicating 

 contamination with surface water. Surface waters which are 

 not contaminated may contain many more organisms, as many 

 as 2,000 per c.c., and often a large number of these (25 per cent.) 

 liquefy gelatine ; such waters are generally found to contain 

 organic matter derived from decaying leaves and other vegetable 

 matter. 



All waters giving evidence of organisms of intestinal origin, 

 and a number of organisms running into many thousands on 

 gelatine, may be condemned as unsatisfactory. 



By the combined information from inspection of the source, 

 chemical analysis and bacteriological examination, a usually 

 reliable opinion can be made of the purity of the water ; it is 

 even more reliable, if the data are compared with those obtained 

 on waters of known purity from the same district and of the 

 same character. 



For other methods of bacteriological examination and for the 

 separation and identification of individual species, works on 

 bacteriology must be consulted. 



