Q U A LIT A TIVE EX AM IN A TION OF WA TER \ 1 3 



be taken up and the fractional amounts say of 0-5, a 2, and 01 c.c. 

 be added to the gelatine. 



{V) Qualitative Examination 



At the time of making the gelatine plates for quantitative 

 examination several agar plates may be made for qualitative 

 purposes. The plates must be poured immediately after inocula- 

 tion of liquefied agar with small quantities of the water, as below 

 40'' C. the agar will re-solidify. When poured the agar plates should 

 be placed on cold stone or metal until the medium has hardened 

 and then incubated at blood heat On the second or third day 

 colonies will have appeared, and these should be studied and 

 subcultured (as pure cultures) on suitable media. 



Valuable facts as to the quality of the water may also be 

 obtained from an examination of the five gelatine plates above 

 referred to, particularly in respect of the liquefying organisms, 

 which should be counted as carefully as any other colonies, and 

 noted separately as well as in the total number of colonies present 

 But in addition to the facts obtained from gelatine and agar plates, 

 other methods must be adopted in order to obtain information 

 respecting the quality of the water. 



Take a sterilised Berkefeld filter, and pump or aspirate 

 through it a litre (1000 c.c.) of the water under examination, and 

 with a sterilised brush transfer the particulate matter which has 



filter wells showing an increase in the number of microbes. In our last Report 

 we pointed out that this would be the inevitable result in the event of an in- 

 creased rainfall." 



A similar report is made monthly and it was found, as the above table shows, 

 that the conditions improved in February, March, and April, but owing to 

 the heavy rains and flood pollution the numbers of bacteria were greatly 

 increased in June. These figures throw an interesting light on the bacterio- 

 logy of water. — See Monthly Report under the Metropolis Water Act^ 1871. 



Another illustration may be quoted. Out of thirty-eight streams entering 

 Lake Vymwy which were bacteriologically examined only one yielded any B. 

 call; many land drains were practically free from organisms except when 

 recei\'ing pollution from neighbouring houses ; the River Severn contained 5000 

 organisms per c.c. two miles above Shrewsbury', but 23,000 per c.c. three miles 

 below. — Second Report of Royal Commission on Sewage Disposal, 1902, p. 99. 



There is- no fixed standard as to how many organisms potable water may 

 contain. It is customary to say that waters containing under 100 bacteria 

 p>er c.c. are potable ; but the figiare is, in fact, altogether arbitrary*, and 

 dependent also upon the kinds of bacteria present. In relation to the 

 milk supply the chief importance of water examination is to detect sewage 

 and surface pollution {B. coli) and not to estimate merely the number of 

 bacteria. 



H 



