APPENDIX 465 



mix and pour into the Petri dishes. Allow the gelatine to set ; and 

 incubate at 22° C. for as long as possible before complete liquefaction 

 occurs. Count the colonies which appear after forty-eight hours incuba- 

 tion (agar), take the average at the period of maximum growth (gelatine 

 4-5 days), multiply up according to the fraction of a c.c. which has been 

 used, and return as so many organisms per cubic centimetre, stating 

 medium, period and temperature of incubation, etc. It is advisable that 

 each quantity of water from which the fractional part is added to the 

 gelatine should be taken up separately, and not that 1 c.c. of water should 

 be taken up and the fractional amounts, say of 05, 0'2, and 04 c.c, be 

 added to the gelatine. If Koch's plates are used they should be allowed 



Fig. 46. — WolfMigel's Counter. 



to set on the levelling apparatus, then placed in the moist chamber for 

 incubation at 22° C, and the colonies counted by means of Wolf hligel's 

 counter (see Figs. 43, 44, and 45). 



{b) Qualitative Examination 



At the time of making the gelatine plates for quantitative examina- 

 tion, several agar, and litmus-lactose agar, plates may be made for quali- 

 tative purposes. The plates must be poured immediately after inoculation 

 of liquefied agar with small quantities of the water, as below 40° C. the 

 agar will resolidify. When poured, the agar plates should be placed on 

 cold stone or metal, and then incubated at blood-heat. On the second 

 or third day colonies will have appeared, and these should be studied 

 and sub-cultured (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, 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 porcelain filter, and pump or aspirate 

 through it 1000-2000 c.c. of the water under examination, and with a 



2 G 



