V] METHODS OF INOCULATION 69 



determine the number of bacteria by means of gelatine 

 plates. For this purpose a definite — i.e. measured — small 

 quantity,! -jV, tV to i cc. (according to the turbidity) of 

 the water after shaking, is added to a gelatine tube ; this is 

 melted in warm water and then poured out into a sterile 

 plate-dish. This gelatine is allowed to set, and after 

 incubation at 20° C. for three or four days the number of 

 colonies that have sprung up are counted, and according to 

 the quantity of the water that has been added to the 

 gelatine for plate-cultivation the number is calculated per 

 I cc. Several points have to be remembered in making 

 an estimate that is to be approximately correct, (i) There 

 ought to be always two plates made, and the number ought 

 to be determined by the average. (2) The sample of water 

 to be tested ought to be well shaken up before withdrawing 

 the required quantity for the plates, in order to make as 

 uniform a distribution of the bacteria in the water as 

 possible. But notwithstanding this sometimes enormous 

 differences will be found in two plates made from different 

 portions ; this is probably due to the fact that in some 



^ For measuring definite small quantities of water or any other fluids, 

 I use a series of glass pipettes on the plan of those added to a h*mo- 

 cytometer : 5 cmm. (if^j), 10 cmm. {xiu), 20 cmm. (^Jj), 50 cmni. (t^), 

 100 cmm. (yV), and 250 cmm. (J), J cc. and I cc. ; each of these 

 pipettes can be fitted with an india-rabber tabe with porcelain or glass 

 mouthpiece ; these latter receive a plug of sterile cotton w ool. The 

 pipettes are sterilised, and when ready for use the tube is fitted on ; of 

 the fluid a little is poured into a sterile watchglass, and of this the re- 

 quired quantity is withdrawn and blown out into the test-tube containing 

 the culture-medium. If a quantity smaller than ^^^ cc. is required a 

 dilution is previously made with a detinite quantity of sterile distilled 

 water ; for instance, if Ttnnr cc. of a given fluid is required, I take 5 cmm. 

 — i.e. ^^ of the fluid — and add this to 5 cc. of sterile distilled water, 

 each I cc. of this would contain I cmm. or xcVtj cc. 



The pipettes can be sterilised either in the liot air-chamber, which is 

 best, or by letting them lie for an hour or so in disinfecting fluid, then 

 empty them, and wash them thoroughly out twice or more times in 

 distilled water. 



