70 MICRO-ORGANISMS AND DISEASE [cHAP. 



waters — e.g. the London waters— there are suspended in the 

 water microscopic masses of organic ddbris loaded with 

 bacteria. If such a mass happens to be in the particu- 

 lar quantity of the water that is added to the gelatine, and 

 after shaking this up the bacterial mass is broken up the 

 resulting number of colonies in the plate may be greatly 

 in excess. 



(3) It ought to be remembered, that the number of 

 bacteria in water is liable to considerably increase as time 

 goes on, for some bacteria — special water bacteria — are 

 capable of living and multiplying in water ; they are capable 

 of thriving even on very small amounts of organic matter 

 present in the water. It is therefore necessary to make the 

 plate-cultivations as early as possible after the water is taken 

 from its stock. It is not necessary to do this on the spot, 

 if the place of work is within a comparatively short distance 

 say if the water can be delivered in the laboratory within a 

 few hours after ; but if water is sent from long distances, 

 when it has to travel many hours by rail in the summer 

 months, then the actual number of bacteria present at 

 starting is not to be measured by that found in plates made 

 say twenty-four hours after. In the cold weather and if the 

 sample of water sent is kept in a cool place, the multipHca- 

 tion in twenty-four hours is not very great. To obviate these 

 errors when water is sent from long distances, it is advisable 

 to keep the sample packed in ice or in cotton wool in a 

 cool place. It is obvious that the bottles in which the water 

 is received and sent should be sterile ; glass stoppered, 

 narrow necked, about two- to four-ounce bottles, sterilised in 

 the hot chamber, are best for this purpose — it does not 

 require to fill the water in vacuum tubes. From a large 

 experience I found that glass-stoppered bottles first well 

 washed out with nitric acid or methylated spirit, then twice 



