EXAMINATION OF AIR, SOIL, AND WATER. 149 



may be said that water containing less than 100 bacteria to the 

 cubic centimetre is very pure water. Water containing 1,000 or 

 more should be filtered. Water containing 100,000 to 1,000,000 

 is contaminated with surface water or sewage. It is necessary to 

 bear in mind that in typhoid fever and Asiatic cholera the excreta 

 contain the bacteria in great numbers, and wells and streams 

 receiving surface water may be contaminated in various ways. 

 The cholera bacillus dies as a rule quickly in distilled water, while 

 it preserves its vitality for a long time in water of a bad quality. 



It is necessary to lay stress upon the fact that a bacterio- 

 logical analysis may show the presence of pathogenic bacteria when 

 their detection is not possible by any other means. They may 

 be present in water in such small numbers that no chemical 

 analysis would detect any contamination, but as they are living 

 organisms capable of increasing in a suitable environment, they can 

 readily be discovered by bacteriological methods. 



The examination of rain water, drinking water, tap water, sea 

 water, various liquids and infusions, by these methods, opens up 

 a wide field for research. Pettenkofer has shown that impregna- 

 tion of water containing many bacteria with carbonic acid diminishes 

 the number of the latter. The examination of waters before and 

 after filtration, or after addition of chemical substances, are matters 

 which require further investigation, though a great deal of work has 

 already been accomplished. The reader will find in Micro- organisms 

 in Water by P. and G. Frankland, a very complete account of this 

 subject with valuable analytical tables. 



