3O Elements of Water Bacteriology. 



They should be cleaned thoroughly before using, by 

 treatment with sulphuric acid and potassium bichromate 

 or with alkaline permanganate of potash, followed by 

 sulphuric acid, dried by draining, and sterilized by dry 

 heat at 160 C. for at least one hour, or by steam at 115- 

 120 degrees for fifteen minutes. If not to be used immedi- 

 ately the neck and stopper should be protected against 

 dust or other contamination by wrapping with lead-foil. 

 For transportation the bottle should be enclosed in a 

 suitable case or box. 



The greatest care must be taken that the fingers do not 

 touch the inside of the neck of the bottle or the cone of 

 the stopper, as the water thereby would become seriously 

 contaminated and rendered unfit for examination. It is 

 well known that bacteria are found abundantly upon the 

 skin, and Winslow (Winslow, 1903) has shown that even 

 B. coli is present upon the hands in a considerable number 

 of cases. 



In order to obtain a fair sample, great precautions must 

 be taken, and these will vary with the different classes of 

 waters to be examined and with local conditions. If a 

 sample is to be taken from a tap, the water should be 

 allowed to flow at least five minutes (if from a tap in regu- 

 lar use) or for a longer period in case the water has been 

 standing in the house service system. In the small pipes, 

 changes in bacterial content are liable to occur, certain 

 species dying and others multiplying. 



If a sample is to be taken from a pump similar pre- 



