STUDY OF WATER. 427 



to be made from it ia the usual way ; the results will 

 often be a pure culture of some single organism, either 

 one of the intestinal variety or a closely allied species. 

 By a method analogous to the latter the spirillum of 

 Asiatic cholera has been isolated from water; and by 

 taking advantage of the effect of elevated temperature 

 upon the bacteria of water, Dr. Vaughan, of Michigan, 

 has succeeded in isolating from suspicious waters a 

 group of organisms very closely allied to the bacillus 

 of typhoid fever. 



The Quantitative Estimation of Bacteria in 

 Water. — Quantitative analysis requires more care 

 in the measurement of the exact volume of water em- 

 ployed, for the results are to be expressed in terms of 

 the number of individual organisms to a definite volume. 

 The necessity for making the plates at the place at 

 which the sample is collected is to be particularly 

 accentuated in this analysis, for the multipliciition of 

 the organisms during transit is so great that the results 

 of analyses made after the water has been in a vessel 

 for a day or two are often very different from those 

 that would have been obtained on the spot. 



Note. — Inoculate a tube containing about ten cubic 

 centimetres of sterilized distilled or tap water with a 

 very small quantity of a solid culture of some one of 

 the organisms with which you have been working, 

 taking care that none of the culture medium is intro- 

 duced into the water-tube and that the bacteria are 

 evenly distributed through it. Make plates at once, 

 and on each succeeding day, from this tube, and deter- 

 mine by counts whether there is an increase or diminu- 

 tion in the number of organisms — i. e., if they are 



