238 Bacteriology of Water 



It is best to count all the colonies developed upon the culture, if 

 possible; but when hundreds of thousands are scattered over it, an 

 estimate made by counting and averaging the number in each of the 

 small squares of some counting apparatus, such as those devised by 

 Wolfhiigel, Esmarch, or Frost. In counting the colonies a lens is 

 indispensable. 



In some cases, where bacteria are exceedingly numerous, as badly 

 contaminated waters, in the study of sewage, in inflammatory exu- 

 dates, and in cultures intended for the preparation of bacterial vac- 

 cines, it is expedient to directly enumerate the bacteria without 

 resorting to the cultivation method, .^^5^ 



where all of the organisms may not grow. ^^^^ 



Excellent methods for the computa- ^l^ ..UtJ-^^Sl^-^str—A 

 tion of bacteria have been devised. That ffl^ Wr^''Br^'^ 



of Winslow and Willcomb* being as ^^^^^^^^ 



"The cover-slips should be boiled in a 10 l^ yj a 



per cent, solution of potassium bichromate in ^^^S 



SO per cent, sulphuric acid and allowed to lie jiffl® 



in this cleansing mixture. Just before using p Ib 



they may be rinsed in 50 per cent, alcohol ||| I 



and dried on a silk cloth, not in the flame. ^li||L ____ 



One-twentieth of a cubic centimeter of water ^E^^^^^^P 



placed on such a cover-slip spreads evenly and f|^^^^3^^^ 



should be allowed to dry in the air without jmim'-^ 



sudden heating. After drying it is fixed by Fig. 84. — Esmarch's instru- 



passing through the flame, covered with Ziehl- ment for counting colonies of 



Neelson's carbol-fuchsin, warmed until steam bacteria in Esmarch tubes, 

 just rises, washed, dried, and mounted. For 



counting the bacteria we use a Sedgwick-Rafter eye-piece micrometer, made 



for the study of the larger micro-organisms in drinking water." Very uni- 

 form results have followed. 



The method of Wrightf was devised for the computation of bac- 

 teria in suspensions used in making tests of the opsonic power of the 

 blood and is given in the chapter upon "The Opsonic Index." 



The majority of the water bacteria rapidly liquefy gelatin, on 

 which account it is better to employ both gelatin and agar-agar in 

 making the cultures. 



In ordinary city hydrant-water the bacteria number from 2 to 

 50 per cubic centimeter; in good pump- water, 100 to 500; in filtered 

 water from rivers, according to Giinther, 50 to 200; in unfiltered river- 

 water, 6000 to 20,000. According to the pollution of the water the 

 number may reach as many as 50,000,000. 



The waters of wells and springs are dependent for their purity 

 upon the character of the earth or rock through which they filter, 

 and the waters of deep wells are much more pure than those of shallow 

 wells, unless contamination take place from the surface of the 

 ground. 



* "Jour, of Infectious Diseases," Supplement, May, 1905, No. i, p. 273- 

 t "Lancet," July 5, 1902. 



