Determination of Bacteria in Water 255 



until steam 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 uniform results have followed. 



The method of Wright* was devised for the computation 

 of bacteria in suspensions used in making tests of the opsonic 

 power of the blood and for use as bacterio-vaccines. The 

 bacteria containing fluid is diluted as may be necessary 

 with an equal volume or 1:2, 1:5, 1:10, with a fluid of which 

 one volume is normal human blood, then spread upon a 

 slide, fixed and stained by Irishman's or some other appro- 

 priate method, placed upon the stage of the microscope, 

 and the number of blood-corpuscles and bacteria counted 

 in each of a number of fields. To facilitate the counting a 

 cross is ruled with a writing diamond, upon a circular cover- 

 glass, which is then dropped into the eye-piece. If there 

 are, for example, ten bacteria to each corpuscle in a 1:5 

 dilution, the calculation is based upon the number of cor- 

 puscles. There being 5,000,000 red corpuscles in i c.mm. of 

 normal blood, there should be 50,000,000 bacteria in i c.mm. 

 of the suspension, but as it has been diluted 1:5, there are 

 only ^ of this, or 10,000,000. 



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. 



Ice always contains bacteria if the water contained them 

 before it was frozen. In. Hudson River ice Prudden found 

 an average of 398 colonies in a cubic centimeter. 



A sample of water when collected for examination should 



be placed in a clean sterile bottle or in a hermetically sealed 



pre-sterilized glass bulb, and must be examined as soon as 



possible, as the bacteria multiply rapidly in water which is 



* "Lancet," July 5, 1902. 



