EXAMINATION OF WATER 707 



AMOUNTS TO BE PLATED, SIZE OF DISHES, ETC. Gelatin. 

 For an ordinary water amounts of O'l, 0'2 and 0*3 c.c. 

 may be plated in Petri dishes of about 10 cm. diameter, 

 preferably done in duplicate. 



Agar. Two plates may be made with 0-1 and 0-2- 

 0-3 c.c., and are preferably duplicated. 



The desired volume of water should be run into the sterile 

 Petri dish by means of a sterile 1 c.c. pipette graduated in hun- 

 dredths. The tubes of gelatin should be melted in a water-bath 

 at a low temperature (40 C.). A tube is taken from the water- 

 bath, wiped to prevent the adherent water running down into the 

 Petri dish, its mouth is singed in the Bunsen flame to sterilise it, 

 and the contents are then quickly poured into the dish and mixed 

 with the water by tilting the dish several times. 



The agar tubes must first be boiled, then cooled to about 45 C., 

 and similarly treated, or surface plates may be made. 



If waters are constantly being examined, it saves trouble to 

 have the gelatin and agar in small flasks or bottles, 30-60 c.c. of 

 the former and 20-40 c.c. of the latter ; a flask or bottle of each 

 will then be used for an examination. 



The amount of the medium in a plate should be 10 c.c. 



In dealing with a river water, and in cases of doubt, 

 additional plates should be prepared with a ten- or 

 hundred-fold dilution of the ''water (made with sterilised 

 tap-water), according to circumstances. 



The counting is done with the naked eye, preferably in 

 daylight, any doubtful colony being determined with the 

 aid of a lens or low power objective. The number of 

 liquefying colonies in the gelatin plates should also be 

 noted. The plates should be inspected daily, in order 

 that the count may be made earlier should liquefaction 

 render this necessary. 



In examining an ordinary drinking-water there is no need 

 ever to dilute. As 1,000 or 1,500 colonies can be counted on a 

 plate, and if the number on a plate should be, owing to crowding, 



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