2o6 PHARMACETITICAL BACTERIOLOGY 



set aside for six hours. Pour off the supernatant liquid and examine 

 the small amount of sedimentary residue microscopically. In order to 

 reduce to a minimum the growth of the contaminating organisms during 

 the time interval above indicated, the containing v^sels should be placed 

 in an ice chest. The method is satisfactory as far as the estimation of 

 inorganic and dead organic water contamination are concerned, but is 

 very unsatisfactory as far as the living contaminations are concerned. 



b. MacDonald Gravitation Method. — Into a vessel containing one to 

 two liters of water, place a watch crystal and let stand for twenty-four 

 hours. Siphon off the water, carefully remove the watch crystal with the 

 sediment and examine microscopically. The deposit in the watch crystal 

 represents the amount of material derived from a volume of water equal 

 to the diameter of the watch crystal times the height of the water column 

 in the vessel. 



c. Kean Sand Filtration Method. — Run loo cc. of water through a 

 funnel with a sand plug (clean fine quartz sand) . Wash the sand carrying 

 the plankton into a watch crystal by means of i cc. of water and examine 

 microscopically. 



d. The Sedgwick-Raf ter Method.- — This is a further development of the 

 Kean method. and has been quite extensively employed in the United 

 States. Sand of definite fineness is to be used and 250 cc. of the water are to 

 run through the filter. A specially constructed counting chamber (50 by 

 20 by I mm., hence holding i cc. of the sediment) is to be used and the 

 counting is to be done by means of a specially ruled micrometer scale 

 and a % inch objective. Bacterial counting cannot be done according to 

 this method, neither can the smaller algae and protozoa be accurately 

 counted. 



e. The Dibdin Double Filtration Method. — Run i liter of water 

 through filter paper. 'Wash residue into a clay filter made by plugging 

 the drawn out end of a piece of combustion tubing with a mixture of baked 

 clay and kieselguhr . Filtration is accomplished by means of suction. The 

 sediment (plankton) forms a compact cylindrical layer in the tube and can 

 be measured and the amount of the sediment per liter of the water stated. 

 The sedimentary plug can then be removed and examined microscopically. 

 This method hkewise precludes the estimation of bacteria and also some 

 other very minute organisms. 



f. Centrifugal Method. — Centrifuge ten to fifty cc. quantities of the 

 water to be examined at a high rate of speed, so as to throw down aU matter 

 in suspension, inclusive of bacteria. Decant or pipette off all but about 

 I cc. of the water, make up to two or three cc. by adding filtered distilled 

 water, shake or mix thoroughly and make the counts by means of a suitable 

 counting chamber. Should actively motile organisms such as paramecia, 



