QUANTITATIVE ESTIMATION OF GERMS. 77 



a clean cover-glass that is free of all grease, forming a 

 film as thin and even as possible. It is dried without 

 the use of a flame. The cover-glass is then placed in 

 ether for a few minutes, b\' which process the fat is ex- 

 tracted. It is then taken out and stained in the usual 

 manner. C. Arens combines the two processes of fat- 

 extraction and staining by placing the cover-glass prepa- 

 ration in a chloroform-methyleneblue solution, of the 

 following composition: 12-15 drops of a saturated alco- 

 holic solution of methyleneblue and 4 c. c. of chloro- 

 form. It is stained from 4 to 6 minutes and washed. 



This simple method is not always successful. The 

 various modifications will be discussed below. 



II. QUANTITATIVE ESTIMATION OF GERAIS. 



For estimating the number of germs contained in milk 

 the gelatine-plate method is employed, using either Koch 

 or Petri dishes. After a stated time the developed colo- 

 nies are counted with the aid of a counting apparatus. 

 Wolffhugl's counting apparatus is a very convenient de- 

 vice where the Koch plates are enip!o)ed. .\s milk usu- 

 ally contains a great number of germs the sample must 

 be properly diluted ; more in sunmier than in winter. Of 

 milk, not rich in germs, i c. c. is diluted with 9 c. c. of 

 sterilized water and i c. c, equal to o.i of this mixture, is 

 then inoculated into 10 c. c. of nutrient gelatine. It is 

 customary to estimate the number in i c. c. The estima- 

 tion of the number of bacteria may serve as an indication 

 of the quality of the so-called sterile milk i^i commerce. 

 Milk is seldom rendered germ-free, for by so doing seri- 

 ous chemical changes very often take place efTecting the 

 appearance, taste, agreeableness and digestibility. 



When the plate culture method is used in isolating 

 and growing the difTerent bacteria contained in the milk, 

 dilutions of the original inoculated tubes must be made 

 in the usual wav. 



