414 



MILK 



of the milk by the formation of air-bubbles which rise slowly 

 because of the viscosity of milk. After 

 dilution this objection, of course, does 

 not hold. 



A slight error is involved in the usual 

 method of diluting, which consists of fill- 

 ing flasks or bottles with water and then 

 sterilizing them. Some of the water 

 evaporates during sterihzation, and the 

 quantity lost is variable according to the 

 method of sterilization, the surface of the 

 water in the flask, and the width of the 

 mouth of the flask. This error can be 

 avoided by sterilizing a quantity of water 

 in a large container and measuring the 

 exact amount required for each dilution. 

 An apparatus for this purpose can easily 

 be designed by an experienced laboratory 

 worker. Freas has described a practical 

 method for distributing sterile water. 

 The water is sterilized in 2-gallon bottles 

 and is connected with the top of a buret 

 by means of a glass tube. Another glass 

 tube from the upper part is connected 

 with a water suction-pump, and at the 

 top is a cock which breaks up the effect 

 of suction when this is indicated. The 

 opening above this cock is filled with 

 cotton to filter the air that enters. At 

 the lower end of the buret is a glass bell 

 to protect the outflowing water from air 

 contamination (Fig. 183). Check plates 

 should always be made to test the ster- 

 ility of the water. 



The composition and reaction of the 

 culture-medium is usually considered of 

 great importance, although closer in- 

 vestigation of this subject is desirable. 

 Some authorities believe that the kind 

 and reaction of the medium are factors 

 of considerable influence on the results, 

 while others hold that they are neg- 

 ligible. Slack, for example, thinks that 

 meat infusion with 1 per cent, agar and a reaction of 1.5 per 

 cent, acid to phenolphthalein gives appreciably higher counts. 



Fig. 183. — Freas' appa- 

 ratus for accurately filling 

 dilution blanks: A, Cot- 

 ton filter; B, stop-cocks; 

 C, connection to suction 

 pump; D, connection to 

 sterile water reservior; E, 

 point of constant level; F, 

 first, 9 c.c. graduation; G, 

 eleventh, 9 c.c. graduation 

 (0-11-99 c.c); H, shield 

 to prevent dust contami- 

 nation. (American Jour- 

 nal of Public Health.) 



