BACTERIA IN THE INDUSTRIES 207 



interfere with the counting, they may be killed by adding a drop or two of 

 ether or chloroform, to the centrifuged material. For the purpose of 

 examining and comparing a number of water samples at one operation, 

 the Stewart-Slack centrifuge head will be found very convenient and time 

 saving. 



2. Milk Analysis. There are three methods of direct examination 

 of milk in use. Two of these require the use of the centrifuge. The 

 direct microscopical examination of milk will convey information as to 

 quality and purity which cannot be obtained through any of the chemical 

 methods. 



a. The Stewart Slack-Method. Two cc. of milk are placed into glass 

 tubes which are then closed at both ends by means of suitable rubber 

 stoppers. The special centrifuge heads made according to the specifica- 

 tions of Dr. Stewart of the Philadelphia Board of Health will hold twelve 

 such tubes. The tubes are then centrifuged at 2,000 to 3,000 revolutions 

 per minute, for ten minutes. The sediment which has become more or less 

 fixed upon the stopper in the lower end of the tube is mixed with a drop 

 or two of water and smeared upon a slide, so as to cover about four square 

 centimeters of space, allowed to dry and stained with methylene blue. 

 The examination is made with the oil immersion and bacteria, pus cells 

 and epithelial cells counted. 



b. The Doane-Buckley Method. Ten cc. of the milk is centrifuged 

 for four minutes at a speed of 2,000 revolutions per minute. The fat is 

 carefully removed; centrifuged for one minute more and the fat again 

 carefully wiped away by means of a small cotton swab. Pipette off the 

 supernatant fat free milk and mix the sediment with two drops of a satu- 

 rated alcoholic solution of methylene blue. Let the stain act for a few 

 minutes, assisted by warmth. Make up to the i cc. mark of the tube by 

 adding water. Examine for body cells by means of the hemacytometer. 



c. The Prescott-Breed Method. Moo cc ' f a thoroughly mixed 

 sample of the milk is spread on a slide, within the ruled lines of just 

 i square cm. The >foo cc - of milk is measured by means of a special 

 graduated capillary tube. Both the pipettes and the ruled slides are to 

 be had in the market. However, both can be made in the laboratory by 

 anyone with ordinary mechanical skill. The slide with the milk smear 

 is set aside to air dry, fixed with alcohol and then stained with methylene 

 blue or some other blood stain. The counting is done with the oil immer- 

 sion, after having determined the area covered by the field of view. 

 Estimations are made of the number of bacteria and body cells per cc. 

 of the milk. 



3. Examination of Tomato Products. The industry of canning 

 tomatoes has reached enormous proportions in the United States within 



