TESTING MILK 327 



surface of the fluid and read off in the graduated neck of a 

 specially constructed glass bottle. 



For making the test 17.6 cubic centimeters of milk is 

 measured out with a marked pipette and put into a test 

 bottle. (See Fig. 77 for the different articles used in making 

 this test). Now 17.5 cubic centimeters of sulphuric acid is 

 measured out and poured into the test bottle in such a way 

 that it washes down any milk on the side of the neck and 

 runs under the milk at the bottom of the bottle. The sulphuric 

 acid curdles the milk at first, but the curdled part is soon 

 dissolved. The sulphuric acid used must have a specific 

 gravity of from 1.82 to 1.83. 



After the acid has been added, the test bottle is shaken by 

 hand with rotary motion. The bottle should be grasped by 

 the neck near the bulb, and care should be taken not to shake 

 any curd up into the neck. The mixing of the acid and milk 

 creates a good deal of heat, and the test bottle becomes quite 

 hot and the contents very black. When the shaking has gone 

 far enough the curdled portion has all dissolved, and the con- 

 tents are uniformly black with no fragments floating in it. 



The test bottle along with others is now set in the pockets 

 of the centrifuge and whirled at a speed of 900 to 1,000 revo- 

 lutions per minute. Most of the small centrifuges are run by 

 hand, while the large ones are operated by steam. The whirl- 

 ing is kept up steadily for five minutes. The revolving is then 

 stopped and each test bottle is filled to the neck with boiling 

 hot water. They are then whirled again for two minutes at 

 the same speed. The machine is again stopped and the bot- 

 tles filled with more hot water up to the 8 or 9 per cent. mark. 

 Again they are whirled for one minute. The fat has now all 

 collected into the neck of the bottle and stands out distinctly 



