EXPERIMENT STATION BULLETINS. 339 



appearance throughout, which is due to one or more of the following 

 causes : — 



(a). Too weak acid. 



(b). Not enough acid, 



(c). Too low temperature of milk or acid. 



(d). Incomplete mixing of milk and acid. 



(e). Uneven or insuflicieut speed of tester, 



3. Grepish huhhics on the surface of the fat column, which are gen- 

 erally due to the presence of carbonates in the water added during test- 

 ing. This condition may be prevented by the addition of a few drops 

 of acid to the water previous to using which precipitates the carbonates. 



CLEANING TEST BOTTLES. 



It is very essential that the test bottles be well cleaned after being used. 

 If not, the test following is apt to be inaccurate, due to fat particles re- 

 maining in them. 



After the work is completed, the contents of the test bottles should be 

 emptied into some non-metallic vessel or an earthen jar, but never into 

 anything metallic because of the strong corrosive action of sulphuric 

 acid on nearly all metals. Then they are easily cleaned by washing in 

 a strong solution of some good washing powder and rinsing in clean, 

 hot water. They will become cloudy, however, if cleaned continuously 

 with this, so it is necessary about once in every five tests to use a solu- 

 tion of potassium bichromate. 



The potassium bichromate solution is prepared by dissolving potassium 

 bichromate crystals in warm Avater, making a saturated solution or one in 

 which the water has as much of the substance in solution as possible. To 

 this is added one-tenth of its volume of sulphuric acid, such as is used 

 in testing. In cleaning the test bottles about 10 cc. of the solution is 

 put into each one; the contents are then heated to a high temperature 

 by placing the test bottles in water which is near the boiling tempera- 

 ture; the test bottles are then shaken until absolutely clean, when the 

 solution is emptied into some receptacle for future use and the bottles 

 are rinsed with hot water ; during the rinsing a test bottle brush is used 

 to insure cleanliness of the bottle neck. 



The potassium bichromate solution may be used several times or until 

 its cleaning proijerties have been entirely used up. 



COMPOSITE SAMPLES. 



A mixture of samples of milk taken from day to day is known as a 

 composite sample. This is done to obviate the great amount of work 

 involved in making daily tests. A jar, usually a pint "Lightning" tin 

 top milk bottle, is provided and in it is placed a small sample of each 

 day's milk, whether it be that from a creamery patron or from the pro- 

 duct of individual cows. Into this is put some preservative to keep the 

 milk sweet up to the time of testing. The fat test gives the average 

 percentage of fat in the milk for the period covered by the mixture of 

 daily samples. 



