AMERICAN ASSOCIATION TEST 103 



neck and the bottle is again whirled for one or two minutes. 

 The bottle is then placed in hot water at a temperature of 130- 

 140 F., after which the reading is taken. It is advisable to use a 

 pair of dividers to measure the fat column. Under this method 

 the reading will be very low possibly .1 per cent to .2 per cent 

 when it should be .4 or .5 per cent. 



A More Rigorous Test. A modification of the foregoing is to 

 take a Q-gram sample by means of a 9 c.c. pipette, that is, a 

 half sample. To this is added about 1 2 c.c. of a fairly strong acid, 

 well up to a sp. gr. of 1.83. The whirling is done in a very high- 

 speed machine and is greatly prolonged fifteen to twenty min- 

 utes the first time, about ten minutes at the second whirling, and 

 about five minutes at the third whirling, or thirty to thirty-five 

 minutes in all. The reading must be doubled, since only a g-gram 

 or half sample is taken to a test. This method will probably 

 double, or more than double, the result. But even under 

 this method the test falls considerably below the results secured 

 by chemical analysis. 



The American Association Test. A close study of the dif- 

 ferent practical methods for the quick determination of the per 

 cent of fat in milk and its products and by-products reveals 

 the fact that there is a general principle running through them 

 all, that is, there are three factors which operate in all of them. 

 These are (i) the use of one or more chemicals to liberate the 

 fat, (2) the heating of the contents of the test bottle in order 

 to liquefy the fat, and (3) the application of centrifugal force. 

 Where sulphuric acid is the chief or only chemical used it gen- 

 erates sufficient heat, through its strong affinity for water; but 

 where it is not used at all or is used very sparingly it becomes 

 necessary to heat the sample in hot water before centrifuging it. 



A number of useful tests have been devised for the quick 

 determination of the per cent of fat in milk, etc. The following 

 are a few of the most outstanding of these: 



(i) The Lactocrite, devised in 1886 by Dr. De Laval, inventor 

 of the cream separator which bears his name. The chemical 

 used was concentrated or glacial acetic acid, containing 5 per 

 cent of concentrated sulphuric acid. It was necessary to heat 



