352 CHEMICAL TESTS AND ANALYSES 



again at full speed for two minutes, fill the test bottles to near 

 the top of the graduation with hot water. Whirl in the centrifuge 

 for one minute. Now set the test bottles in the water bath at 

 135 degrees F. for five minutes. The test is now ready to be 

 read. The figures on the test bottles represent per cent. In the 

 case of the 8 per cent standard milk test bottle the sub-divisions 

 represent tenths per cent. Read from the bottom of the lower 

 curve to the top of upper curve of the fat column, including the 

 meniscus in the reading. 



Gravimetric Method Paper Coil. 



"Make coils of thick filter paper, cut into strips 6.25 by 62.5 

 c.m., and thoroughly extract with ether and alcohol, or correct 

 the weight of the extract by a constant obtained for the paper. 

 From a weighing bottle or weighing pipette, transfer about 5 

 grams of milk to the coil, care being taken to keep the end of 

 the coil held in the fingers, dry. Dry the coil, dry end down, on 

 a piece of glass at the temperature of boiling water; transfer to 

 an extraction apparatus and extract with absolute ether or pe- 

 troleum ether boiling at about 45 degrees C. ; dry the extracted 

 fat and weigh." 



Roese-Gottlieb Method. 



" Weigh 10-11 grams of the milk into a Rohrig tube or some 

 similar apparatus, add 1.25 c.c. of concentrated ammonium hy- 

 droxid (2 c.c. if the sample is sour) and mix thoroughly. Add 

 10 c.c. of 95 per cent alcohol by volume and mix well. Then add 

 25 c.c. of washed ether and shake vigorously for thirty seconds, 

 then 25 c.c. of petroleum ether (redistilled slowly at a tempera- 

 ture below 60 degrees C.) and shake again for thirty seconds. 

 Let stand twenty minutes, or until the upper liquid is practically 

 clear. Draw of? as much as possible of the ether-fat solution 

 (usually 0.5-0.8 c.c. will be left) into a weighed flask through a 

 small quick-acting filter. The flask should always be weighed 

 with a similar one as a counterpoise. Re-extract the liquid re- 

 maining in the tube, this time with only 15 c.c. of each ether, 

 shake vigorously thirty seconds with each and allow to settle. 

 Draw off the clear solution through the small filter into the same 

 flask as before and wash the tip of spigot, the funnel and the 

 filter with a few c.c. of a mixture of the two ethers in equal parts. 



