ESTIMATION OF FAT — WERNER-SCHMID METHOD. 



117 



The author prefers to add to the ethereal solution an equal 

 bulk of petroleum ether, and wash with water containing just 

 sufficient ammonia to neutralise the free acid. 



Stokes later introduced a new form of tube, in which the 

 middle portion is narrowed for greater accuracy of measurement 

 of the ethereal layer (Fig. 9). 



Yarrow points out that it is absolutely necessary when using 

 Stokes' tubes to read the volume before and after pipetting off 

 a known volume at the same temperature. He has observed 

 that a tube which after pipetting showed 2'1 c.c. read, after 

 30 minutes and at a higher temperature, .3"4 c.c. — a very serious 

 difference. 



Allen proposes to di-aw off as much ether as possible, to add 

 a further supply, to di-aw that off as completely as could be done, 

 and to continue washing in this way till all the fat was separated 

 from the aqueous layer. 



!\Iijliiiari and Stokes have both described forms 

 of apparatus in which the ethereal solution can be 

 completely removed from the aqueous layer without 

 the necessity for pipetting it off. 



Stoki's apparatus has tlio. advantage that tlic 

 globules of etlirr and water during separation have 

 but a short distance to travel, and the separation is 

 complete In a much shorter time than in the longer 

 tube, which was not deviseil for the purpose of 

 complete extraction where rapidity of separation is 

 important. 



The author prefers to dilute the milk with an 

 equal bulk of water before heating with hydro- 

 chloric acid, as there is then no tendency for the 

 formation of a fluffy-looking la}-er at the point of 

 junction. He finds that it is necessary to wait for ten minutes 

 at least after the ether has apparently separated from the aqueous 

 layer, in order to allow the tine globules of water to settle out of 

 the ether. 



Analysts who have used this method are generally agreed that 

 it gives results practically identical with that of Adams ; it is 

 a question whether the drawing oft' of an aliquot portion of 

 the ether is to be preferred to the extraction of tlie whole of the 

 fat. In the first case, there is a tendency to be slightly low, 

 owing to the fact that the ether, which dissolves in the aqueous 

 portion, retains a minute proportion of fat; in the other, the 

 tentlency is to be somewhat high, as the water which dissolves 

 in the ether dissolves a small amount of substances other than 

 fat , iu either case, however, the error introduced is very small 

 and may usually be neglected. 



Fi'j,. 9. 

 Stokes' Tube. 



