SPECIFIC GRAVITY 69 



several times with absolute ether. The ether so used is poured 

 over the coil and cone, previously placed in the extraction 

 apparatus, and, after the volume of solvent has been increased, 

 the apparatus is connected with a condenser and heated in a 

 water bath at about 45 C. After four or five hours extraction 

 the ether is distilled off and the fat dried to constant weight. 

 The removal of the ether is facilitated by drawing a current of 

 air through the flask by means of a vacuum pump. It is nec- 

 essary that the ether used in this process should be perfectly 

 dry, as otherwise small quantities of milk sugar and salts are 

 extracted with the fat. 



This is the official method of the Society of Public Analysts 

 of Great Britain and one of the official methods of the Amer- 

 ican Official Association of Agricultural Chemists. 



Total Solids. These may be determined either directly 

 by drying to constant weight or indirectly by calculation from 

 the fat content and the specific gravity. 



Direct Method. Five grams of milk are weighed into a 

 shallow platinum or quartz dish and after all visible liquid has 

 been driven off on the water bath, the dish and contents are 

 dried to constant weight in a steam oven. Ignited sand or 

 asbestos may be used to facilitate the drying process. 



Ash. The residue from the determination of the total 

 solids may be ignited at a low temperature until white and the 

 residue weighed, or a fresh portion of 20 c.cms. evaporated 

 with the addition of 6 c.cms. of nitric acid, and ignited until 

 free from carbon at a temperature just below redness. The 

 former method is the more convenient and the latter the more 

 accurate one. 



Specific Gravity. This is determined either by a lac- 

 tometer, a Westphal balance, or the ordinary specific gravity 

 bottle. The lactometer method is the simplest and quickest, 

 but, owing to the comparatively short space occupied by each 

 graduation (usually 1) and the opalescence of the liquid the 

 degree of accuracy obtained is low. 



The gravity is usually expressed as the excess weight of 



