82 SCIENCE AND PRACTICE OF DAIRYING. 



If in the same sample of milk two determinations of the total solids be 

 carried out, it is quite possible that, despite the greatest care, a difference 

 of plus or minus -15 per cent may be obtained. This difference may be 

 chiefly ascribed to the peculiar behaviour of the dissolved milk-sugar when 

 being dried, as has been already described in paragraph 7. The experi- 

 mental errors in the determinations of the total solids may therefore 

 amount to plus or minus *15 per cent. 



If the exact percentage of fat and the specific gravity of milk be 

 obtained, the percentage of total solids can be calculated from the formula 

 given in 11. The correctness of this determination is as great or greater 

 than the indirect determination, and can be used in corroboration. 



Determination of the Percentage of Fat. For this purpose the residue 

 obtained in the determination of the total solids can be utilized. It is 

 better, however, to weigh out 10 to 12 grams of milk in the way pre- 

 viously described, using a roomy porcelain dish, about 10 centimetres 

 in diameter, with as much sand as will perfectly absorb the milk, and 

 then to place this on the water-bath. In order to prevent the milk from 

 sticking firmly to the porcelain basin, it should be stirred with a small 

 sharp -edged glass stirrer. As soon as the mass shows a tendency to 

 become cohesive, the whole should be stirred and all the little lumps 

 broken up before they become hard, so that eventually one obtains a 

 uniform coarse powder. If this does not become baked to the slightest 

 extent after remaining 15 minutes undisturbed in the water-bath, it is 

 rubbed with a small porcelain pestle, which is allowed to stand in the 

 middle of the basin. It is retained 15 minutes longer in the water-bath; 

 the powder is then carefully removed, every single particle being cleared 

 from the vessel on to a Swedish filter -paper which contains no fat, 

 shaped in the form of a cylinder, and resting on glazed paper. It is then 

 introduced into the tube of a Soxhlet fat - extraction apparatus. The 

 paper cylinder is made by wrapping a piece of filter-paper cut at right 

 angles twice round a wooden cylinder, the diameter of which is about 

 4 mm. less than the diameter of the extraction tube, and then placing 

 on the level surface of the wooden cylinder a piece of paper of similar 

 diameter to the roll, bending this, and smoothing down the surface as 

 one would close a packet. It is unnecessary to use a plug of cotton wool 

 under the coil in the extraction apparatus. It is better to place some 

 cotton wool, free from fat, above the coil, to prevent any washing out 

 of the powder by the falling drops of the ether. In order to prevent 

 the opening of the syphon at the base of the extraction cylinder from 

 being closed by the coil, a ring made out of a strip of pure tin 3 to 4 mm. 

 broad is used. The upper surface of the cylinder should be at least 

 3 mm. under the highest point of the syphon bend of the extraction 



