74 PUBLIC HEALTH CHEMISTRY 



for half an hour, and weigh. The increase in weight is the 

 amount of total solids in 2 c.c. If the specific gravity of 

 the milk is known, the weight of 2 c.c. is readily calculated, 

 and the percentage of total solids is then worked out. 

 Some analysts weigh 5 grm. of milk into the dish, and 

 thus avoid using the specific gravity. 



Ash. The dried solids are incinerated at as low a heat 

 as possible, and, on cooling, the dish is reweighed and the 

 percentage calculated. It averages in normal milks about 

 073, and should not fall below 07. Watering makes it 

 less. Test ash for effervescence ; if marked, suggests 

 addition of a carbonate. 



Fat. This is a very important determination. Several 

 methods. 



Werner-Schmidt Method. Ten c.c. of the milk are 

 pipetted into a Stokes' tube, and strong HC1 is added to the 

 20 c.c. mark. The mixture is now heated in the water- 

 bath, or carefully over a flame, until it turns a brown 

 colour. Now cool the tube and its contents in water, and 

 then add ether to the 50 c.c. mark. Cork the tube firmly, 

 and mix the contents well by inverting slowly ten to 

 twelve times. Set the tube down in an upright position to 

 allow the ether to separate and become clear. Pipette off 

 10 c.c. or 15 c.c. of the ether into a weighed platinum 

 capsule, or other dish, evaporate the ether on water-bath 

 at 6o C, dry at ioo C, cool and weigh. The increase of 

 weight is weight of fat dissolved in the amount of ether 

 taken by pipette. Now observe number of c.c. of ether 

 still in the tube, plus f of the fluffy layer, separating the 

 clear ether from the dark liquor. Calculate by proportion 

 amount of fat dissolved in this quantity of ether, and add 

 the result to former ; the sum gives the amount of fat 

 derived from 10 c.c. of milk sample. Express result as 

 weight of fat per 100 grm. of milk. To do this specific 

 gravity of sample must be observed. 



Example. Twenty c.c. of ether removed from tube 

 gave a residue of 0-286 grm. Ether still in tube (plus f 

 fluffy layer) 2-8 c.c. Therefore as 20 : 22-8 : : 0-286 : x 

 0-326 grm. of fat in 10 c.c. of milk sample. But specific 

 gravity of sample was 1030-5, hence 10 c.c. weigh 10-305 



