78 ANALYSIS OF MILK. 



for an hour or two on a water-bath, and dry in a water-oven till 

 constant in weight. 



The residue thus obtained shows no signs of browning, and a 

 ■constant weight, which shows no appreciable change on pro- 

 longed drying,, can be obtained. The "total solids" by this 

 method are somewhat hygroscopic, and care must be exercised 

 in weighing. 



Macfarlane's Method. — Macfarlane uses " chrysotile " or 

 Canadian asbestos for this purpose ; this substance cannot be 

 ignited, being a hydrated mineral, and on treatment with water 

 affords a very sensible amount of soluble alkali ; this causes a 

 loss of weight owing to its action on the milk, and for this reason 

 chrysotile is not so satisfactory as the Italian asbestos. The 

 residue obtained by drying on " chrysotile " is very distinctly 

 brown, and the results are much lower than those given by other 

 methods. 



Adams' Method. ^Adanis uses a paper coil, which is first 

 dried at 100° C. and weighed, for the estimation of total solids. 

 The results thus obtained are frequently low, owing probably 

 to the presence of alkaline salts. 



Duclaux has proposed the use of sponge, and Ganntner of 

 wood fibre ; but these substances have never come into general 

 use. 



Drying. — The author has found that by evaporation in vacuo 

 ■over sulphuric acid good results are obtained if the milk be 

 spread on blotting-paper or on asbestos ; the result is slightly 

 higher than when the drying is performed at 100° C. 



In order to shorten the time of drying, Gerber and Eaden- 

 hausen experimented with acetic acid and alcohol. They found 

 that by coagulating the casein with these substances a skin no 

 longer formed on heating, and the time of evaporation was 

 materially shortened. For this reason the use of acetic acid, or 

 alcohol, or a mixture of the two, has been largely adopted for 

 the estimation of total solids. A much greater browning of the 

 total solids takes place, and constant results are quite impossible 

 of attainment when acetic acid or alcohol is used ; by a some- 

 what close adherence to arbitrary conditions as to time of drying 

 very fair results may, however, be obtained in this way in a 

 short time, but the method cannot be recommended where 

 accuracy is of importance. Revis proposes the use of acetone. 



It may sometimes be of importance to estimate the water 

 driven ofi, instead of deducing it from the diSerence between 

 the percentage of total solids and 100. To accomplish this, 

 about 4 grammes of asbestos should be placed in a U-tube, and, 

 after drying by passing a current of dry air, this should be weighed. 

 About 5 grammes of milk should be weighed in, and the U-tube 



