100 ESTIMATION OF TOTAL SOLIDS AND ASH. 



Drying. The author has found that by evaporation in vacua 

 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 Raden- 

 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 

 which is far more satisfactory. 



It may sometimes be of importance to estimate the water 

 driven off, instead of deducing it from the difference 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 

 suspended in a beaker of water. This is connected with another 

 weighed U-tube filled with pumice moistened with strong sul- 

 phuric acid, and provided with a bulb in which the bulk of the 

 water can condense. A current of air (or preferably, hydrogen) 

 dried by sulphuric acid should be passed through the tubes, and 

 the water in the beaker boiled. After about three hours' heating, 

 the sulphuric acid tube should be removed, and, after cooling, 

 weighed. The increase of weight of the sulphuric acid tube will 

 give the weight of the water in the milk taken. 



It is not advisable to dry the total solids at temperatures 

 exceeding 100 C., as the decomposition of the residue by heat 

 is increased at higher temperatures. 



Drying Apparatus. For the drying of total solids the 

 following conditions may be laid down for the drying 

 apparatus : 



(1) The temperature must not exceed 100 C. (212 F.). 



(2) The moisture must be removed as soon as it is converted 

 into vapour. 



The usual form of water-oven consists of a water-jacketed 

 metal box with a door to it ; very little provision is made for 

 the removal of the moisture, as no current of air is allowed to 

 circulate through the whole of the interior. 



