TOTAL SOLIDS. 99 



Haddon have observed that formic acid is produced by heating 

 milk to the temperature of boiling water, and Johnstone has 

 found that formic acid added to milk had an enormous influence 

 on the results. The results obtained by the estimation of total 

 solids by the evaporation of 1 gramme spread over a large sur- 

 face, from which the water was very quickly driven off, were 

 always slightly higher than when 5 grammes were used, and 

 evaporation was very much slower. 



In order to secure rapid evaporation, the milk has been spread 

 over a large surface by the use of sea sand and other granular 

 substances. Vieth has found that evaporation on sand gives 

 practically the same results as direct evaporation in a basin. 



Babcock's Method. Babcock has used asbestos as a medium 

 over which to spread the milk. The method as adopted by the 

 Association of Official Agricultural Analysts (of America) is 

 described elsewhere. The author has found Babcock's method 

 most satisfactory, and finds it convenient to operate as follows : 

 Place about 3 grammes of fine asbestos fibres in a small platinum 

 basin, and ignite strongly (preferably in a mufne). The asbestos 

 should be soaked in hydrochloric acid, and thoroughly washed 

 before use ; when ignited and shaken with water containing a 

 few drops of phenol- phthalein no red colour should be produced. 

 After weighing, add about 5 grammes of milk, and weigh again 

 as quickly as possible to the nearest milligramme. Place the basin 

 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. Adams 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. 



