TOTAL SOLID.S. 77 



in an oven at 100° C. till the weigtt is constant. This method 

 has been adopted by the Society of Public Analysts. It is not, 

 however, possible to attain absolute constancy, as a decomposition 

 due to heating takes place, and the weight continually sUghtly 

 diminishes on further heating ; for this reason the weight i* 

 usually considered constant when less than 1 milligramme per 

 hour is lost on further drying. The method gives very good 

 results, and duplicates agree closely ; but it is doubtful whether 

 the results represent accurately the true total solids of the milk. 

 First, an absolutely constant weight is not attained ; and, next, 

 the residue is markedly brown, indicating decomposition ; the 

 point taken as constancy is really a point where it may be assumed 

 that the bulk of the water is driven off, and comparatively little 

 decomposition has taken place. 



The author has found by taking smaller quantities of milk 

 — about 1 gramme — and spreading this over a large surfaee 

 during evaporation, that a nearly white residue is obtained, .uhI 

 constancy of weight can be attained. It appear.s probable that 

 the decomposition to which the browning is due takes place 

 during the heating of the milk before evaporation is concluded. 

 In support of this view it may be noted that C'azeneuve and 

 Haddon have observed that formic aeiil is pidduced b>' ln'ating 

 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 bv the evaporation of 1 gramme spread over a large sur- 

 face, from which the water was veiy ((uickly driven off, were 

 always slightly higher than when 5 grammes were used, when 

 e\'aporation 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, \ieth has found that evaporation on sand gives 

 practically the same results as direct e\'aporation in a 

 basin. 



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

 for spreading the milk over. The method as adopted by the 

 Associatiim 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 follow-s : — 

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

 basin, and ignite strongly (preferably iu a muffle). The asbestos 

 should be soaked in hydrochloric acid, and thoroughly washed 

 before use ; when ignited and shaken with water containing a 

 few drops of phenolphthalein no red colour should be produced. 

 .\fter weighing, add about 5 grammes of milk, and again weigh 

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



