ACTION OF HEAT. 393 



that meta-phenylene-diamine gives very characteristic results, 

 if a small quantity of amyl alcohol be added to dissolve and 

 concentrate the light blue colouring-matter formed. Saul re- 

 commends " ortol," which is a mixture of quinol with ortho- 

 methyl-amino -phenol . 



Wilkinson and Peters have used benzidine, Lefhnann employing 

 it in the form of the hydrochloride. 



Near 100 C. calcium salts in small amount are deposited, and, 

 by keeping at this temperature for some time, slight oxidation 

 sets in, with the production of traces of formic acid and a marked 

 reduction of the rotatory power of the milk-sugar ; a brown 

 colour is produced at the same time. A deposition of salts, 

 and perhaps, also of albumin takes place on the fat globules, 

 which increases their mean density, causing them to rise slowly 

 to the surface, when the milk is afterwards cooled ; during 

 the heating the fat globules are expanded and may somewhat 

 coalesce. 



If the surface of the milk be exposed freely to the air, a skin 

 forms at temperatures exceeding 60 C. This has been stated 

 to consist of casein, but has not the properties of this substance ; 

 it is partly of a protein character, and there is some reason to 

 suppose that it is an oxidation product. It contains all the con- 

 stituents of the milk in a concentrated form. The taste and 

 smell of milk are changed by heating to above 70 C. 



It is not known how far the action of heat on milk affects its 

 digestive qualities. Milk which has been heated is curdled less 

 readily by rennet than fresh milk, but there are good grounds 

 for the view that this is due to a change in the distribution of the 

 calcium salts as well as possibly to a change in the casein. It 

 has been claimed that sterilised or boiled milk is more easy of 

 digestion than unboiled milk, but this, again, is possibly due to 

 the fact that it is not curdled so easily in the stomach, and does 

 not produce so firm a clot. There appears to be no evidence 

 that healthy adults digest boiled milk either more or less readily 

 than unboiled milk. In one respect boiled milk is less to be 

 preferred than fresh milk. From the evidence adduced by 

 Barlow, and since fully confirmed, it seems that children fed 

 exclusively on sterilised milk have a scorbutic tendency. It has 

 long been known that the absence of fresh food of any descrip- 

 tion is a predisposing cause of scurvy, but no substance has yet 

 been identified as the vitamine which confers anti-scorbutic 

 properties. 



It is of considerable importance to be able to distinguish 

 between fresh milk, on the one hand, and " pasteurised " or 

 " sterilised " milk, on the other. 



