75 
1895 - 96 .] Mr D. F. Harris on Chemistry of Milk. 
they are pure caseinogen, but that they are fat-particles weighted 
down or loaded by caseinogen till they are of exactly the same 
specific gravity as milk-plasma, from which, therefore, they never 
rise, nor can be separated by centrifugalisation. 
If they were pure caseinogen, skimmed milk should contain no 
fat ; but it does contain fat ; its particles stain with osmic acid like 
full milk. In any case, if any of these particles were pure or “ naked” 
fat, there would be nothing to prevent their coalescing, the milk 
would not be the emulsion it is. Thus skimmed milk gives very 
little fat compared with cream (into which all the large globules 
have risen), because all the very smallest particles only are left in 
it, but it gives as much casein as cream, because associated with 
these myriads of specks of oil is some not inconsiderable burden of 
proteid, enough to raise their specific gravity from that of oil to 
that of milk plasma. 
The envelope-theory appears to have several things to recom- 
mend it : — 
(1.) The permanent emulsion, which milk is. 
(2.) The analogy from artificial emulsions, where egg-albumin or 
gum is made the coating of oil-globules. 
(3.) Cohesiveness of these oil particles in precipitates and clots, 
amounting in certain cases, e.g ., in old shrunken clots, to extreme 
tenacity ; naked oil-globules could not exhibit this property. 
(4.) The action of an alkali in apparently dissolving off something, 
so that the fat can then be dissolved by ether, &c. Even without 
ether the globules can now coalesce into large drops, which have 
lost the spherical appearance and dark periphery of fat-globules. 
(5.) The fact that something is broken or ruptured in “ churning” 
milk, whereby the oil-globules accumulate and caseinogen is ap- 
parently liberated ; for butter-milk contains a larger percentage of 
casein than milk skimmed. This is just what we would expect, 
for instead of fat being taken out of it as cream with its associated 
caseinogen, all the fat is left in it, but disintegrated there, and after- 
wards removed, having set free in the plasma its associated proteid, 
which, being the caseinogen of the cream, comes to be added in 
analysis to the casein of what would otherwise have been skimmed 
milk whose percentage of casein is thus considerably raised. 
Hence, I think, we are justified in only saying at present that 
