7 6 Proceedings of Royal Society of Edinburgh. [sess. 
both the largest and the smallest particles in milk contain fat in 
direct ratio to their bulk, and caseinogen in indirect ratio to the 
same; that the caseinogen is associated with the fat in a most 
intimate fashion, yet one which can be destroyed by a mechanical 
process such as “churning.” 
II. On Caseinogen — its Chemistry and Coagulation. 
Following Halliburton, the term casein is reserved for the chief 
proteid of milk when it has been rendered visible as a clot or 
precipitate by the action of an enzyme, usually rennin of rennet, 
sometimes bacterial ferments. 
Acetic acid is said merely to precipitate the caseinogen ; preci- 
pitation also is the effect of saturating milk with MgS0 4 . 
I studied the chemistry of “pure caseinogen” with Ringer’s 
solution or suspension, made by grinding up the washed acetic acid 
precipitate with fine prepared chalk and throwing the mixture into 
a couple of litres of distilled water. The chalk particles fall to the 
bottom, some of the large globules rise to the top, while the water 
in between contains the smaller globules and granules. We thus 
obtain an odourless, opalescent, neutral liquid of varying specific 
gravity (depending on the amount of water used) which, unless 
sterilised by boiling, rapidly becomes putrid and clotted. Steril- 
ised and well corked, it keeps for a fortnight or more. It boils at 
103° C., and like milk, especially in an open dish, it skins over; 
this pellicle can be nothing else than a surface dessication of 
“ caseinogen.” 
As in milk this proteid is entirely precipitated by saturating 
with MgS0 4 , a chemical and not a mechanical process, inasmuch 
as such precipitation entirely fails when ground glass, fine sand, 
dust, or chalk is agitated with the liquid. The various familiar 
proteid precipitants, as might be expected, entirely precipitate 
caseinogen both pure and in milk ; corrosive sublimate does not 
appear to precipitate it in milk. The corrosive mineral acids (in 
minim doses) precipitate caseinogen both in milk and when “ pure,” 
excess of the acid in many cases effecting a partial solution. Acetic 
acid of only 10 per cent, tends in excess to dissolve its precipitate. 
Picric acid does not precipitate it in milk, but does in pure solu- 
