6 INTRODUCTION. 



(3) The mucoid protein can only be separated with the fat 

 globules if the density of the serum is increased (by the addition 

 of cane sugar) till it is greater than the density of mucoid protein 

 (1-0228) ; this proves that it is independent of the fat globules. 



Milk is regarded by others as an emulsion, and they see no 

 reason why an emulsion of fat containing ether should not exist 

 of the same nature as that of fat alone ; these regard the fact that 

 while a small quantity of ether does not extract the fat to any 

 extent from milk, a large quantity does so with a much nearer 

 approach to completeness, to favour the idea that a membrane 

 does not exist round the globules. If milk is precipitated with 

 a solution of nitrate of mercury, which coagulates all the protein 

 of milk, the whole of the fat is removed from suspension, even 

 if it exceeds the protein in weight many times. That this is 

 not due to the precipitation of substances distributed through- 

 out the solution, and the enclosing of the fat therein, is shown 

 by the fact that when the casein, which is equally distributed 

 throughout the solution, is precipitated by means of rennet 

 a considerable proportion of the fat is not enclosed. This fact 

 can hardly be used as an argument that a membrane exists, 

 as the two modes of precipitation differ essentially ; the mercury 

 precipitate commences to settle immediately, leaving the solution 

 clear, while rennet gradually reduces the milk to a semi-solid 

 mass, which does not yield a precipitate until the whole has 

 received considerable agitation. As a strong argument against 

 the existence of a membrane may be cited the possibility of 

 preparing artificial emulsions of fats in a finely divided state 

 with milk from which the natural fat has been removed ; these 

 emulsions partake very largely of the character of milk. Emul- 

 sions of a similar nature can be prepared with other substances, 

 and their behaviour in a great number of respects resembles 

 that of milk. The general consensus of opinion among chemists 

 who may be regarded as authorities on this point is that the fat 

 in milk is not surrounded by a membrane, and, therefore, that 

 it is a true emulsion. There is very little doubt that a layer 

 exists round each fat globule ; this is probably formed by an 

 attraction due to surface energy, a force akin to that which 

 causes the phenomenon known as capillary attraction. Much 

 of the evidence which has been taken as proof of the existence 

 of a membrane round the globules is only evidence of the presence 

 of a layer of some sort, but not necessarily membranous. 



Bauer concludes from surface tension experiments that a solid 

 layer exists round the fat globules, and that this probably con- 

 tains some fat. 



The author and S. 0. Richmond have obtained evidence that 

 the fat globules in milk solidify when cooled below their melting 



