8 INTRODUCTION. 



being coagulated by heat. Other proteins have been described 

 in milk, but many of them are only decomposition products of 

 casein or albumin, which were formed during the process adopted 

 for the removal of the other proteins. Evidence has been adduced 

 of a globulin in milk ; Wroblewski states that there is a protein 

 which he terms opalisin present in small quantities in cow's milk, 

 and in very much larger amounts in human milk ; Storch's 

 mucoid protein has already been referred to. Bechamp has 

 described a starch-liquefying enzyme, and lately Babcock 

 and Russell have separated a proteolytic enzyme, and there 

 also exist other enzymes. 



The casein in milk is not in a state of true solution ; it is 

 probably in a state described by Picton and Linder as " pseudo- 

 solution." They have shown that this state is due to the exis- 

 tence of particles in the solution not sufficiently large to settle 

 under the influence of gravity, but which will interfere with 

 the passage of light ; they can also be separated by a current 

 of electricity, by subjecting milk to the influence of great centri- 

 fugal force, or by passage of the solution through a porous jar. 

 They show also that there is no sharp dividing line between 

 crystalloids and colloids in solution, substances in pseudo-solu- 

 tion, and substances in suspension. In milk we have the four 

 states represented the fat is in suspension, the casein in pseudo- 

 solution, the albumin in solution as a colloid, and the milk-sugar 

 in solution as a crystalloid ; these four states are probably due 

 to the size of the conglomerates of molecules or particles. 



Salts. The salts of milk are not yet fully studied ; the presence 

 of chlorides, phosphates, and sulphates of sodium, potassium, 

 calcium, and magnesium is generally admitted. Salts of organic 

 acids are also present ; Henkel has described citric acid, and 

 Bechamp acetic acid, but this latter result is not universally 

 accepted. Bechamp also maintains that the casein and albumin 

 exist in milk as salts ; there is much to recommend this view. 

 A solution greatly resembling milk can be prepared in which 

 casein undoubtedly exists combined with a base, while it has 

 not been found possible to dissolve casein to an appreciable 

 extent unless an alkali be present ; milk does not taste sour 

 until an appreciable acidity has developed ; at about the same 

 point it curdles on heating ; it is proved that this is due to the 

 acid developed displacing the alkali from its compound with 

 casein. It is also found impossible to coagulate the albumin 

 in milk unless a certain amount of free acid is added, and this 

 fact accords well with the theory of Bechamp. Soldner has also 

 adduced evidence in proof of this view. 



Besides the constituents enumerated above, milk contains 

 traces of other compounds ; among these may be mentioned 



