ALBUMINOUS COMPOUNDS. 57 



is very readily precipitated by those acids, although they do not form combina- 

 tions with it, and it is made to coagulate by contact with an excessively small 

 portion of rennet ; whilst neither of these reagents has any effect upon ordinary 

 Albumen. 1 The analyses given by different chemists of the ultimate composi- 

 tion of Casein, differ quite as much from each other, in regard to the propor- 

 tions of Oxygen, Hydrogen, Carbon, and Nitrogen, which it contains, as they 

 do from the analyses of Albumen already cited ; and the only positive diversity 

 between the two substances, that can be detected by ultimate analysis, seems to 

 be in the absence of phosphorus from casein, and in the smaller proportion of 

 sulphur which it includes. Casein, like albumen, never occurs in nature in an 

 isolated form, but is intimately blended with other substances; and it is specially 

 remarkable for the quantity of Phosphate of Lime which is incorporated with 

 it, as much as 6 per cent, of this earthy salt being usually obtainable from it. 

 It is in the milk of the Mammalian female, that we meet with Casein in the 

 greatest abundance; and it must be formed in that secretion either from the 

 albumen of her blood, or at the expense of her solid tissues. When introduced 

 as food into the stomach of her offspring, it is there completely coagulated, and 

 seems to be reduced back in the digestive process to the condition of Albumen, 

 as it also is when used as food in its coagulated state by the adult. Casein has 

 been obtained in considerable quantity from the blood of puerperal women, in 

 whom the secretion of milk has been checked ; but it seems doubtful if true 

 casein is ever a normal constituent of the blood. Various experimenters have 

 affirmed its presence ; but their results do not for the most part seem trust- 

 worthy, since they are invalidated by certain fallacies which have been pointed 

 out by Prof. Lehmann (op. cit. vol. i. p. 380). The existence of Casein in 

 ordinary blood-serum has been more recently affirmed by Dr. Panum ; but his 

 experiments are scarcely satisfactory, since it appears by no means impossible 

 that what he represents to be Casein may be nothing else than the neutral 

 albuininate of soda ( 20), which is precipitated, as we have seen, by the addi- 

 tion of water, and which may also be thrown down by acetic acid. 3 It is inte- 

 resting to remark, however, that from the latter inquiries of Prof. Lehmann, it 

 seems by no means improbable that the Vitellin, which forms the albuminous 

 body of the yolk of the egg, and which has been described as a peculiar com- 

 pound intermediate between albumen and fibrin, is in reality a mixture of Albu- 

 men with Casein. " The amorphous dark granules of the yolk/' he says, " are 

 pure casein free from alkali, which, like ordinary casein, is rich in phosphate of 

 lime ; while in the proper intercellular fluid of the yolk, there is no casein, but 



1 It is shown by Prof. Lehmann, however, that under certain circumstances Albumen 

 may be precipitated by acetic acid ; and that even the rennet-test is liable to fallacies. 

 It is very commonly supposed that the coagulation of Casein in milk, when treated with 

 rennet, is due to the action of rennet as a ferment upon the sugar of milk, the lactic acid 

 thus formed being the real precipitant of the casein. The experiments cited by Prof. 

 Lehmann, however, render it probable that the action of rennet upon casein does not re- 

 quire the intermediation of lactic acid. (Op. cit. pp. 375-381.) 



2 It has been ascertained by Dr. Panum, that pure Casein from milk is readily soluble 

 in water containing a certain quantity of phosphate of soda, but is precipitated when this 

 solution is too much diluted ; and this fact he applies to the explanation of the turbidity 

 which he has found to be frequently produced in ordinary blood-serum by the addition of 

 acetic acid, or by dilution with water, or by both means combined ; this reaction being 

 supposed by him to take place under the following conditions. The (supposed) Casein, 

 united with soda, is held in solution by the salts which are present, especially phosphate 

 of soda ; if its quantity be very large, the solvent power of the salts is sufficiently weakened 

 by the addition of water to occasion its precipitation ; and the addition of acetic acid, 

 without dilution, will produce the same effect by withdrawing its soda ; but if its quantity 

 be smaller, its precipitation can only be accomplished by the dilution of the solution, and 

 the addition of acetic acid, conjointly. See Dr. Panum's Memoir in the "Bibliothek fur 

 Laager," for Jan. 1850, and the "London Journal of Medicine," July, 1850. 



