870 APPENDIX. 



min and other proteids. The jelly produced by the action of caustic potash on white 

 of egg, spoken of in Class I. , 1 , is alkali-albumin ; the similar jelly produced by 

 strong acetic acid is acid-albumin. One of the most productive methods of obtain- 

 ing alkali-albumin is that introduced by Lieberkiihn, 1 and consists in adding a strong 

 solution of caustic potash to purified white of egg until the above-mentioned jelly 

 is obtained. This is then cut into small pieces, and dialyzed until quite white. The 

 lumps are then dissolved by heating on the water-bath, and the alkali-albumin pre- 

 cipitated by the careful addition of acetic acid. 



Both alkali- and acid- albumin are with difficulty precipitated by alcohol from 

 their alkaline or acid solutions. The neutralization precipitates, however, become 

 coagulated under the prolonged action of alcohol. 



The body "protein," described by Mulder, appears, if it exists at all, to be closely connected 

 with this body. All subsequent observers have, however, failed to confirm his views. 



The rotatory power of alkali-albumin varies according to its source ; thus 

 when prepared by strong caustic potash from serum-albumin, the rotation rises 

 from 56 (that of serum-albumin) to 86 ; for yellow light. Similarly prepared 

 from egg-albumin, it rises from 38. 5 to 47 ; and if from coagulated white of 

 egg, it rises to 58.8. Hence the existence of various forms of alkali-albumin is 

 probable. 



In addition to the methods given above, alkali-albumin may be also readily obtained by shak- 

 ing milk with strong caustic soda solution and ether, removing the ethereal solution, precipitat- 

 ing the remaining fluid with acetic acid and washing the precipitate with water, cold alcohol and 

 ether. 



The most satisfactory method of regarding acid- and alkali-albumin is to con- 

 sider them as respectively acid and alkali compounds of the neutralization precip- 

 itate. We have reason to think that when the precipitate is dissolved in either an 

 acid or an alkali, it does enter into combination with them. The neutralization pre- 

 cipitate is in itself neither acid- nor alkali-albumin, but may become either, upon 

 solution in the respective reagent. 



It is probable that several derived albumins exist, 2 differing according to the proteid from 

 which they are formed or possibly according to the mode of their preparation, and that each of 

 these may exist in its correlative forms of acid- and alkali-albumin ; but the whole subject re- 

 quires further investigation. 



Acid-albumin, prepared by the direct action of dilute acids on native albumin, or 

 on muscle-substance, contains sulphur, as shown by the brown coloration which ap- 

 pears when the precipitate is heated with caustic potash in the presence of basic 

 lead acetate. Alkali-albumin, at all events as prepared by the action of strong caus- 

 tic potash or soda, does not contain any sulphur ; and the acid-albumin, prepared 

 by the solution in an acid of the neutralization precipitate from such an alkali-albu- 

 min solution, is similarly free from sulphur. 



3. Casein. 



This is the well-known proteid existing in milk. When freed from fat, and in 

 the moist condition, it is a white, friable, opaque body. In most of its reactions 

 it corresponds closely with alkali-albumin ; thus it is readily soluble in dilute acids 

 and alkalies, and is re-precipitated on neutralization ; if, however, potassic phos- 

 phate is present, as is the case in milk, the solution must be strongly acid before 

 any precipitate is obtained. 



Various reactions have at different times been assigned to casein as distinguishing it from the 

 closely allied body alkali-albumin. Later researches have, however, in most cases cast so much 

 doubt on these differences that the identity or non-identity of casein and alkali-albumin must 

 still be left an open question, the discussion of which would be out of place here. 



Casein, as occurring in milk, has had several reactions ascribed to it, as characteristic ; but 

 these lose their importance on considering that milk contains, in addition to casein, other sub- 

 stances, such as potassic phosphate, and a number of bodies which yield acids by fermentation. 

 The presence of potassic phosphate has an especial influence on the reactions of casein. In the 

 entire absence of this salt, acetic acid in the smallest quantities, as also carbonic anhydride, gives 

 a precipitate ; but if this salt is present, carbonic anhydride gives no precipitate, and acetic acid 

 only one when the solution is acid from the presence of free acid, and not from that of acid 

 potassic phosphate. 8 



1 PoggendorflPs Annalen, Bd. Ixxxvi., S. 118. 



2 Morner. Pttviger's Arch., Bd. xvii. (1878), S. 468. 



a See Kiihne, Lehrb. d. Physiol. Chem., 1868, S. 565. 



