CHEMICAL BASIS OF THE ANIMAL BODY. 867 



This last reaction is useful, not only on account of its exactness, but also because the reagents 

 used produce no decomposition of other bodies which may be present ; and hence after filtration 

 the same fluid may be further analyzed for other substances. Additional methods of freeing a 

 solution from proteids are : acidulating with acetic acid and boiling, avoiding any excess of the 

 acid ; precipitation by excess of alcohol ; in the latter case the solution must be neutral or faintly 

 acid. Hoppe-Seyler 1 recommends the employment of a saturated solution of freshly precipitated 

 ferric hydrate, in acetic acid ; this is added to the solution, and on boiling the whole of the pro- 

 teids are precipitated as well as the ferric salt, the latter as a basic acetate. Briicke's method of 

 removing the last traces of proteids from glycogen solution is also of use (see Glycogen). Precip- 

 itation of the last traces of proteids by means of hydrated oxide of lead at a boiling tempera- 

 ture 2 may be also employed. 



[Solutions may entirely be freed from proteids by one of the following methods : 

 (1) Add acetic acid to faint acidity, then tannic acid; (2) render acid with hydro- 

 chloric acid, then add iodide of mercury and potassium ; (3) render decidedly acid 

 with hydrochloric acid, then precipitate any proteids in solution with phospho- 

 tungstic acid. ] 



Proteids may be conveniently divided into classes. 



CLASS I. Native Albumins. 



Members of this class, as their name implies, occur in a natural condition in 

 animal tissues and fluids. They are soluble in water, are not precipitated by very 

 dilute acids, by carbonates of the alkalies, or by sodium chloride. They are coagu- 

 lated by heating in solution to a temperature of about 70 C. If dried at 40 C. , 

 the resulting mass is of a pale yellow color, easily friable, tasteless, inodorous, and 

 soluble. 



1. Egg-albumin. 



Forms in aqueous solution a neutral, transparent, yellowish fluid. From this 

 it is precipitated by excess of strong alcohol. If the alcohol be rapidly removed 

 the precipitate may be readily redissolved in water ; if subjected to longer action 

 a coagulation occurs, and the albumin is then no longer thus soluble. Strong 

 acids, especially nitric acid, cause a coagulation similar to that produced by heat 

 or by the prolonged action of alcohol ; the albumin becomes profoundly changed 

 by the action of the acid and does not dissolve upon removal of the acid. Mer- 

 curic chloride, argentic nitrate, and lead acetate, precipitate the albumin, forming 

 insoluble compounds of variable composition with it : the precipitants may be re- 

 moved by means of sulphuretted hydrogen and the albumin again obtained, appar- 

 ently unaltered, in solution. 



Strong acetic acid in excess gives no precipitate, but when the solution is con- 

 centrated the albumin is transformed into a transparent jelly. A similar jelly is 

 produced when a strong caustic potash is added to a concentrated solution of egg- 

 albumin. In both these cases the substance is profoundly altered, becoming, in the 

 one case, acid ; in the other, alkali-albumin. 



The specific rotatory power of egg-albumin in aqueous solution is, for yellow 

 light 35.5. Hydrochloric acid, added until the reaction is strongly acid, in- 

 creases this rotation to 37.7. The formation of the gelatinous compound with 

 caustic potash is at first accompanied by an increase, but this is followed by a 

 decrease of rotation. 



Preparation. White of hen's egg is broken up with scissors into small pieces, 

 diluted with an equal bulk of water, and the mixture shaken strongly in a flask 

 till quite frothy ; on standing, the foam rises to the top, and carries all the fibres 

 in whose meshwork the albumin was contained. The fluid from which the foam 

 has been removed, is strained, and treated carefully with dilute acetic acid as long 

 as any precipitate is formed ; the precipitate is then filtered off, and the filtrate 

 after neutralization purified by dialysis and then concentrated at 40 to its original 

 bulk. 



2. Serum-albumin. 



This form of albumin resembles, to a great extent, the one previously described. 

 The following may suffice as distinguishing features : 



1. The specific rotation of serum-albumin is 56; that of egg-albumin is 

 35.5, both measured for yellow light, 



i Op. cit., 8. 227. 2 Hofmeister, Zeitschr. f. Physiol. Chem., Bd. ii. (1878), S. 288. 



