1168 PROTEIDS. 



Class VII. Lardacein or amyloid substance. 



Insoluble in water, dilute acids and alkalis and saline solu- 

 tions. Converted into members of Class II. by strong acids 

 and alkalis. 



The Chemistry of the several Proteids. 

 Class I. Native Albumins. 



1. Egg-albumin. 



As obtained in the solid form by evaporating its solutions 

 to dryness at 40°, preferably in vacuo, it forms a semi-trans- 

 parent, brittle mass, of a pale yellow colour, tasteless and 

 inodorous. Dissolved in water it yields a clear neutral colour- 

 less solution. This solution coagulates on heating, but the 

 temperature at which the coagulation takes place varies con- 

 siderably with the concentration and is largely dependent upon 

 the presence or absence of salts. The more commonly observed 

 temperature is 70 — 73°, but it is stated that coagula may also be 

 obtained at 54° and 63°. The more dilute the solution is the 

 higher is the temperature at which it coagulates, thus finally 

 resembling a solution of albumin from which the salts have 

 been removed by dialysis. When precipitated from solution 

 by excess of alcohol it is readily coagulated by the precipitant, 

 so that it is now usually insoluble in water. In this respect 

 it differs somewhat characteristically from serum-albumin which 

 is not so immediately though it is ultimately coagulated by the 

 action of alcohol. 



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 b}^ the 

 action of the acid and does not dissolve upon removal of the 

 acid. Mercuric chloride, nitrate of silver and lead acetate, 

 precipitate the albumin, forming with it insoluble compounds 

 of variable composition. 



Strong acetic acid in excess gives no precipitate, but when 

 the solution is concentrated the albumin is transformed into a 

 transparent jelly. A similar jelly is produced when 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. 



According to recent researches egg-albumin may be obtained 

 in a crystalline form by slow evaporation of its solutions in 

 presence of neutral ammonium sulphate. The separation takes 

 place at first in the form of minute spheroidal globules of 



