702 PROTEIDS. [App. 



2. With Millon's reagent they give, when present in sufficient 

 quantity, a precipitate, which turns red on heating. If they are only 

 present in traces, no precipitate is obtained, but merely a red colouration 

 of the solution. 



3. If mixed with some concentrated solution of sodic hydrate, 

 and one or two drops of a solution of cupric sulphate, a violet colour is 

 obtained, which deepens in tint on boiling. 



The above serve to detect the smallest traces of all proteids. The 

 two following tests may be used when there is more than a trace 

 present, but do not hold for every kind of proteid. 



4. Render the fluid strongly acid with acetic or other acid, and 

 add a few drops of a solution of ferrocyanide of potassium ; a pre- 

 cipitate shews the presence of proteids. 



5. Bender the fluid, as before, strongly acid with acetic acid, add 

 an equal volume of a concentrated solution of sodic sulphate, and 

 boil. A precipitate is formed if proteids are present. 



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 analysed 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 proteids 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 

 solutions is also of use (see glycogen). Precipitation of the last traces of proteids by 

 means of hydrated oxide of lead at a boiling temperature 2 may be also employed. 



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 alkalis, or 

 by sodium chloride. They are coagulated by heating in solution to a 

 temperature of about 70 C. If dried at 40 C., the resulting mass is of 

 a pale yellow colour, 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; 



1 Op. cit. S. 227. 



2 Hofmeister, Zeitsch. f. pliysiol. Chem., Bd. n. (1878) S. 288. 



