ALBUMINOIDS. 215 



alcohol, which has been called "gliadin" or "vegetable 

 gelatin," to which the stickiness of gluten is due, and 

 another portion, insoluble in alcohol, which has received 

 the name "vegetable fibrin." According to Martin 

 gluten is not present in flour, but is produced by a fer- 

 ment from other proteids during the treatment with 

 water. 



Good wheat flour contains 10 or 11% of gluten, the 

 poorer varieties 8 to 10 %. Barley, oat. and rye flours 

 contain very little of the gliadin, their gluten consisting 

 mainly of vegetable fibrin ; hence they do not yield such 

 sticky paste nor such spongy bread as does wheat flour. 



The complex character of albumen may be gathered from 

 the various formulae which have been proposed as most nearly 

 representing its composition, thus: Lieberkiihn proposed 

 C 72 H 1;il N ls O. 22 S, Schiitzenberger C 240 H 8 y. 2 N,;s0 75 S 8 , and Harnack 

 CaoiHaaaNgaOfleSg. 



Proteids are all optically active, being laevo-rotatory. They 

 are precipitated by acids and potassium ferrocyanide, by basic 

 lead acetate, by mercuric chloride, by copper sulphate, by satu- 

 ration of the liquid with ammonium sulphate or magnesium 

 sulphate, or by alcohol. 



The following are general qualitative tests for proteids : 



1 . Heated with strong nitric acid they yield a yellow coloura- 



tion which becomes orange on the addition of alkali 

 (xantho-proteic reaction). 



2. Millon's reagent (an acid solution of nitrate of mercury) 



gives a precipitate which turns red on heating. 



3. With excess of strong sodium hydrate solution and a small 



quantity of copper sulphate a violet colour is obtained, 

 becoming deeper on warming. 



4. Adamkiewicz's reaction. The proteid is dissolved in gla- 



cial acetic acid and strong sulphuric acid is added, when 

 a violet colour is produced. It has lately been shown* 

 that the acetic acid only produces this reaction if it con- 

 tains as an impurity, glyoxylic acid, HCO.COOH, and 



* Hopkins and Cole, Proc. Roy. Soc. 1901, 21. 



