116 SUBSTANCES CONTAINING NITROGEN. 



with remarkable ease by that acid which exists in the ripe grape, so it is 

 in the interior of plants. Where sugar occurs in connection with an acid 

 in the juice of a plant, it is grape sugar in whole or in great part, be- 

 cause in the presence of an acid body cane sugar cannot permanently ex- 

 ist, but is gradually transformed into the sugar of grapes. It thus ap- 

 pears also why fruits so readily enter into fermentation, and why, even 

 when preserved with cane sugar, they will, in consetiuence of the acid 

 they retain, slowly change the latter into grape sugar, and thus induce 

 fermentation.* 



§ 8. O/" substances which contain Nitrogen. — Gluten, Vegetable 

 Albumen^ and Diastase. 



The substances described in the preceding sections consist of carbon, 

 hydrogen, and oxygen only, and of them the great bulk of the vegeta- 

 ble productions of the globe consists. But there are certain other sub- 

 stances occurring along with starch and sugar, into which nitrogen enters 

 as a constituent, and which, though not formed in the vegetable king- 

 dom in very large quantity, are yet of such interest and importance in 

 other respects, as to make it necessary shortly to advert to them. 



1^. Gluten. — When the flour of wheat is made into a dough, and this 

 dough is washed with water upon a fine sieve, a milky liquid passes 

 through, from which starch gradually subsides. Tliis has been already 

 slated. But on the sieve, when the water ceases to go through milky, 

 there remains a soft adherent, tenacious, and elastic substance, which 

 can be drawn out into long strings, has scarcely any colour, taste, or 

 smell, and is scarcely diminished by washing either with hot or with 

 cold water. This substance is the gluten o( wheat. The flour of other 

 kinds of grain also yield it by a similar treatment, though generally in 

 much smaller quantity. This appears from the following table : — 



The grain of 



Wheat contains 8 to 35 per cent, of gluten. 

 Rye .... 9 to 13 " " 



Barley ... 3 to 6 " " 



Oats .... 2 to 5 " " 



When the moist gluten is dried in the air or at the temperature of 

 boiling water, it diminishes much in bulk, and hardens into a brittle 

 semi-transparent yellow substance resembling horn or glue. In this state 

 it is insoluble in water, but dissolves readily in vinegar, in alcohol either 

 cold or hot, and in solutions containing caustic potash, or soda, [the 

 common pearl-ash or soda of the shops boiled with quick-lime.] 



2°. Vegetable Albumen. — To the white of egg the name of albumen 

 (albus, white) has been given by chemists. It jwssesses the well known 

 property of coagulating or of forming a white solid insoluble substance, 

 when it is heated either alone or after being mixed with water. 



When the starch has subsided from the milky liquid which passes 



* Milk also, in favourable circumstances, as when kept at a temperature of 100° F., tfn- 

 dergoes fermentation, and in some countries of Asia a spirituous liquor is prepared from 

 mares' and asses' milk^ In this case the milk first becomes sour, then the acid thus form- 

 ed converts the milk sugar into grape sugar, and finally this sugar enters into fermenta- 

 tion. This takes place more readily in consequence of the presence of the decomposing 

 cheesy matter (casein) of the milk — as is shown by the fact that the introduction of a small 

 quantity of the curd of milk into a solution of grape sugax will cause it to ferment. 



