SUBSTANCES PRODUCED BY PLANTS. 57 



under a stream of water, we effect a separation of its parts ; 

 the starch contained in it is carried through the sieve and 

 j:fradually deposited from the water, while in the sieve a 

 tibrous matter is retained. If we boil the clear liquid from 

 which the starch has fallen down, a froth or curdy matter 

 forms on its surface, which, from its resemblance to the coa- 

 gulated white of egg, scientifically termed albumen, has been 

 named vegetable albumen. Though this substance exists in 

 plants in much smaller quantities than those we have lately 

 been considering, yet we find it invariably present in their 

 juices. It performs an important part in contributing to 

 the nourishment of animals, and is distinguished from starch, 

 gum, and sugar, by containing nitrogen, which is an essential 

 constituent of flesh (8), and also a small amount of sulphur 

 and phosphorus. In all its leading chemical characters, it 

 agrees with animal albumen.* 



74. Gluten. If you place some wheat-flour in a muslin 

 bag and knead it with your fingers under a stream of water, 

 so long as the water is rendered milky, you will find upon 

 opening the bag that the flour has diminished in bulk, and 

 that there remains a gi-ey, adhesive, elastic matter, which, like 

 birdlime, can be drawn into threads. This substance which 

 does not wash away is called gluten, and is found in con- 

 siderable amount in all vegetables, and especially in those 

 parts of our cultivated plants which we value for food. From 

 the milky fluid which runs through the bag starch will subside, 

 and by pouring off" the clear liquid after the deposit has taken 

 place, and boiling it, white flakes of albumen will separate. 

 The gluten of wheat consists chiefly of a substance termed 

 by Liebig vegetable fibrine, which, like albumen, approaches 

 closely to the fibre of muscle in its composition. It contains 

 about 15 per cent of nitrogen and a small amount of sulphur. 



75. Vegetable Casein. When peas are bruised in a mortar 

 and the pulp then mixed with a considerable proportion of 

 water, and strained through a piece of muslin, a milky liquid, 

 from which starch is gradually deposited, passes through the 

 sieve. If, when the liquid has become clear, it be decanted and 

 boiled, no coagulation takes place as when albumen is present, 



• The characteristic smell of rotten eggs is produced by the sulphur 

 contained in the albumen or white coming off united with hydrogen in 

 the form of a gas named sulphuretted hydrogen. The black stain which 

 stale eggs produce on silver spoons is also occasioned by this gas. 



