206 COMMON SUBSTANCES 



formed when coal is distilled in making illuminating 

 gas. NH 3 is a gas, but it dissolves very easily in water. 

 A solution of NH 3 in water forms NH 4 OH, ammonium 

 hydrate. This is a strong alkali; somewhat weakened 

 with more water, it is used as "household ammonia." 

 Two salts of this base are common, NH 4 C1, famil- 

 iarly called sal ammoniac, and NH 4 NO 3 , ammonium 

 nitrate. A solution of sal ammoniac in water is used in 

 many kinds of voltaic cells. 



239. Cellulose. Cellulose may be described as the 

 chief substance which makes up the structure of plants. 

 It is found in many different forms, though its chemical 

 composition does not change. All wood fiber, trunks of 

 trees, their branches, roots, stems, veins of leaves, and 

 parts of fruits are composed largely of cellulose ; also 

 such fibers as cotton, flax, and hemp. The substance 

 cellulose is a carbohydrate ( 219) having the symbol 

 C 6 H 10 O 6 . It is formed by the activity of the plants, 

 largely from the H 2 O and CO 2 that the soil and the 

 atmosphere furnish. 



240. Starch. Starch is a carbohydrate having the 

 symbol C 6 H 10 O 5 . It is made by the action of plants, 

 and is found throughout the vegetable world ; seeds of 

 all sorts contain starch, and some plants store up large 

 masses of it, as sago and tapioca. Starch is prepared in 

 large quantities from corn and from potatoes. It forms 

 an important food for man, both in its prepared state 

 and as cereals, barley, oats, wheat, rye, rice, etc. In 

 cold water, starch is usually not soluble ; but in hot 

 water it partly dissolves, forming a paste. 



