302 Organic Substances and Formations. 



it grows contains. It takes from the fluid or solution what it 

 requires for its development and reproduction, and in so .doing 

 determines certain decompositions and recompositions, which 

 are termed fermentation, and which give rise to special 

 products. In more complicated organic beings a great series 

 of processes, more or less similar, are going on, and every 

 organ is engaged in a series of transactions with fluids and 

 gases, one result of the whole being the orderly building up, 

 and the equally orderly taking to pieces, of the various parts 

 which make up the entire structure. 



Starches and sugars are among the most interesting pro- 

 ducts of vegetable life. Common starch consists of carbon, 

 oxygen, and hydrogen (the proportion being C 12 H 10 O 10 ); 

 and if the elements of water (oxygen and hydrogen) be added, 

 starch sugar is the result. There are many varieties of sugar, 

 and many substances allied to sugar, and belonging chemically 

 to the same group. They consist of the same elements com- 

 bined in different proportions. Cane sugar crystallizes in four 

 or six-sided rhombic prisms ; grape sugar in cubes or square 

 tables; sugar of milk in four-sided prisms. Fruit sugar is not 

 crystallizable. Gums are much like sugars in composition, and 

 abundantly found in plants. Gum arabic has the same compo- 

 sition as cane sugar, C 12 H n 1X , and may be converted into 

 sugar by the action of dilute sulphuric acid, which first 

 removes one equivalent of water, and turns it into dextrin, 

 a substance in properties between gum and starch. 



Vegetable jelly (pectin) gives their gelatinous character to 

 the juice of many fruits, and pectose, which may be converted 

 into it, abounds in carrots, turnips, etc. 



Silica, or flint, pure in white quartz crystals, is often met with 

 by the microscopist in vegetable bodies, such as wheat straw, 

 the glazed surface of canes, the spicules of certain sponges, and 

 so forth. In these cases it has been deposited by organic 

 agency in an organic structure. Silica is composed of silicon 

 and oxygen, and is one of the most abundant constituents of 

 the earth's crust. Silica or silicic acid crystallizes in six-sided 

 prisms, terminated by six-sided pyramids. It requires an 

 intense heat to melt it, and is insoluble in all the acids except 

 hydrofluoric. One of its modifications is slightly soluble in 

 water, and it may be precipitated in a gelatinous form from 

 solutions of silicates of soda and potash. Diatoms, and many 

 other organized beings, can separate from water the minute 

 quantity of silica necessary for their hard structures. When 

 the microscopist is in doubt whether a substance is silica, or 

 carbonate, or oxalate of lime (the last common in plants as 

 raphides), he should apply a drop of muriatic acid. If lime it 

 will be dissolved, if silica not. Oxalate of lime crumbles to an 



