SILICA 



5381 



SILK 



ern mountain ranges. Rye, oats, wheat, barley, 

 potatoes and beets are raised in large quantities. 

 Silesia was also an important manufacturing 

 state, holding first rank among the Prussian 

 provinces in the weaving and spinning of flax. 

 Population in 1910, 5,225,962. 



Related Subjects. The reader is referred to 

 the following articles in these volumes : 

 Frederick, subhead Prussia 



Frederick II Versailles, Treaty of 



SILICA, sil'ika, one of the most widely dis- 

 tributed substances in the earth. Sand, quartz, 

 gravel and sandstone are composed almost en- 

 tirely of silica. It occurs also in granite, clay, 

 slate, mica, feldspar and other rocks. It is 

 found in the stalks of grasses and grains, to 

 which it imparts strength and toughness. It 

 also imparts similar qualities to the quills of 

 birds and to living sponges. 



Silica is a chemical compound of silicon (which 

 see) and oxygen. It occurs both in crystal- 

 line and in amorphous (irregular) forms; that 

 is, without definite structure. Amorphous silica 

 is a soft, white powder, insoluble in water and 

 all acids except hydrofluoric acid. Quartz is 



the best example of crystallized silica. Many 

 precious stones, like amethyst, jasper and cat's- 

 eyes, are crystallized silica delicately colored 

 in various shades and tints. Other precious 

 stones, like opal, onyx, agate and chalcedony, 

 are amorphous silica variously colored. Flint 

 is an amorphous silica colored black or dull 

 brown. 



Related Subjects. The reader is referred to 

 the following articles in these volumes: 



Crystallization 



Gems 



Quartz 



Sand 

 Sandstone 



SILICON, sil'ikon, a nonmetallic element, 

 the most abundant in nature with the excep- 

 tion of oxygen, although it does not occur un- 

 combined. Natural silica, or silicon oxide, is 

 the principal constituent of quartz, flint, sand, 

 amethyst, agate, jasper, opal and many other 

 rocks, in most of which it occurs in the form 

 of crystals. Silicon occurs also as a brown pow- 

 der, which will burn in air and chlorine, and is 

 soluble in alkalies. The chemical symbol for 

 silicon is Si. See SILICA. 



THE STORY OF SILK 



ILK, a lustrous fiber produced by the 

 most useful insects known. The threads that 

 form the beautiful and costly silk fabrics of 

 our markets are taken from cocoons spun by 

 thousands of little creatures whose ceaseless 

 activity makes possible one of the world's 

 great textile industries. Silk fabrics are among 

 the handsomest and richest textiles used by 

 mankind, and such is the esteem in which they 

 have always been held that writers use the 

 term as a symbol of the luxury and elegance of 

 the most splendid civilizations. 



The Silkworm. The humble creator of this 

 rarest of textile fabrics is the silk moth, an in- 



sect with a short, blunt body, stout legs and 

 broad, white wings, marked by several black 

 lines running parallel with the borders. Its 

 spread wings measure about two inches from 

 tip to tip. The female moth lays her eggs in 

 the early summer, depositing from 200 to 500 

 bluish eggs about the size of a pin's head and 

 gluing them together with a gummy secretion. 

 In a wild state, the moth lays her eggs on the 

 mulberry tree; but under cultivation, she is 

 provided with bits of paper or muslin. 



The eggs hatch during the following spring, 

 the larvae being about a quarter of an inch 

 long when they emerge. They are voraciously 



