SILICON 



SILK 



451 



form has a specific gravity of about 2'9, and is otily 

 attacked with difficulty by potash or hydrofluoric 

 acid. The amorphous form exists naturally in 

 opal, and is obtained artificially as gelatinous 

 silica, Ac. ; it differs from the former in its specific 

 gravity, being about 2 '2, and in its beinp rapidly 

 dissolved by potash and by hydrofluoric acid. Pure 

 silica (as it occurs in rock-crystal, for example^ 

 Ls perfectly transparent and colourless, anu is 

 sufficiently hard to scratch glass. The heat oi 

 the oxyhydrogen blowpipe is required for its fusion, 

 when it melts into a transparent glass, capable 

 of bein^ drawn out into elastic threads. Perfectly 

 pure silica in its amorphous form may be obtained 

 by various chemical processes. If a solution of 

 silicate of potash or soda be treated with hydro- 

 chloric acid, the silicic acid separates as a hydrate, 

 and on evaporating this to dryness, and treating 

 it with boiling water, silicic acid remains as an 

 amorphous powder, which, after being washed, 

 dried, and exposed to a red heat, may be regarded 

 as chemically pure. The hydrateu silicic acid 

 mentioned in the al>ove experiment is soluble in 

 water, and (more freely) in acids and alkalies. 

 The solubility of hydrated silicic acid in water 

 accounts for the presence of silicic acid in mineral 

 springs and in the geysers of Iceland, as well 

 as for its gradual separation from these waters 

 in the form of petrifactions. That silica or silicic 

 acid is a true acid (although a feeble one) is ob- 

 vious from its uniting with bases, especially those 

 which are capable of undergoing fusion, and 

 forming true salts, known as silicates. These 

 silicates occur abundantly in nature, all the 

 forms of clay, felspar, mica, hornblende, augite, 

 serpentine, &c. being compounds of this descrip- 

 tion. 



Most of the silicates are fusible, the basic 

 silicates fusing more readily than those which are 

 either neutral or contain an excess of acid. 

 Excepting the silicates of the alkalies, no silicates 

 are soluble in water. The anhydrous, neutral, 

 and acid silicates of the earths resist the action of 

 all acids except the hydrofluoric. 



Silica derives ite name from the Latin silex, 

 'flint,' of which it is the essential constituent, 

 and is largely employed in the manufacture of 

 glass, china, and porcelain. For these pur- 

 poses it is obtained in a finely comminuted 

 state by heating flints or portions of colourless 

 quartz to redness, and plunging them in cold 

 water. The silica splits up into a friable mass, 

 which may be easily ground to a fine powder. 

 The use of silica in giving firmness and rigidity to 

 various parts of the animal organs is exemplified 

 in its free occurrence in the quill-part of the 

 feathers of birds, in the shields of certain infusoria, 

 and in the spicula occurring in sponges ; while its 

 similar use in the vegetable kingdom is seen in its 

 more or less abundant presence in the stalks of the 

 grasses, more particularly in the cereals and in the 

 bamboo (where it is especially deposited about 

 the joints, and is known as Tabasheer), in the 

 EniiiseUe, &c. 



Silicon may be made to combine with several 

 other elements besides oxygen, but, with the excep- 

 tion of silicofliioric acid, these compounds are of no 

 practical value. Thus, silicon and hydrogen form 

 a hydride of silicon, a colourless and spontaneously 

 inflammable gas. Nitride of silicon is a bluish 

 fibrous body, while sulphide of silicon is a white 

 iirtliy powder. Silicon unites with chlorine, 

 bromine, and probably iodine and fluorine, in two 

 proportions corresponding to its oxygen com- 

 pounds. Fluoride of silicon, SiF 4 , is a colourless 

 pungent gas, liqiiefiahle under strong pressure, and 

 solidifying at - 220, inflammable, and a non-sup- 

 porter of combustion. It is obtained by heating 



powdered glass with twelve times its weight of oil 

 of vitriol, and when a stream of this gas is trans- 

 mitted through water a reaction takes place ; two 

 atoms of water and three atoms of the fluoride of 

 silicon yielding silicofluoric acid, H 2 SiF 6 , which 

 remains in solution, and silicon, which is deposited. 

 A saturated solution of this acid forms a very sour 

 fuming liquid, which does not directly attack glass, 

 but if allowed to evaporate on it causes erosion 

 from the fluoride of silicon becoming evaporised, 

 and free hydrofluoric acid being left. A dilute 

 solution is sometimes employed in the laboratory 

 as a precipitant of potash, which it throws down 

 in a transparent gelatinous form. With salts of 

 baryta it gives a white crystalline precipitate. It 

 combines with bases to form salts. 



For soluble glass, a silicate of soda or potash, 

 see GLASS, Vol. V. p. 245 ; and for silicate cotton, 

 see SLAGS. 



Sillqne (Siliqua), in Botany, the fruit of the 

 Cruciferse. See FRUIT, Vol. V. p. 19. 



_ Silisl ria. a town of Bulgaria, is situated on the 

 right bank of the Danube, here li mile wide, about 

 70 miles NW. of Varna. Owing to ite strategic 

 position it has for many centuries been a more or 

 less formidable fortress, especially since the 14th 

 century, under the dominion of the Turks. The 

 Roman Durostorum, it was captured by the 

 Russians under Sviatoslaff in 867, but was re- 

 covered in 971 by the Byzantine emperor, John 

 Zimisces. It has been repeatedly besieged by the 

 Russians. They destroyed the works after captur- 

 ing it in 1810 ; but the fortifications were rebuilt 

 more strongly than before, and offered a stout 

 resistance to the Russian attacks in 1828-29. In 

 1849 it was made a stronghold of the first class, and 

 was rendered almost impregnable by the addition 

 (1853) of twelve detached forte on the south and 

 east. On the outbreak of the Crimean war the 

 Russians laid siege to it with an army of from 

 60,000 to 80,000 men, but were compelled to 

 retreat after thirty-nine days. In 1877, again, it 

 successfully defied the troops of the czar. The 

 Congress of Berlin in 1878 decreed that the forti- 

 fications should be dismantled ; but this has not 

 been given effect to. Pop. (1893) 11,710, who 

 weave cloth, tan leather, and grow vegetables. 



Stilus Italicus, a minor Latin poet, was born 

 in 25 and died in 101 A.D. At an early age he 

 became a prominent forensic orator, was consul 

 the year of Nero's death (69), became a familiar 

 friend of Vitellius, and was afterwards proconsul 

 in Asia. He was a devoted student of Cicero and 

 Virgil, and owned their estates at Tusculum and 

 Naples. In old age, finding himself labouring 

 under an incurable disease, he starved himself to 

 death. His epic poem, Pnniea, in seventeen books 

 and about 14,000 lines, has come down entire, and 

 remains a monument of industry, of patient imita- 

 tion, not of poetic creation. Scipio and Hannibal 

 are ite Achilles and Hector, ite ./Eneas and Turnus ; 

 and every episode in his great originals is slavishly 

 reproduced and degraded to a dead level of literary 

 mediocrity. 



The poem was discovered by Poggio about 1416, and 

 the editin princept appeared in 1471. Editions are by 

 Krnesti (1791) and Lemaire (1823). 



Silk. The Chinese appear to be the first people 

 who applied themselves to sericulture, although 

 lome claim for the Tussnr silk of 

 India the earliest silk fibre used. 

 The words Seres used by Theo- 

 >hanes and Serinda by Procopius were in all 

 probability so used to indicate that part of the 

 East, which was no doubt China, where the silk 

 ndustry existed at a very remote period. Ptolemy 

 was the first to use the word Series for China, or 



Copyright 1892. 1897, and 

 1MO In the U. S. 1)7 J. B. 

 Llppfncott Gompauj. 



