S I L 



It 19 a fine white powder, harfh to the touch, and defti- 

 tute of tafte or fmell. Its fpecific gravity is 2.6. 



Silica is not afted upon by the air, nor changed by moif- 

 ture. When ftriftly pure, no ordinary heat will fufe it. 

 It does however put on appearances of fufion, when heated 

 by the aid of a blowpipe and oxygen gas. It does not 

 combine with oxygen, fulphur, phnfphorus, carbon, or 

 azote. 



The two fixed alkalies do not affeft it in the cold, but 

 ■when fufed with it in a filver crucible they combine, forming 

 compounds, which have all the appearance of glals. Indeed, 

 when the two bodies are in one proportion, they form the 

 purell and mod perfeft glafs. When the alkali is to the 

 filica as three to one, the compound is foluble in water. 

 This folution has been called the liquor of Jlints. Any of 

 the acids are capable of combining with the alkali, and pre- 

 cipitating the filica in a ftate of purity, as v/e have before 

 obferved in the procefs for obtaining filica. 



If an excefs of acid be added to the folution, part of the 

 filica is diflblved. This is more efpecially the cafe with the 

 muriatic acid. A very dilute folution of filica in potafh, in 

 a veilel covered with paper, was left at reft by profeflbr 

 Seigling for eight years. Cry ftals of iilica were found in the 

 fluid, and the furface had a tranfparent crull upon it, ftrong 

 enough to allow the veilel to be inverted without fpilling the 

 liquid. Some of the cryftals were found to be pure Iilica, 

 in groups of tetrahedral pyramids. There were alfo cryftals 

 of fulphate and carbonate of potafh. The former were fo 

 hard as to ftrike fire with fteel. 



When fluoric acid is diftiUed from a glafs veflel, or from 

 any other fubftance containing filica, this acid aflumes a 

 complete gafeous form. The moment it is abforbed by 

 water, a proportion of fihca is precipitated. In this cafe, 

 the gas which comes over is a compound of filica and the 

 acid, which may be called fluat of filica. The liquid acid 

 is alfo found to hold filica in folution. It was from this 

 folution, after ftanding two years, that Bergman obtained 

 cryftals of pure filica. They were of a cubic form, with 

 three angles truncated. They were not fo hard as rock- 

 cryftal. 



The boracic and phofphoric acids have no aftion upon 

 filica in the cold, but unite with it by fufion, forming tranf- 

 parent vitreous fubftances. 



Silica does not combine with any of the metals, but it 

 combines with many of their oxyds, forming compounds, 

 which are called glaffes, enamels, or porcelains, according 

 to their appearance. 



With the oxyd of lead it forms the glazing of common 

 pottery ; with oxyd of iron, a dark green or black glafs. 



We have already fpoken of its combination with the 

 alkalies which belong to this clafs of compounds. 



When a folution of lime or barytes in water is added to 

 a folution of filicated potafli, or liquor of flint, thofe 

 earths become precipitated, forming a peculiar compound. 



A fimilar combination takes place when a folution of 

 filica in potafti is added to a folution of alumine in the fame 

 alkali. 



Although the fuCbility of filica is impraaicable at the 

 heat of our hotteft furnaces, yet its combination with 

 other earths is fufible, though at a very high temperature. 

 Equal parts of lime and filica fufe into a mafs between 

 porcelain and enamel at 150^ of Wedge wood. We hence 

 fee the ufe of lime in fmelting iron ores which abound with 

 filex. 



Silica appears to be the moft abundant of the earths, 

 forming the greateft proportion of the primitive rocks, and 

 the bafis of the terrettrial globe. 



I2f 



S I L 



Although it has not been direftly proved to be a metallic 

 oxyd, there is the moft prefumptive reafon for thinking it 

 fo. From the fmall adion which acids had upon it, com- 

 pared with the other earths, fir Humphrey Davy at firtt 

 lufpefted it to be an earth already combined with an acid, 

 and made fome attempts, by the aid of Galvanifm, to verify 

 this idea, but without fuccefs. He fuled filica with iron 

 by the Galvanic battery, and obtained a mafs which afforded 

 filica when diflblved by an acid with water. He alfo heated 

 filica to whitenels, and on bringing potaflium in contaft, a 

 compound was formed of filica and potalh, but a number of 

 black particles were diffufed through the mafs, which 

 fir Humphrey Davy thought were conductors of elec- 

 tricity. They did not aft upon water ; but when an acid 

 was added, an effervefcence took place. They alfo burnt 

 in a ftrong heat, affording a white fubftance, which had the 

 charafters of filica. 



Little is known of the proportions in which fiUca com- 

 bines with other bodies. Dalton gives its atom as forty, 

 five times heavier than hydrogen : and fir Humphrey 

 Davy ftates the number for filica at fixty-one, which, re- 

 duced to Dalton's ftandard, would be 30.J. But neither 

 of thefe is to be depended upon. 



SILICEOUS, in Mineralogy^ denotes corapofed princi- 

 pally of filex. 



SiLlCEOL'S Schyius, in Mineralogy and Geology, the hom- 

 ftone flate of fome geologills ; flinty flate of Jamefon ; a 

 rock of the nature of (late, but containing a great portion 

 of filiceous earth. It frequently occurs in beds in clay-flate, 

 and fometimes forms entire mountains, which are either 

 homogeneous or porphyritic, containing cryftals of felfpar, 

 and forming porphyritic flinty flate. The colour confifts 

 of various ftiades of grey, but it is fometimes red, approach, 

 ing the nature and colour of jafper. It is generally tra- 

 verfed by veins of quartz, but rarely, if ever, contains me- 

 tallic veins. It is extremely hard, and breaks with difficulty ; 

 the fragments are fttarp.edged, and more or lefs tranflucent 

 in minute portions. The Lydian ilone, which is ufed as a 

 teft, or touch-ltone, for determining the purity of metals, i» 

 nearly allied to filiceous fchiftus. This rock paffes, by 

 gradation, into clay-flate ; and when the filex predominates, 

 into hornftone or chert. 



Siliceous Earth, in ylgrlculiure, fuch as is conflituted of 

 filiceous materials. 



Siliceous Marie, that which is of a fandy or filiceou.3 {| 

 nature. See Marle. 



SILICERNIUM, among the Romans, a funeral fupper,. 

 which is otherwife called exequium. 



SILICIATE, in Mineralogy, a term lately introduced by 

 profeffor Berzelius, to denote the combination of filex 

 with other earths or oxyds, in which the filex is fuppofed ttv 

 aft as an acid. Thefe fubftances he denominates filiciates. 

 As this view of the aftion of filex tends to throw confiderable 

 light OR various procefles in the mineral kingdom, it is but 

 juftice to our own countryman, Mr. Hume, a refpeftable 

 fcientific and prafticable chemift in London, to ftate, that 

 fo early as 1805, he had, in Mr. Park's Chemical Catechifm, 

 allerted the aftion of filex as an acidifying principle, and 

 fubfequeutly in the Philofophical Magazine for 1808, he 

 diftinftly and perfpicuoufly defcribed various proceft'es, both 

 natural and artificial, in which filex performed the part of 

 an acid : this he attributed to the great proportion of 

 oxygen contained in it. As filex is by far the moft abun- 

 dant fubftance in the compofition of the globe, either pure 

 or in combination with the other earths, its peculiar pro. 

 perties, and the part which it performs in the fuccefiive 

 changes that take place in the mineral kingdom, are a fub- 



jeft 



