782 SILICA 



air can enter the lantern than is required for the maintenance of proper com- 

 bustion. 



With reference to the cost of artificial light from oil, obtained by the Silber system, 

 and compared with the cost of gas, we have the evidence of Mr. William Valentino, 

 who says : 



4 The principle involved in the construction allows of the burning either of mineral, 

 vegetable, or animal oils, and of oils which hitherto have never been burnt to advantage, 

 such as the so-called heavy mineral oils. 



4 The same principle is equally applicable to ordinary moderator lamps, with the 

 additional advantage that largely-increased lights can now be constructed on the prin- 

 ciple of the moderator Limp burning colza or mineral oils. 



4 The light produced by the burners constructed on these new principles is whiter 

 and steadier than any light I am acquainted with; and the increase in the illuminating 

 power, as compared with that from ordinary burners hitherto in use, is fully equal to 

 from 40 to 50 per cent, without any increased consumption of oil. 



4 My experiments show that a light can be produced from mineral oils 40 to 50 per 

 cent, cheaper than the same light from coal-gas, costing 3s. Qd. per 1,000 cubic fuet. 



4 Experience has shown that the burning of properly-purified mineral oil has no 

 detrimental effect upon animal and vegetable life.' 



It is known that the volatilisation of colza or other vegetable oil cannot be properly 

 effected if the oil is allowed to ascend in excessive quantity to the top of the wick. 

 On this account, instead of allowing the oil to overflow the top of the wick-case, as it 

 does in ordinary moderators. Mr. Silber drills holes through the case about a quarter 

 of an inch below the top. The overflow in the Silber lamps takes place by means of 

 these holes, and combustion is carried on with only a small portion of the wick raised 

 above the case. By this arrangement also, the wick is preserved for a much longer 

 time than when it is exposed to the atmosphere as in ordinary moderators. 



Signal-lamps for railway and other purposes are constructed of two parts : the outer 

 casing, or lantern ; and the inner part, or lamp proper. Generally speaking, the lantern 

 is a fixture, from which the lamp is withdrawn to be trimmed and lighted. In the Silber 

 signal-lamp, the lamp proper is so portable that a man can carry three or four in his 

 hands at once, either lighted or unlighted, in any weather. Whilst thus carried the 

 lamp remains effectually closed ; but as soon as it is in its place within the lantern, it 

 is opened .it the top by the falling of a self-acting handle, so as to allow the escape of 

 the products of combustion. This self-acting handle is a very important matter, 

 because it prevents mistakes, which frequently happen with other lamps, from the 

 man forgetting to open the top. Such negligence causes the lamp to go out as soon 

 as it has consumed the limited supply of air enclosed in it, and then of course, although 

 the lamp is supposed to be in action, no signal is displayed, and serious mischief may 

 result. 



Another advantage is, that the oil -reservoir of these lamps drops through the inner 

 shell of the lamp, and is kept cool by being exposed to wind and weather, while the 

 flame is perfectly protected. 



SXXiESXAZr I.INENS. See FLAX and LIKEN. 



SXIiBX. Quartz, or pure flint. See SILICA. 



SILICA or SllilCIC ACID. SiO*. This substance exists nearly pure in rock- 

 crystal, chalcedony, opal, agate, and many other minerals ; and it is an important 

 constituent of a very large class of minerals. See AGATE ; OPAL ; QUARTZ. 



It may be obtained perfectly pure by precipitation from any of its combinations. 

 Silicic acid forms a class of salts termed silicates, which are generally formed by 

 fusing silicic acid with the bases. Those silicates in which the acid predominates are 

 insoluble in water, and constitute the different varieties of glass. See GLASS. 



Kecent researches have shown that crystallised silica exists in nature under three 

 distinct forms: (1) Quarts, crystallising in the hexagonal system, with sp. gr. 2'6; 

 (2) Tridymite, crystallising in the same system, but with different parameters, and with 

 sp. gr. 2 - 3 ; (3) Asmanite, belonging to the rhombic system, and having a sp. gr. of 2-24. 



Some curious natural deposits of silica are found in nature. Way discovered at 

 Farnham large deposits of silica, in the condition in which it is readily soluble in hot 

 solutions of caustic potash, or soda. These beds are situated at the base of the chalk 

 formations, between the Upper Greensand and the Gault Clay. Mr. Way proposed 

 to employ those beds as a convenient source of silicate of lime for agricultural pur- 

 poses. He found that a mixture of slaked lime witli the powdered rock, when made 

 into a thin paste and loft for some weeks, is entirely converted into silicate of lime. 

 The action is promoted by the presence of 2 or 3 per cent, of carbonate of soda ; the 

 latter appearing to act as the carrier between the silica and the lime. Similar deposits 

 had been previously found by Sauvage in the Department des Ardennes. 



Siliceous deposits are often formed from warm springs. In the Island of Terceira 



