SILHOUETTE SILICATE. 



255 



cooling, the appearance of glass. This glass dis- 

 solves in water. If to an aqueous solution of it 

 we add as much muriatic acid as will saturate the 

 alkali, and concentrate the solution sufficiently, 

 the silica assumes the form of a white translucent 

 jelly. This characterizes silica. If we evaporate 

 the whole to dryness, and wash off the salt of 

 potash from the dry mass, the silica remains behind 

 in the state of a very fine powder. Silex consists 

 of about 48 silicon, and oxygen 52. It may be sub- 

 jected to a very violent heat, without suffering any 

 change; there is no difficulty in causing it to melt, 

 however, before the compound blow-pipe. Though 

 silex does not redden vegetable blues, yet it enters 

 into definite compounds with the different bases, 

 and forms saline compounds, which arc distinguished 

 by the name of silicates, and is hence called silicic 

 acid by some writers on chemistry. Like other 

 weak acids, it is capable of entering into a great 

 variety of combinations with bases. The combina- 

 tions which it forms are frequently so intimate, 

 that no other acid is capable- of removing the base 

 and setting the silica afr liberty. The different 

 kinds of glass consist of two or more silicates 

 melted together. Silicon burns vividly when 

 heated in chlorine gas, and the compound formed 

 is a colourless liquid, which is a chloride of silicon. 

 It evaporates almost spontaneously, in the form of 

 a white vapour, when exposed to the open air. It 

 boils at a temperature below 212. It reddens 

 litmus paper very strongly. When dropped into 

 water, it swims on the surface of that liquid. It 

 is gradually dissolved in the water, but deposits, at 

 the same time, a little silica. Silicon unites with 

 fluorine, and forms an acid gas, which is called 

 fluosilicic acid. It is easily obtained by mixing 

 together fluor spar and glass, or quartz, both in fine 

 powder, in a small retort, and adding a sufficient 

 quantity of sulphuric acid, to form the whole into 

 a semi-fluid mass. When heat is applied to this 

 mixture, fluosilicic acid comes over in the form of 

 a transparent invisible gas. This gas is rapidly 

 absorbed by water, while silica is deposited in a 

 gelatinous state. Its specific gravity is 3-6. It 

 consists of fluoric acid 17'6, silica 27'2. When 

 potassium is heated in it, combustion takes place, 

 and a chocolate-coloured substance is formed, which 

 differs in its nature, according to the proportion of 

 potassium employed. Fluosilicic acid gas combines 

 with twice its volume of ammoniacal gas, forming 

 a volatile salt. Silicon and carbon combine, when 

 they come in contact in a nascent state. The car- 

 buret formed is a dark brown powder. Sulphuret 

 of silicon is formed by heating silicon in the vapour 

 of sulphur, and the union is attended with the 

 phenomena of combustion. It is a white, earthy- 

 looking substance, which is instantly converted by 

 the action of water into sulphureted hydrogen and 

 silica. Silicon is more allied to carbon and boron 

 than to the metals. 



SILHOUETTE is the representation of the 

 outlines of an object filled with black colour, in 

 which the inner lines are sometimes slightly drawn 

 in white. The name comes from Etienne de Sil- 

 houette, French minister of finance in 1759. He 

 strove by severe economy to remedy the evils of a 

 war which had just terminated, leaving the country 

 in great exhaustion. At the end of nine months, 

 he was obliged to leave his place. During this 

 period, all the fashions in Paris took the character 

 of parsimony. Coats without folds were worn ; 

 snuff-boxes were made of plain wood ; and, instead 



of painted portraits, outlines only were drawn in 

 profile, and filled with Indian ink, &c. All these 

 fashions were called d la Silhouette; but the name 

 remained only in the case of the profiles, because 

 the ease with which they may be drawn, or cut 

 out of black paper, makes them popular, though, 

 considered as works of art, they have little value. 

 Some faces those with a marked profile are easily 

 taken in this way, whilst others lose their character 

 entirely, particularly those whose traits are well har- 

 monized. In general, the extremes of expressions, as 

 the expression of great savageness or sternness, great 

 mildness or obstinacy, deep reflection, or great stu- 

 pidity, are those most easily given in this mode. 

 But the expression of blended imagination, thought, 

 and goodness of disposition, is not easily conveyed 

 in profile. On the whole, the silhouette expresses 

 more the original disposition of the mind, than its 

 cultivated character. These representations may 

 be taken very well from the shadow of a person on 

 a paper held on the wall ; and in order to make the 

 shadow more steady, it is well to rest the head on 

 a book or the like, put between the face and the 

 paper. The paper is then cut according to the out- 

 line of the shadow, and the outer surface pasted 

 on black paper. The likeness can be taken still 

 better, and of any size, by means of an instrument 

 called a pantograph. A frame with a glass, on 

 which paper slightly oiled and well dried is fastened, 

 attached to a chair on which the person sits, whose 

 likeness is to be taken, is also of much advantage. 

 The invention of the silhouette, in ancient times, 

 is said to have given rise to the art of painting. 

 This invention is ascribed to the daughter of the 

 potter Dibutades, who drew the outline of her 

 lover's shadow on the wall. The time of this in- 

 vention may be placed at the renewal of the Olym- 

 pic games, shortly before the expulsion of the Bac- 

 chiades from Corinth, about 776 B. C. Sicyon 

 and Corinth were the first places where painting 

 flourished. Crato of Sicyon, Philocles of Egypt, 

 and Cleanthes of Corinth, are mentioned as inven- 

 tors of the monochromes, they having filled the 

 outline with colours. The silhouettes were soon 

 applied to large objects; thus Saurias of Samos 

 drew the shadow of his horse on a wall. The esti- 

 mation of these drawings with the ancients, the 

 beauty and delicacy with which they were executed, 

 may still be seen from the Etruscan vases. 



SILICATE OF MANGANESE; the name of a 

 chemical family in mineralogy, which consists of 

 two mineralogical species, viz. the Fowlerite and 

 the manganese spar, or siliceous oxide of manganese. 

 The Fowlerite occurs in feld-spar-looking crystals, 

 differing, however, in the valve of its angles, from 

 feld-spar. Its angles are 95 0', 121 0', and 113 

 0'. Its hardness is the same as feldspar ; specific 

 gravity 3-5 to 3-8; colour flesh-red. It is fusible 

 with difficulty, and tinges borax red. It consists ot 



Silex, 



Protoxide of manganese, 



Peroxide of iron, 



Water, 



29-48 



5058 



13-22 



3-17 



The manganese spar occurs massive, fine, granular, 

 and, rarely, somewhat fibrous; colour rose-red; 

 lustre intermediate between pearly and resinous; 

 translucent; hardness but little inferior to feldspar 

 specific gravity, 3-5. Heated before the blow-pipe, 

 it becomes dark-brown, and melts into a reddish 

 globule. It tinges borax hyacinth-red. It con- 

 sists of 



