SILICON CALCIUM. ] 



CHEMISTRY. 



3SO 



fashioning into the shape of the articles required : l>nt 

 as, in the process of baking, the material contracts nearly 

 one-third in bulk, allowance is made in the size of the 

 article as it leaves the hands of the potter. Of all im- 

 plements of manufacture, the potter's wheel seems to 

 have undergone the least alteration by the lapse of time ; 

 for, on comparing the instrument in use at the present 

 <lay with those employed three thousand years ago, as 

 illustrated in ancient drawings and sculpture, their 

 identity is almost complete. 



The process of firing or baking is one requiring the 

 greatest care, because, if the material have been impro- 

 perly mixed, the different parts will contract unequally, 

 and the vessel will be destroyed. Each article, as it is 

 produced by the potter, is removed to a kiln fitted with 

 shelves, on which the vessels to be baked are arranged. 

 A fire is then kindled in the centre of the kiln, and by 

 it the mass of the articles is aggregated and made into 

 a compact body. The vessels so baked are allowed to 

 cool gradually, and so to assume their proper shape 

 and consistency. In this state, however, they are 

 porous, and unfit for domestic use. The next process, 

 therefore, is that of glazing, by means of which the 



j external surface is converted into a kind of glass. This 

 is effected by washing the surfaces of the vessels by 

 means of a mixture of powdered felspar, borax, (be., 

 fur the bettor class of goods, and of common salt for 

 common goods. The articles are again removed to a 

 kiln, and by means of an intense heat the surface is 

 fused, and a glaze formed thereon. On cooling, the 

 pottery is then ready for sale. If patterns such as we 

 observe in china and plates, <tc., be required, such are 

 painted on before glazing ; and in that process they are 

 " burnt in" on the surface of the ware. 



The preceding remarks afford a general insight into 

 the manufacture of all kinds of ceramic ware. Our 

 limits proscribe us from entering into minute details. 

 Those of our readers who may be desirous of full infor- 

 mation on the subject, and yet have not the opportunity 

 of visiting a pottery, may trace every part of the pro- 

 cess in the brick-field, which is a rough but accurate 

 type of every class of the manufacture. 



Alum. Alumina, in combination with an alkali and 

 sulphuric acid, gives rise to a series of compounds 

 known in commerce as alums. The manufacture of 

 alum is always carried on in close proximity to the place 

 where the slate or suitable clay is found ; and at Catnp- 

 sie and Hurlet, near Glasgow, immense quantities of the 

 article are produced. The slate is first roasted, by which 

 it is reduced into powder. Exposed to the atmosphere, 

 a chemical decomposition ensues. Sulphuric acid is 

 produced by the oxidation of the sulphide of iron, and 

 so a sulphate of alumina is formed. Potash being added 

 thereto, a double salt results, forming a sulphate of 

 potash and alumina, which is the ordinary alum of com- 

 merce. Instead of potash, soda or ammonia may be 

 siibntituted with a similar result. 



The chief uses of alum are those in dyeing ; for the 

 alumina which it contains has the power of attracting 

 the colouring matter of vegetable bodies. For this 



( reason it has been termed a mordaunt. The principle 

 of its employment for this purpose may be readily under- 

 stood by adding an infusion of many vegetable colouring 

 substances to one of alum, and on precipitating the alu- 

 mina by means of an alkaline carbonate, a coloured pre- 

 cipitate will be produced. In the process of dyeing, the 

 yarn or cloth is first steeped in a solution of alum, 

 and subsequently boiled in one of the colouring matter, 

 by mean* of which the colour is precipitated in a solid 

 form, and fills up the minute tul>es of which the fibres 

 of cotton and wool are composed. Alum is also em- 

 ployed in medicine as an astringent. The other com- 

 binations of alumina do not require special notice. 



SILICON. 



WB have placed this substance amongst those pro- 

 ducing earths, rather as a matter of convenience silica, 

 iU oxide, being one of the most common constituents of 



rocks and soils. It may be obtained from pure silex, 

 which is a combination of silicon with oxygen, by heat- 

 ing the silex with the potassium. It is a powder of a 

 dark-brown colour, and, in some respects, is more analo- 

 gous to carbon than to the metals. Its symbol is Si, 

 and its equivalent 21 ~3. 



Silica, silex, or silicic acid, is found in its purest state 

 as rock-crystal ; it is also the chief constituent of common 

 flints, and of the sand of the sea-shore, ibc. It is com- 

 posed of one equivalent of silicon with three of oxygen, 

 Si Oj. It may be prepared artificially by fusing sand 

 with four times its weight of carbonate of soda : the 

 product is afterwards to be dissolved in water and 

 hydrochloric acid, and the solution is evaporated to dry- 

 ness, when a white insoluble powder is afforded, which is 

 the silica. It has a gritty f>. 3ling in the mouth, and is hard 

 to the touch. Silica, in a crystallised state, is often to 

 be noticed on the outside of some kinds of grasses. A 

 chloride of silicon may be produced by similar means 

 mi'iitioned for obtaining chloride of aluminium, silicon 

 being substituted for silica in the process. Silicic acid 

 unites with soda, ifec., to form silicates; and, by such 

 means, a soluble glass is formed, which has been much 

 used as a preparation for coating stone liable to decay 

 by atmospheric influences. The application of silica 

 promises to be of great importance ; and it has been 

 proposed to arrest the progress of decomposition now 

 going on in the stone used in building the Houses of 

 Parliament, which is rapidly being disintegrated by a 

 variety of causes. 



The most important use of silica, in a commercial 

 point of view, is that of the manufacture of glass of all 

 kinds, of which it is an essential ingredient. For this 

 urpose, sand is fused with soda, together with a little 

 oxide of lead, to give it fusibility. The "metal," as it 

 is called, is kept in a state of fusion for some hours, 

 until all the ingredients are completely mixed. The 

 colour of glass varies, and has a greenish tint if oxide 

 of iron be present. Colon. -ed glass is produced by burn- 

 ing various metallic oxides into ordinary glass at a 

 high temperature. The oxides used for this purpose 

 have already been named in our previous pages, under 

 the heads of their individual metals. (See Iron, 

 Copper, Cobalt, Chromium, Manganese, <fec.) If a 

 proper proportion of alkali be employed, ordinary glass 

 is unacted on by air and moisture. The lapse of time, 

 however, indicates that a very slight action takes place 

 on most kinds of glass, although the progress is very 

 slow. Artificial gems are produced by melting together 

 metallic oxides, silica, and alkalies ; and to so great a 

 perfection has this art arrived, that some - productioas 

 of this kind almost equal the true stone in their beauty 

 of colour and other qualities. Hydro-fluoric acid is tho 

 only one in which silica is soluble, and it is accordingly 

 employed in etching on glass surfaces. 



CALCIUM. 



Tins metal, which is at present very rarely obtained, 

 may be produced by heating lime with potassium or 

 sodium. Its equivalent is 20, and the symbol Ca. It 

 is 'highly possible that if the attention of chemists 

 were turned to ita production in quantities, another 

 valuable addition would be made to our stock of useful 

 metals. The general properties of calcium are but little 

 known. 



The well-known earth, lime, is an oxide of the metal, 

 Ca O ; and it may be obtained by heating chalk (its 

 carbonate) to redness in an open vessel. By this, the 

 carbonic acid is driven off : access of air is required, be- 

 cause the gas is liable to form a heavy superstratum if 

 the operation be carried on in a closed vessel. On the 

 large scale, lime is produced by burning chalk or lime- 

 stone in a kiln. Soft and hard lime are respectively 

 produced by the use of these two materials. 



Lime is used for a great variety of purposes. With 

 sand, it forms mortar, after being slaked ; during which 

 process, a great amount of heat is set free : sufficient, in- 

 deed, to set fire to wood. It frequently happens that 



