Cooley's Cyclopaedia of Practical Receipts Volume Ii Part 209

The test for a silicate consists in fusing the suspected body with sodic or pota.s.sic carbonate, heating the residue with acid, and evaporating to dryness. If the residue be then treated with hot water the silica remains undissolved in the form of a white powder, which will yield a colourless bead when fused with sodic carbonate upon a piece of platinum foil before the blowpipe flame. If silica be fused with borax it becomes slowly dissolved, forming a clear, colourless bead.

Chapman contests Plattner's opinion that, when silicates are fused with phosphor salt, the 'silica skeleton' that results is especially due to the presence of alkalies or earthy bases.

Chapman says: "It is true enough that silicates in which these bases are present exhibit the reaction; but as other silicates--practically all, indeed--exhibit the reaction also, the inference implied in the above statement is quite erroneous.

"The opalescence of the gla.s.s arises entirely from precipitated silica.

"If some pure silica (or a silicate of any kind), in a powdered condition, be dissolved before the blowpipe flame in borax until the gla.s.s be saturated, and some phosphor salt be then added, and the blowing be continued for an instant, a precipitate of silicate will immediately take place, the bead becoming milky white (or, in the case of many silicates, opaque) on cooling. This test may be resorted to for the detection of silica in the case of silicates, which dissolve with difficulty in phosphor salt alone, or which do not give the p.r.o.nounced 'skeleton' with that reagent."[162]

[Footnote 162: Chapman on 'Blow-pipe Reactions.']

_Prop., &c._ A fine, white, tasteless, infusible powder, insoluble in all acids, after being heated, except the hydrofluoric; requires the heat of the oxyhydrogen blowpipe for its fusion; approaches the precious stones in hardness; soluble in strong alkaline solutions; its salts are called SILICATES. Sp. gr. 266. See GLa.s.s, GLa.s.s, SOLUBLE, &c.

=Silica, Hydrates of.= By pouring a dilute solution of sodium silicate into a considerable excess of hydrochloric acid the whole of the silica is retained in solution, together with the chloride of sodium formed by the action of the hydrochloric acid upon the soda. By subjecting this solution to dialysis (see DIALYSIS) the hydrochloric acid and chloride of sodium are removed, whilst the hydrate of silica is left behind, in solution, in the dialyser. Graham recommends a stratum of the liquid four tenths of an inch in depth, to be subjected for four or five days to dialysis, the water in the outer vessel to be changed every twenty-four hours.

If the solution so obtained be carefully evaporated down in a flask, and any drying of the silicic acid at the edges of the liquid being prevented, a solution may be obtained containing 14 per cent. of silica.

The solution has a very feebly acid reaction, and is without taste or colour. It cannot be preserved in the liquid state for more than a very few days, even in well-closed vessels, but becomes converted into a transparent gelatinous ma.s.s, which separates from the water. Hydrochloric acid, as well as small quant.i.ties of caustic potash or soda, r.e.t.a.r.d the coagulation.

When the solution is evaporated _in vacuo_ at 59 F. over sulphuric acid, a l.u.s.trous transparent gla.s.s is left behind, which consists of 22 per cent. of water, which closely accords with the formula SiO_{2}H_{2}O.

By the action of moist air upon silicic ether a transparent gla.s.sy hydrate was obtained by Ebelmen, to which this chemist a.s.signed the formula 2SiO_{2},3H_{2}O. Two hydrates of silica were obtained by Fuchs, one having the formula 3SiO_{2},H_{2}O, the other, 4SiO_{2},H_{2}O.

=Silicic Chloride.= _Syn._ SILICIC TETRACHLORIDE. (SiCl_{4}.) This compound is rarely, if ever, obtained by the direct method, viz. by heating silicon in chlorine, but by the following indirect process:--A paste is made of finely divided silica, oil, and charcoal, and heated in a covered crucible. The fragments of the charred substance (consisting of silica and carbon) are then placed in a porcelain tube, which is raised to a red heat in a furnace, and during the ignition of the fragments a current of dry chlorine is pa.s.sed into and over them; the silicic chloride which is thus formed being made to distil over into a bent tube surrounded by a freezing mixture of ice and salt, whereby it becomes condensed.

Silicic chloride is a very voluble and strongly fuming gas, transparent and colourless, with an irritating and pungent smell. It is immediately decomposed by water into hydrochloric acid and hydrated silica, which deposits in the vessel.

=Silicic Fluoride.= _Syn._ SILICIC TETRAFLUORIDE. (SiF_{4}.) This gas is best prepared by heating in a capacious flask or retort equal parts of finely powdered fluorspar and white sand, or gla.s.s with ten or twelve times their weight of strong sulphuric acid. This gas must be collected over mercury, and in jars that are free from the least trace of moisture.

Silicic fluoride is a colourless gas, with a very pungent odour, fuming strongly in the air, and neither burning nor supporting combustion.

Faraday succeeded in liquefying it under great pressure, and Natterer states that at a temperature of -220 F. it may be solidified. By water it is partially decomposed and partially dissolved, yielding silicic acid and hydrofluosilic acid.

With twice its volume of ammoniacal gas silicic fluoride combines to form a crystalline volatile. Silicic hydride has an acid reaction.

=Silicic Hydride.= (H_{4}Si?) To procure this gas silicide of magnesium is decomposed with cold diluted hydrochloric acid.

The silicide of magnesium may be prepared as follows:--Mix intimately 40 parts of fused magnesic chloride, 35 of dried sodic silico-fluoride, and 10 of fused sodic chloride; these are mixed in a warm, dry tube, with 20 parts of sodium in small fragments, and thrown into a red-hot Hessian crucible, which is immediately covered, the operation being finished when the vapours of sodium cease to burn.

Silicic hydride becomes spontaneously ignited in the air, and in doing so gives off white fumes, which consist of amorphous silica (SiO_{2}). A cold body, such as a piece of porcelain or gla.s.s, introduced into the flame, becomes covered with a brown deposit of reduced silicon. Pa.s.sed into solutions of cupric sulphate, argentic nitrate, and palladium chloride, this gas throws down the metals, in most cases combined with silicon.

=SILICOFLUORIC ACID.= See FLUOSILICIC ACID.

=SIL'ICON.= Si. _Syn._ SILICIUM. An elementary substance forming the base of silica.

This element was first obtained by Sir Humphry Davy, by acting upon silica with pota.s.sium. It is now procured much more easily by the decomposition of silico-fluoride of pota.s.sium, at an elevated temperature, with pota.s.sium or sodium. By heating a mixture of fluorspar and ground flints with sulphuric acid a gaseous tetra-fluoride of silicon is formed, which, being partially soluble in water, yields an acid solution of the tetra-fluoride. Caustic potash is then added to the acid solution of the tetra-chloride until it becomes neutralised, and the sparingly soluble silico-fluoride of pota.s.sium thus formed is thoroughly dried and mixed in a gla.s.s or iron tube with eight or nine tenths of its weight of pota.s.sium or half its weight of sodium, and heated. The following equation explains the reaction that takes place:

2KF,SiF_{4} + 2K_{2} = Si + 6KF.

The resulting ma.s.s, consisting of pota.s.sic fluoride, and reduced silicon in partial combination with the excess of pota.s.sium, is treated with cold water, when a copious evolution of hydrogen gas ensues, owing to the decomposition of the water by the excess of pota.s.sium. The pota.s.sic fluoride is got rid of by washing with cold water, its entire removal being indicated by the water ceasing to have an alkaline reaction on test paper, whilst amorphous silicon is left behind in the form of a brown powder.

Another method by which silicon may also be procured is by pa.s.sing the vapour of silicic chloride over heated pota.s.sium or sodium, placed on a porcelain tray in a gla.s.s tube. In this operation it is advisable to protect the lining of the tube with thin plates of mica.

The silicon obtained by the above processes is known as _amorphous silicon_, and, as already stated, occurs as a brown powder. It is dull in colour, and, being heavier than water, as well as insoluble in it, sinks in that fluid. It is a non-conductor of electricity, is unaffected by nitric or sulphuric acid, but dissolves readily in hydrofluoric acid, and in a warm solution of caustic potash. It burns with great brilliancy when heated in air or oxygen, and becomes converted into silica, which, owing to the great heat of combustion, fuses, and thus forms a superficial crust over the unburnt silicon. A _crystalline_ variety of silicon may also be procured by heating the brown amorphous silicon, already described, intensely in a platinum crucible, with exclusion of air. This crystalline silicon so obtained is much darker in colour than the amorphous, and also considerably denser; besides which it differs widely in properties from the latter. It will not take fire if heated strongly in the air or oxygen, even if before the blowpipe flame. Its density is such that it sinks in strong sulphuric acid, and hydrofluoric acid fails to dissolve it, although it is soluble in a mixture of hydrofluoric and nitric acids. It does not become oxidised, even if fused with pota.s.sic nitrate or chlorate, unless a white heat is obtained, when it burns brilliantly, giving rise, on so doing, to the formation of silica.

In addition to the above, a _graphitoid_ form of silicon, occurring in plates, has been described by Deville and Wohler. These chemists obtained this last modification from an alloy of silicon and aluminium, which was treated in succession with boiling hydrochloric and hydrofluoric acids.

The plates of silicon which are left have a metallic l.u.s.tre, and a sp.

gr. of 249. The graphitoid bears a great resemblance in properties to the crystalline silicon. It is a conductor of electricity. Like the crystalline variety, it dissolves in a mixture of hydrofluoric and nitric acids, although slowly, but, unlike the crystalline, it undergoes no change when heated to whiteness in a current of oxygen.

Deville states that silicon requires a temperature between the melting points of iron and steel to fuse it. He effected its fusion in a platinum crucible lined with lime, the platinum crucible being then placed in a clay crucible, which was then exposed to intense heat in a wind furnace.

By pa.s.sing the vapour of silicic chloride over pure aluminium, placed on a porcelain tray, and raised to an intense heat, the aluminium becomes volatilised as aluminic chloride, whilst the silicon remains behind in crystals possessing a reddish l.u.s.tre. These crystals occur in regular six-sided prisms, terminated by three-sided pyramids, derived from the octohedra, and are so hard that gla.s.s may be cut by them, in the same way as by the diamond.

With oxygen silicon forms only one oxide, silica, described above.

=SILK.= As an article of clothing, as far as "roundness of fibre, softness of texture, absence of attraction for moisture, and power of communicating warmth, are concerned, silk is greatly superior to both linen and cotton; moreover, it gives the sensation of freshness to the touch which is so agreeable in linen. But, with all these advantages, silk (when worn next the body) has its defects; on the slightest friction it disturbs the electricity of the skin, and thus becomes a source of irritation.

Sometimes, it is true, this irritation is advantageous, as causing a determination of blood to the surface; but when this action is not required, it is disagreeable, and quite equal, in a sensitive const.i.tution, to producing an eruption on the skin. I have seen eruptions occasioned in this manner, and, when they have not occurred, so much itching and irritation as to call for the abandonment of the garment."

(Eras. Wilson.)

Silk is characterised by its fibres appearing perfectly smooth and cylindrical, without depressions, even under a magnifying power of 160.

Its fibres (even when dyed) acquire a permanent straw-yellow colour when steeped in nitric acid of the sp. gr. 120 to 130. The fibres of white or light-coloured silk are similarly stained by a solution of picric acid. A thread of silk, when inflamed, shrivels and burns with difficulty, evolves a peculiar odour, and leaves a bulky charcoal. By these properties silk is distinguished from cotton and linen.

Cotton, wool, and silk may be easily distinguished from each other by means of the microscope.

The cotton fibre will be seen to consist of only one cell; wool (as well as hair and alpaca) is made up of numerous cells in juxtaposition; whilst silk fibre is similar to the secreted matter of spiders and caterpillars.

[Ill.u.s.tration: FIG. 1-3.]

The silk fibre (fig. 1) is smooth, cylindrical, devoid of structure, not hollow inside, and equally broad. The surface is glossy, and only seldom are any irregularities seen on it. If it is desired to detect in a woven fabric the genuineness of the silk, it is best to cut a sample to pieces, place it under water under the object-gla.s.s of a microscope, magnifying 120 to 200 times, covering it with a thin piece of gla.s.s. The round, glazed, equally proportioned silk fibre (fig. 1), is easily distinguished from the unequalled and scaled wool fibre (W in fig. 2), and from the flat, band-like, and spiral cotton fibre (B, fig. 3). Under the microscope also the mixture of inferior with superior fibres of silk can be easily detected.

Black silk, the weight of which has been augmented by extensive sophistication, is not uncommon in English, French, and German markets. It is known as 'weighted' or 'shotted' silk, and very frequently contains no more--and frequently less--than one third of its weight of silk, the remaining two thirds consisting, according to Persoz, of a combination of iron salts, with some astringent substance, salts of tin, and cyanides. It is easily distinguishable from genuine silk by its want of elasticity and tenacity, and its much greater combustibility. Persoz found a specimen of this adulterated silk to yield, upon incineration, more than 8 per cent.

of ferric oxide.

The cleaning and renovation of articles of wearing apparel made of silk are matters requiring some care. No silk goods look well after being washed, however carefully it may be done; and this method should, therefore, never be resorted to but from absolute necessity. It is recommended to sponge faded silks with warm water and curd soap, then to rub them with a dry cloth on a flat board, and afterwards to iron them on the wrong side with an ordinary smoothing iron. Sponging with spirit, benzol, or pure oil of turpentine, also greatly improves old silk, and is often preferable to any other method. The odour of the benzol pa.s.ses off very quickly, that of the turpentine after exposure for a few days. When the ironing is done on the right side thin paper should be spread over the surface to prevent 'glazing.' See DYEING, GILDING, &c.

=Silk Material, a New.= The 'Textile Manufacturer' contains the following:--The utilisation of new substances as raw material for manufactures is a distinguishing feature of the scientific investigations of the nineteenth century. One of the most recent suggestions is the result of the researches of Herr Tycho Tulburg, an eminent German naturalist, on the products of the mussel. It will be remembered it was from one of the mussel species the famous purple dye was in past ages obtained, and this colour gained an imperishable renown from its being adopted by the Roman emperors, and the imperial purple became the symbol of sovereignty. In these latter days animal products have been displaced by aniline dyes, and there is no likelihood of their regaining their former celebrity. The researches of Tulburg have not, however, been in the direction of dyes, but in the adaptation of animal products other than the silkworm for silk yarns. The mussel (_Mytilus edulis_) fastens itself to the rocks by strong threads, called by naturalists _byssus_, and it is this substance which it is proposed to utilise for the manufacture of silk. The material is of a silky texture and very tough, and the experiments that have been made prove that it is well adapted to be made into yarn. Already the _Pinna_, one of the mussel tribe, has been already manufactured into fabrics, although it is not of general use, nor at present of much commercial value, and the same obstacles to the use of the _byssus_ of the common mussel are apparent. Notwithstanding the abundant supply of this popular sh.e.l.l fish, it is difficult to see how a sufficient quant.i.ty of _byssus_ can be collected to enable manufacturers to purchase the raw material at rates low enough for a marketable remuneration on the manufactured article. But the records of industrial progress testify to greater difficulties than these having been successfully overcome; and should the commercial value of the new material be satisfactorily demonstrated, there is no doubt some agency will be developed whereby the requisite supply may be obtained.

At present it is sufficient to notice the discovery that has been made, and to welcome another instance of the results of scientific labour being for the advantage of manufacturers.

=SILK'WORM, Diseases of.= Silkworms are liable to a disease known as _pebrine_, which Pasteur has shown to be due to the presence, on the body, the egg, and in the blood of the insect, of peculiar parasitic corpuscles.

Pasteur states that the black specks which const.i.tute these bodies are very easily distinguishable in the moth of the silkworm, but that in the earlier stages of its development, such as in the egg and worm condition, the detection of them becomes difficult, if not impossible. Pasteur further adds that sound moths produce sound eggs, and unsound moths the reverse, and that although the unsound eggs show no sign of disease, they never give rise to healthy worms.

Pasteur advises the silk cultivator, therefore, to ensure breeding from healthy moths at starting, and to abandon the old and useless precaution of hatching apparently healthy eggs.

The loss resulting from the silkworm disease in Italy may be seen from the following tables, which are calculated for bales of 102 lbs.

weight:[163]--

[Footnote 163: 'British Manufactory of Industries,' Stamford.]