Properties.—Soluble glass forms a viscid solution, which when concentrated becomes turpid and opalescent : it has an alkaline taste and reaction. The solution mixes in all proportions with water. When the density of the solu tion is 1.25, it contains nearly 28 per cent. of glass ; if the concentration be carried beyond this point, it becomes so viscid that it may be drawn out in threads like molten glass. Finally, the liquor passes to the state of a vitreous mass, whose fracture is conchoidal ; it then resembles common glass, except in hardness. When the solution is applied to other bodies, it dries rapidly at common temperatures, and forms a coat like a varnish. Solublewhen dried does not undergo any perceptible change when exposed to the air, nor does it attract from it either moisture or carbonic acid; neither has the carbonic acid of the atmosphere any well marked action on the concentrated solution ; but when a current of carbonic acid is passed through the solution, the glass is decomposed, and hydrate of silica deposited. But a weak solution becomes turbid on exposure to the air, and is after a time decomposed wholly. When the glass is impure, an efflorescence is formed after a while, which may be pro duced either by the carbonate and hyposulphate of potassa, or by chloride of potassium. Soluble glass dissolves gradually without residuum in boiling water; but in cold water the solution is so slow as to have led to a belief that it does not dissolve at all. It however never becomes entirely insoluble, except when it contains a much larger proportion of silica, or when it is mixed with other bodies, such as the earths, metallic oxides, &c., with which double or triple salts are formed, as is the case in the common glasses. Soluble glass which has been exposed to the air, and is afterwards submitted to the action of heat, swells and cracks at first, and melts with difficulty ; it then loses about 12 per cent. of its weight. It therefore contains, even when solid, a considerable quantity of water, which it does not lose when simply dried by exposure to the air. Alcohol precipitates it unaltered from its solution in water. When the solution is con centrated, but little alcohol is required for precipitation, and it need not be highly rectified. Pure soluble glass may therefore be easily obtained from an impure solution by the use of alcohol. The alcohol being added, the gelatinous precipitate is permitted to settle • the supernatant liquor is decanted, the precipitate collected, rapidly stirred after the addition of a little cold water, and subjected to pressure. In truth, however, this process is attended with some lose, for even cold water will rapidly dissolve the precipatated glass in consequence of its minute division. The acids decompose the solution of glass. They also act upon it when solid, separating the silica in the form of powder.
properties of soluble glass fit it for numerous and varied applica tions. It has been used in the theatre of Munich as a means of safety from fire. All sorts of vegetable matter, wood, cotton, hemp, linen, paper, &c. are, as is well known, combustible; but in order that they shall burn, two conditions are requisite, an elevated temperature, ana free contact of air, to furnish the oxygen necessary for their transformation into water and carbonic acid. When once set on fire, their own combustion develops the beat necessary to keep up the chemical action, provided they be in contact with air. If deprived of such contact, and made red hot, they will, it is true, yield inflammable volatile pro ducts, but the carbon which is left will not burn, as it is deprived of air, and thus the combustion will stop of itself. Such is the part which all the fixed fusible salts are capable of performing, if they be, in addition, composed of substances incapable of yielding their oxygen at a low red heat, to either carbon or hydrogen. These salts melt as the vegetable matter becomes heated ; they form upon it a coat impenetrable to the air, and either prevent'altogether, or limit its combustion. The phosphate and borate of ammonia have such a cha racter, but they are so readily soluble in cold water, as to be liable to objections which cannot be urged against soluble glass. Although soluble glass is of itself a good preservative from fire, it fulfils the object better when it is mixed with another incombustible body in powder. In this case the solution of glass acts in the same manner as the oil of painters. The several coats have more body, become more solid, and more durable ; and if the substance which is added be of proper quality, coagulate by the action of fire into a strongly adhesive crust. Clay, whiting, calcined bones, powdered glass, &c. may all be employed for this but we cannot yet say with certainty which of them is to be preferred. A mixture of clay and whiting appears to be better than either used separately. Calcined bones form with soluble glass a very solid and adhesive mass. Litharge, which, with the glass, makes an easily fusible mix ture, does not give a product fitted for coating wood, as the mixture contracts In drying ; it therefore cracks, and is easily separated. Flint glass and crude
soluble glass are excellent additions. The latter ought to be exposed to the air after it is pulverized, in order to attract moisture. If it be mixed with the solution, and be applied to any body whatever, it in a short time.forop a coating as hard as stone, which, if the glass be of good duality, is unalterable by exposure, and resists fire admirably. The scones of iron and lead, felspar, fluor, may all be employed with soluble glass ; but experience alone can decide which of these substances is best, and in what proportion they are to be employed. We should advise that the first coat should always be a simple solution of the glass ; and that a similar solution be applied over coats composed of its mixture with other substances, particularly when such a coat is uneven and rough. The last named substances form a solid and durable coating, which suffers no change by exposure to the air, does not involve any great expense, and is readily applied ; but, in order that it may not fail, particular care is to be taken both in preparing and employ ing it. In order to cover wood and other bodies with it, the solution must be made of a pure glass, for otherwise it would effloresce and finally fall off. However, a small degree of impurity is not injurious, although after a few days a slight efflorescence will appear; this may be washed off by water, and will not show itself a second time. When a durable covering is to be applied to wood, too strong a solution must not be employed at first ; for in this case it will not be absorbed, will not displace the air from the pores, and in consequence will not adhere strongly. It is a good plan to rub the brush several times over the same place, and not to spread the coating too lightly. For the last coats a more concentrated solution may be employed; still it must not be too thick, and must be spread as evenly as possible. Each coat must be thoroughly dry before another is applied; and this will take, in warm and dry weather, at least twenty-four hours. After two hours the coat appears to be dry, but is still in a state to be softened by laying on another. The same inconvenience will then arise, which occurs when a thick coat of a concentrated solution is applied ; the coat will crack, and does not adhere. This, however, is only the case when potassa is the base of the glass, for that formed from soda does not appear to crack. In applying soluble glass to the woodwork of the theatre at Munich, 10 per cent. of yellow clay (ochre?) was added. After six months, the coat had suffered but little change ; it was damaged only in a few places where it had need of some repair. This arose from a short time only having been allowed for the preparation and application of the glass, and they were therefore done without proper attention. When this mode is employed for preserving a theatre from fire, it is not enough to cover the woodwork, it is also necessary to preserve the scenery, which is still more exposed to danger. None of the methods yet proposed for this purpose appears as advantageous as soluble glass, for it does not act upon vegetable matter, and completely fills up the spaces between the thread ; it fixes itself in the web in such a way that it cannot be separated, and increases the durability of the fabric. The firmness which it gives to stuffs does not injure them for use as curtains, because it does not prevent them from being easily rolled. So far as the painting of scenes is concerned, the glass forms a good ground for the colours. To pre vent the changes which some colours, Prussian blue and lake for instance, might undergo from the alkaline matter, it will be necessary, before painting, to apply a coat of alum, and then one of whiting. There is no great difficulty in applying soluble glass to cloths ; still this operation is not so easy as might at first be imagined. It is not sufficient to coat or dip them in the solution ; they still require after this operation to be subjected to pressure. This object might perhaps be best attained by passing them between rollers lunged in the solution. When a cloth is only coated with soluble glass, and put into the fire, it will remain incandescent after it is taken out. This is not the case when it has been properly impregnated with this solution. A still better purpose is answered in this base, when litharge has been added to the solution. The stuff in drying yields to the shrinking of the mixture, and becomes inseparable from it, which is the reverse of what happens when it is applied to wood. A single part of litharge in fine powder is sufficient for fourteen parts of concentrated liquor. Soluble glass is capable of many other applications, and particularly as a cement; for this use it is superior to all those which have hitherto been employed for uniting broken glass, porcelain, &c. It may be used in place of glue or Isinglass in applying colours, although when employed by itself it does not make a varnish which will preserve its transparency when in contact with air.