Royal Institution. 375 



a preference of one substance to combine with a certain other sub- 

 stance instead of a third, but, in a great number of instances at 

 least, this substance will combine with both according to certain 

 proportions, whenever the whole of the affinities can be brought into 

 play at the same time. 



April 7. — On Silica and some of its apphcations to the Arts. 

 By the Rev. J. Barlow, M.A., F.R.S., Vice-President, and Sec. R.I. 

 Silica is one of the most abundant substances known. Quartz, 

 common sand, &c., flint, chalcedony, opal, &c„ and a variety of sand 

 described by Mr. J. T. Way*, may respectively be taken as examples 

 of crystallized and un crystallized silica. Under all these forms 

 silica is capable of combining with bases as an acid. Heat is how- 

 ever essentially necessary to effect this combination, a combination 

 of which all the well-known silicates, whether natural, as felspar, 

 mica, clay, &c., or artificial, as glass, slags, &c., are the results. The 

 common forms of insoluble glass are produced by the union of 

 silica with more than one base. But, when combined with an 

 alkaline base only, silica forms a soluble glass, the degree of solu- 

 bility of which depends on the proportion which the silicic acid 

 bears to this alkaline base .... This soluble silicated alkali (or 

 water-glass) may be prepared by various processes. If sand be 

 used, 15 parts of fine sand, thoroughly incorporated with 8 parts of 

 carbonate of soda, or with ten of carbonate of potash, and one of 

 charcoal fused in a furnace, will produce a silicated alkali which is 

 soluble in boiling water. Messrs. Ransomes obtained this silicated 

 alkali by dissohang broken flints in a solution of caustic alkali at 

 a temperature of 300° Fahr. And more recently, Mr. Way has ob- 

 served that the sand which he has described wiU combine with 

 caustic alkali at boiling heat, also producing a water-glass. 



This water-glass has been applied to several important purposes, 

 three of which were specially noticed. 



I. To protect Building -stones from decay. — The stone surfaces 

 of buildings, by being exposed to the action of the atmosphere, be- 

 come liable to disintegration from various causes. Moisture is 

 absorbed into their pores. The tendency of their particles to sepa- 

 rate, in consequence of expansion and contraction, produced by 

 alternation of temperature, is thus increased. Sulphurous acid is 

 always present in the atmosphere of coal-burning cities, and cannot 

 but corrode the calcareous and magnesian ingredients of oolites and 

 dolomites. It is true that good stone resists these sources of injury 

 for an indefinite time, but such a material is rarely obtained. As a 

 preventive of destruction, whether arising from physical or chemical 

 causes, it has been proposed to saturate the surfaces of the stones 

 with a solution of the water-glass. 



It is well known that the affinity of silica for alkali is so feeble 

 that it may be separated from this base by the weakest acids, even 

 by carbonic acid. According to the expectation of those who 

 recommend the silification of stone, the carbonic acid of the atmo- 

 • Quarterly Journal of Chemical Society, July 1, 18.5.3, and Journal of 

 Iloyal Agricultural Society, vol. xiv. part I . 



