CHEMICAL SCIENCE. 2G7 



hydraulic lime. A mortar of fat lime repeatedly wetted with a solution of 

 alkaline silicate is transformed into hydraulic mortar. Lastly, with the glassy 

 silicate and lime more or less energetic hydraulic cements can be produced 

 at will, which will be found very useful in countries where only fat limestones 



exist. 



Silicification. From observing the great affinity of lime for silica when set 

 free in a nascent state from its compound with potash, Mr. Kuhhnann was led 

 to study the action of the silicates of potash and soda upon the calcareous 

 stones, upon chalk in particular. He observed that by placing some chalk 

 in contact with a solution of silicate of potash in the cold, a portion of the 

 chalk is transformed into silico-carbonate of lime, whilst a corresponding 

 portion of potash is displaced; that the chalk hardens gradually in the air 

 and acquires a greater hardness than that of the best hydraulic cements; if 

 the chalk is made into a paste with the silicate, it will adhere strongly to 

 bodies, to the surface of which it is applied. Thus a cement was discovered, 

 capable of being employed in restoring public monuments and in the manu- 

 facture of cornice-work. Pushing his experiments further, he ascertained 

 that chalk, when plunged into a solution of silicate of potash, was capable 

 of absorbing a considerable quantity of silica; by exposing it alternately and 

 repeatedly to the action of the silicious solution and to that of the air, he 

 found that this stone acquired in time a great hardness on the surface, and 

 that the hardening, which was at first superficial, penetrated gradually to the 

 centre. 



This silicification of the stone (this is the name given by Mr. Kuhlmann 

 to this transformation) is due to the decomposition of the silicate of potash 

 by the carbonate of lime on the one hand, and by the carbonic acid of the 

 air on the other. A solution of silicate of potash when left to the air gives 

 origin, in fact, after some time, to a gelatinous and contractible deposit of 

 silica and to a stratum of carbonate of potash. In course of time the deposit 

 of silica acquires sufficient hardness to scratch glass. T\vo balls of chalk of 

 the same diameter and of the same nature were silicified under the same con- 

 ditions ; the one was exposed to the free action of the air, and acquired more 

 hardness than the second, which was kept under a bell-glass in an atmos- 

 phere deprived of carbonic acid. In silicification, therefore, as long as the 

 stone is porous enough to continue absorbing silicate of potash, a sort of 

 hydrated silico-carbonate of lime is formed, which hardens by gradually 

 losing its water of hydratation, besides a contractible layer of silica which 

 adds to the hardness of the stone. The carbonate of potash produces on 

 the surface an almost imperceptible exudation, which diminishes gradually 

 and at last disappears entirely, without having in the least altered the sur- 

 face of the stone; by means of hydro-fluosilicic acid Mr. Kiihlmann has 

 succeeded in getting rid of the inconvenience which might result from this, 

 and even in adding to the hardness of the stone. Calcareous stones thus 

 prepared acquire a compact grain, and a lustrous appearance, and become 

 capable of receiving a fine polish. The hardening is singularly assisted by 

 heat; and calcareous porous stones, on being plunged into a high-pressure 

 boiler containing a bath of silicate of potash, presented, as soon as they 

 were withdrawn from this immersion, all the character of compact sili- 

 cious limestones, without the least intervention of the carbonic acid of 

 the air. 



From limestones Mr. Kuhlmann passed on to porous stones, and has suc- 

 ceeded in showing that the action of the carbonic acid of the air upon 



