CHEMICAL SCIENCE. 281 







due, viz., the Roman Cements, which contain caustic lime when fresh, and the 

 Portland Cements, which do not contain caustic lime. 



Fuchs has satisfactorily shown that the hardening of Roman cement, apart 

 from the production of carbonate of lime, is essentially due to the production 

 of basic silicate of lime (3 CaO, 2 SiO), by the combination of an acid sili- 

 cate, or of free soluble silica with caustic lime. 



The hardening of Portland cement under water, is, on the contrary, due to 

 the decomposition of a silicate, consisting of three or four equivalents of 

 base combined with one equivalent of silica, with the production of caustic 

 lime, and such compounds of lime with silica and with alumina as may be 

 produced in the wet way. 



Consequently, the hardened Portland cement contains the same substances 

 as hardened Roman cement ; but they are produced in a different way, in the 

 hardening by the action of water. 



Mr. Winckler has made a series of analyses of different specimens of Port- 

 land cement, and comes to the conclusion that the silica may be replaced by 

 alumina or peroxide of iron ; that the presence of alumina does not interfere 

 with the hardening of the cement, but renders it less capable of resisting the 

 action of carbonic acid ; and that the presence of peroxide of iron renders 

 the cement less hard and less durable. 



He also found that, during the hardening of Portland cement, lime is 

 gradually abstracted by water. The composition of the cement, after this 

 action had gone on for some days, was found to correspond closely with the 

 formula 3 CaO, SiOs + CaO, Ah Os; and he regards this compound as the 

 final result of the action of water upon Portland cement. 



He considers the presence of magnesia in Portland cement injurious, owing 

 to the fact that the tribasic silicate of magnesia and lime is not decomposed 

 by water. 



The presence of alkalies in the cement seems to accelerate the hardening, 

 probably by facilitating the penetration of water into the mass. 



SOLUBLE GLASS, WATER GLASS, MINERAL GUM. 



Soluble silicate of soda is now largely manufactured in England, and sold, 

 under the above names, for various important commercial purposes. Long 

 known to chemists, it has within a recent period attracted the attention of 

 manufacturers, who find it possesses properties highly useful, and susceptible 

 of a great variety of applications. It is prepared most cheaply from quartz- 

 sand and soda-ash. The former is very finely ground, and then dissolved in 

 a heated solution of the alkali. The solution is then concentrated to 20, 30, 

 or 50 of Twaddle, and sold of the strength best adapted for the purpose con- 

 templated. A large demand has recently arisen for it as a size for calicoes, 

 etc., in lieu of British gum, the glaze being superior, and the price (6 to 7 

 a ton) being lower. 



It is also in demand for soap-making; being itself a detergent, and mixed 

 with common soap, prevents the usual loss by evaporation, and increases the 

 weight. It has also been used for saturating timbers, scenery, dresses, and 

 other inflammable material ; being itself of a highly incombustible nature, 

 and thus protecting other substances to a great extent from the action of fire. 

 In contact with lime, it consolidates, and is partly converted into silicate of 

 lime. Hence it has been employed, especially in Berlin, in the process called 

 " Stereochrome," a revival of fresco painting on the walls of buildings; the 



24* 



