CHEMICAL SCIENCE. 205 



is greater than for either of the other kinds of artificial stone ; (2) its 

 being subject to a white efflorescence of salt and a green stain from 

 damp, both of which take away from its value for ornamental pur- 

 poses, for which it is otherwise admirably adapted. 



The mechanical and chemical principles involved in these different 

 contrivances are as follows : in terra cotta the material is a kind of 

 clay, purer and more free from foreign substances than common clay, 

 and mixed with dust from pottery already made. The manufactured 

 article is thus a superior fire brick. The burning produces little 

 chemical change or metamorphosis, but the condition after burning is 

 so far different that ordinary exposure will not bring back the original 

 texture of clay. Of closer texture than brick, there is less absorp- 

 tion from the surface ; but in ornamental work there are always 

 flaws enough to render frost following rain dangerous and injurious. 

 In other respects the material itself is little more liable than brick to 

 injury from exposure. 



In cement the raw material is carbonate of lime, with a certain but 

 variable proportion of foreign substances, of which clay or silicate of 

 alumina is an important and even an essential part. All the varieties 

 of cement stone, such as the stones called septaria and other nodules, 

 agree in this. On burning this material the limestone is converted 

 into lime, and the condition and proportion of the foreign material 

 determines the value of the resulting cement. It is called hydraulic 

 cement, as setting with almost any required rapidity when properly 

 mixed with water, and this in damp air, during rainy weather, and 

 even under water, absorbing no more water than is necessary for con- 

 solidation. Under various names, pozzuolaua, Roman cement, Par- 

 ker's cement, Atkinson's cement, etc., this valuable material has been 

 used from time immemorial, and is especially adapted for making 

 concrete where a larger proportion of foreign substances is introduced. 

 As an artificial stone, although it hardens on exposure, its composi- 

 tion is too irregular to justify a very extended use. In the process of 

 setting the lime first mixes with water and becomes hydrate of lime, 

 and is then rapidly converted into silicate of lime, adhering strongly 

 in thin films to itself and to foreign bodies with which it is in contact. 



The silicious stone of Mr. Ransome consists of sand and foreign 

 substances* worked up into a paste with the fluid silicate of soda. If 

 left to dry in the air it would fall to powder, but being exposed to a 

 high heat in a kiln a chemical action takes place. The alkali of the 

 silicate of soda " combines with an additional quantity of silica sup- 

 plied by the sand, etc., with which it is incorporated, and becomes 

 converted into an insoluble glass, firmly agglutinating all the various 

 particles together into a solid, compact substance." No sensible con- 

 traction takes place in burning, and cracks rarely occur. 



The resistance to weather offered by these three kinds of artificial 

 stone may be thus stated : 1 . Terra cotta, contracting irregularly in 

 the kiln, is subject to cracks and flaws, into which water penetrating 

 and expanding during frost, a peeling and splitting of the material 

 naturally follows. It is almost certain, from the nature of the case, 

 that delicate and ornamental work should be more liable to such 

 injury than straight work and plain surfaces. 2. Cement, owing to 

 the want of homogeneity in the raw material, is also very subject to 

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