. The Civil engineer and architect's journal, scientific and railway gazette. Architecture; Civil engineering; Science. 188 THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. [JvNE, be any reason why the gj'psums should differ from the carbonate of lime in this respect. Indeed, we find that the law holds good with the English pypsums, for the Derby stone makes a stronger plaster than that of Newark, just as it is harder than the latter. Dumas a^ees witli Guy Lussac, in supposing that no other chemical action takes place with the gypsum, than the evolution of its water of crystallisation. In Paris, the
. The Civil engineer and architect's journal, scientific and railway gazette. Architecture; Civil engineering; Science. 188 THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. [JvNE, be any reason why the gj'psums should differ from the carbonate of lime in this respect. Indeed, we find that the law holds good with the English pypsums, for the Derby stone makes a stronger plaster than that of Newark, just as it is harder than the latter. Dumas a^ees witli Guy Lussac, in supposing that no other chemical action takes place with the gypsum, than the evolution of its water of crystallisation. In Paris, the mode of using plaster is to employ it pure and free from mixture. The very low price at which it is sold, and the comparatively high price of sand, dispense with the motives of economy which render mixtures almost indispensable in our case. The town of Paris pays for its municipal works, at the rate of 12s. 9gd. per ton of plaster, whereas it cannot yet be had in London for less than about iOs. per ton. Whilst the practice in France is to use plaster pure, I am disposed to think that the mixture of sand, so far from being prejudicial, is even desirable, if confined within reasonable limits. We find that in reassuming the state of liydrated sulphate of lime, the plaster goes through an imperfect crystallisation; and this action is accompanied by a singular re- arrangement of the molecules. This causes the plaster to swell when used alone, and to such an extent, that it is impossible even to finish a ceiling close up to a wall at once. Now the introduc- tion of a body so full of inequalities as the coarse, sharp sands, must afl'ord room for the free action of this expansion; and, at the same time, the facettes of the sand must offer, as is were, nuclei, which cannot but be favourable to the crystallisation. It is, doubt- lessly, on these principles that we can explain the superiority of the plaster containing the wood brees, which does become harder than the purer plasters, if u
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