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on the Principle of Cementitious Architecture.

VIII. On Calcarcotis Cements.

To the Editor of the Quarterly Journal.

Sir,

Having for nearly thirty years experienced practically'theimperfections of the various cements in use in England, I havebeen led into a variety of examinations of them, and do nothesitate to communicate to you (as a portion of the papers onmy design for New London Bridge), an account of a series ofexperiments which will, in my opinion, contribute essentiallyto a knowledge of the subject.

The first endeavour at investigation was made by a com-parison of various burnt clays obtained in the neighbourhoodof London and in Kent, with the imported pozzolano, as soldin London : but the imported material was so variable in itsnature, that little resulted beyond a knowledge that it pos-sessed more calcareous matter, and that it was more uncertainand variable in the sizes of the grains, than that of Britishmanufacture.

The next endeavour was to ascertain what practically werethe best sizes of the particles to be used with lime, and in whatstate and what species of lime entered best into combinationwith them.

It appeared that either the foreign or British pozzolano, ifreduced into a very fine powder, lost considerably its powerof adhesion, though it was more plastic. It necessarily was in-ferred that the greater the variety of dimension of the par-ticles, the greater would be the entanglement of the asperities,and consequently the greater the adhesion. Of the mortarmade, it also appeared that the finer the lime could be ground,the more perfect would be the combination, and the harderthe mortar obtained; because the hard particles of pozzolanobeing in a state of actual contact, no compression was likelyto take place; and which, in fact, upon the subsequent investi-gations, proved to be the case*.

My reasons for trying the pozzolano were these.—I con-ceived that I should have two causes for the induration of themortar: one, the disposition which many burnt materials haveto unite intimately with lime; the other, the variety of formwhich the fracture of burnt clay produces: the real difficultywhich existed, was the obtaining a perfect knowledge of thebest state, and the best mode of indurating properly the clay

* The facts here inferred agree, mutatis mutandis, with the results of theexperiments of Dr. Higgins and others, on the composition of commonmortar:—see Dr. Thomson’s Inorganic Chemistry , vo ). i. p. 440 .—Edit.

itself.