Mr. White on Calcareous Cements. 6\ 



will, in my opinion, contribute essentially to a knowledge of the 

 subject. 



The first endeavour at investigation was made by a comparison 

 of various burnt clays, obtained in the neighbourhood of London 

 and in Kent, with the imported Pozzolano, as sold in London ; but 

 the imported material was so variable in its nature, that little re- 

 sulted beyond a knowledge that it possessed more calcareous mat- 

 ter, and that it was more uncertain and variable in the sizes of the 

 grains, than that of British manufacture. 



The next endeavour was to ascertain what, practically, were 

 the best sizes of the particles to be used with lime, and in what 

 state and what species of lime entered best into the combination 

 with them. 



It appeared that either the foreign or British ]PozzolanQ, if re- 

 duced into a very fine powder, lost considerably its power of adhe- 

 sion, though it was more plastic. It necessarily was inferred, that 

 the greater the variety of dimension of the particles, the greater 

 would be the entanglement of the asperities, and, consequently, the 

 greater the adhesion. Of the mortar made, it also appeared that 

 the finer the lime could be ground, the more perfect would be the 

 combination, and the harder the mortar obtained, because the 

 hard particles of the Pozzolano being in a state of actual contact, 

 no compression was likely to take place ; and which, in fact, upon 

 the subsequent investigations proved to be the case. 



My reasons for trying the Pozzolano were these. I conceived 

 that I should have two causes for the induration of the mortar ; 

 one, the disposition which many burnt materials have to unite in- 

 timately with lime ; the other, the variety of form which the frac- 

 ture of burnt clay produces ; the real difficulty which existed was, 

 the obtaining a perfect knowledge of the best state, and the best 

 mode of indurating properly the clay itself. For if the burning of 

 the clay were such as to cause vitrification, an imperfect mortar 

 was made ; perfect glass, scoria, and pumice-stone, produced very 

 inferior mortar ; perfect Pozzolano appeared to be made when a 

 chalky clay was so indurated by fire as to put on the appearance 

 of a commencing vitrification only. 



E 2 



