334< Mr. White's Design Jar the Nexo London Bridge, 



state and what species of lime entered best into combination 

 with them. 



It appeared that either the foreign or British pozzolano, if 

 reduced into a very fine powder, lost considerably its power 

 of 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 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 pozzolano 

 being in a state of actual contact, no compression was likely 

 to take place ; and which, in fact, upon the subsequent investi- 

 gations, proved to be the case*. 



My reasons for ti-ying the pozzolano were these. — I con- 

 ceived that I should have two causes for the induration of the 

 mortar : one, the disposition which many burnt materials have 

 to unite intimately with lime ; the other, the variety of form 

 which the fracture 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 was such as to cause 

 vitrification, an imperfect mortar was made; perfect glass, 

 scoria, and pumice-stone produce very inferior mortar; per- 

 fect pozzolano appeared to be made when a chalky clay was 

 so indurated by fire as to put on the appearance of an inci- 

 pient vitrification only. 



Fi7-st Experiment, tried 9fh of August, 1824. — A pier three 

 feet four inches wide, one foot ten inches and a half thick, 

 five feet eight inches high, was built on the 9th of Jul}', 1824, 

 and was composed of hard sound London burnt stock bricks, 

 and mortar of one part ground lime, one part and a half 

 sand, and two parts rough pozzolano. It was elevated by 

 applying screw-jacks on each side, course by course, be- 

 ginning at fifteen courses, or three feet nine inches from the 

 bottom : this elevation from its foundation took place without 

 any separation of the courses until the screw-jacks were placed 

 two courses, or six inches from the top, when these two courses 

 separated from the remainder of the pier. 



The remainder of the pier was then tlu'own down, when it 

 separated at eight courses from the foundation, and showed 

 that the middle was not dry. 



This experiment was tried in the presence of Mr. Brunei, 

 Mr. MathewWyatt, Mr. Smirke, and many other gentlemen. 



* The facts here inferred agree, mtitatk miitaiidin, with tiic results of the 

 experiments of Dr. Iliggins and otiicrs, on the composition of coninioii 

 mortar: — sec Dr. Thomson'- InorKsinic Clicniistrv, vol, i. p. MO. — Edit. 



Second 



