No. VI. — District of the Bay of Baja. 97 



What may be the principle of the astonishing hardness of 

 the mortar made with this material is a curious subject of in- 

 quiry. According to the analysis of Bergman it consists of 65 

 to 60 parts of silex, 19 to 20 of argillaceous matter ; 5 to 6 

 of lime, and 15 to 20 of iron. It is, as we have just remark- 

 ed, harsh to the feel and brittle. It has a specific gravity of 

 2.570 to 2.788, but rarely 2.8 ; it has an earthy smell, and is 

 not diffusible in water unless when heated. It does not ef- 

 fervesce with acids. When hot it is magnetic. The iron may 

 justly be considered as its intrinsically important ingredient, 

 and Mr Kirwan ingeniously supposes its peculiarity to exist 

 in its pure and magnetic form, by which it is rendered capable 

 of decomposing the water with which it is mixed, and which, 

 therefore, accounts for its rapid absorption of that fluid. The 

 clay contributes greatly to give it a plastic tenacity, and, ac- 

 cording to this theory, the principal effect of the lime is to 

 favour chemical action and solution by its evolution of heat. 

 This supposition is rendered the more probable by the cir- 

 cumstance, that the best imitation of Pozzuolana mortar was 

 found by Mr Smeaton to consist in mixing granulated parti- 

 cles of the sparks of iron from forges with the other ingre- 

 dients of cement. * This may also lead to some conclusions 

 on the original cause of the differences observed between the 

 tufas and the Pozzuolana, with which they are occasional- 

 ly interstratified near their surface. The ingredients are un- 

 doubtedly almost the same ; but in the one case, the iron, hav- 

 ing already exerted its adhesive influence, has combined the 

 loose sandy particles into the form of a rock, and is itself 

 transformed into an oxide incapable of repeating the operation ; 

 while, in the other, the volcanic sand, fresh from the igneous 

 focus, was probably deposited by the eruption of some neigh- 

 bouring volcano, just as the hills, produced by submarine ac- 

 tion, were emerging from the waves. -|- This might, I think, 

 explain most of the peculiarities of this singular substance, 



* The distance from which real Pozzuolana must be brought has given 

 rise to several other imitations of it. One by M. Delahaye-Dumeny was 

 secured some years ago by patent in France. — Ann. des Mines, xxviii. 384. 



+ See Number III. of these Notices. 



NEW SERIES. VOL. II. NO. I. JAN. 1830. G 



