40 SOME NOTES ON ANCIENT MORTARS. 



sea sands derived from qaartz rocks. The matrix consisted 

 almost entirely of carbonate of lime with a very small trace 

 of hydrate of magnesia. When it was treated with very 

 dilute hydrochloric acid there was left a porous, but very 

 coherent skeleton of sand and burnt clay cemented together 

 with calcium silicate, probably formed by the action of the 

 caustic lime on the clay or pozzuolana, and which was readily 

 decomposed by more concentrated acid. The following is 

 the empirical analysis : — 



Per cent. 



Lime (CaO) 11-20 



Magnesia (MgO) ....... 1-82 



Alumina (Alg O3) ...... . 12-92 



Oxide of iron (Feg O3) 6-58 



Silica (pure quartz sand) (SiOo) .... 26-71 



Combined silica (SiOs) 21-04 



Sulphuric anhydride (SO3) trace 



Carbon dioxide (CO2) . • . • • .8-62 



Water . . 10-60 



Alkalies , ... "51 



The water present was nearly all, if not all, hydroscopic, 

 and was readily driven off. The rational composition of 

 this and other ancient mortars can only be to a very great 

 extent mere question of guesswork, and every one may have 

 a different theory. 



When the foundation for the new post office of St. Mar- 

 tin-le- Grand was dug, a fine portion of old London wall was 

 discovered. This splendid piece of Roman work measured 

 about 120 feet in length of solid blocks of Kentish Rag 

 inters tratified pretty regularly at every fifth coarse by a 

 double, or near the top triple layers of red tiles, and bound 

 together by a white mortar which for strength and endur- 

 ance far surpassed that used in the present day. This 



