SOME NOTES ON ANCIENT MOKTAHS. 43 



found considerable traces of hjdrated silica. It occurred to 

 Mr. Roberts that possibly the hardening or setting of mor- 

 tar might be due to some chemical action occurring between 

 the lime and the silica when these ingredients were mixed, 

 whereby some proportion of the silica was caused to assume 

 the gelatinous form. This being then incorporated by the 

 usual mixing process, subsequently solidified, binding the 

 whole bulk with a hard network of silica. A series of ex- 

 periments were made, and the general conclusions arrived at 

 may be summarized as follows : — 



(1) Practically no gelatinization of silica occurs in the 

 manufacture of mortar. 



(2) Under the ordinary conditions of access of air the 

 lime in mortars becomes gradually dehydrated, absorbs car- 

 bonic acid, and forms neutral carbonate. 



(3) The absorption of carbonic acid is very slow. 



(4) A slight action takes place between the lime and the 

 silica, although very small. 



(5) Although even the small proportion of diy silicates 

 slightly increases the hardness of a mortar, the ordinarily 

 sufficient hardness of mortar is obtained by simple dehydra- 

 tion and carbonation. 



These conclusions appear to be confirmed by the fact that 

 lime already containing a small proportion of carbonate is 

 preferred to pure lime for making mortar. 



