896 PROCEEDINGS OF SECTION J. 



is to see that the lime and sand are thoroughly and intimately 

 amalgamated. 



Mortar, when made with ordinary lime, is at its best a 

 mechanical mixture, and the difficulty of thoroughly mixing 

 wet lime and sand together is known to all builders. 



Cement mortar (hydraulic lime and sand) on the other 

 hand can be thoroughly mixed in a dry state, and forms, by 

 the addition of water, a chemical mixture, containing within 

 itself, as already stated, all the elements for its own hardening. 

 This hardening takes place throughout the whole mass simul- 

 taneously. It sets not only on the outside when the hydro- 

 silicates of lime, alumina, and iron is additionally strengthened 

 by the carbonic acid gas there absorbed by the lime from the 

 atmosphere, but it hardens likewise in the interior where 

 the setting, caused by the formation of the hydrosilicates, 

 goes on indefinitely, improved, however, in all cases by 

 moisture. 



The simultaneous hardening of the cement mortar thus 

 enables the builder (without waiting for it to dry out like 

 lime mortar) to pile upon it immense weights, as there is 

 present no free water to ooze out and cause a settling or 

 sliding, the true cement in setting refusing all water not 

 chemically combined. 



It is evident, then, that for all buildings having any pre- 

 tensions to importance, it is advisable to use mortar made 

 from hydraulic lime. The Avell-known American " Rosen- 

 dale " Cement is a natural cement, and is about equivalent in 

 analysis and hydraulic properties to the Victorian stone. 



Natural cement is made by burning in an ordinary draw 

 kiln hydraulic limestone or cement rock at a moderate heat, 

 and grinding the calcined product to fine powder. 



Mr. F. W, Rudler, F.G.S. in his essay on dolomite, speaks 

 thus of the stone : — " This mineral is essentially a double 

 carbonate of lime and magnesia, containing in the normal 

 varieties carbonate of lime 54-35 ; carbonate of magnesia 

 45'65 ; a percentage composition corresponding to the 

 formula CaO.C02 - Mg. O.CO^ (Ca.Mg.2C0»). The 

 relative proportion of the two carbonates is, however, subject 

 to considerable variation, the species passing on the one hand 

 into carbonate of magnesia. 



The presence of protoxide of iron replacing the isomorphous 

 oxides lime and magnesia often causes the mineral to assume 

 a brown colour on exposure to the atmosphere, when such 

 varieties are distinguished as brown spar." 



