678 NEW YOEK STATE MUSEUM 



The "burning of hydraulic lime must be carried on very slowly. 

 The higher the percentage of total silicates, the lower must be 

 the temperature of burning, for under no circumstances should 

 the material be allowed to sinter as it does in the case of Portland 

 cement. Su€h overburned pieces slake very slowly. Further- 

 more, the burning should only go to the point of driving off the 

 COg. The best hydraulic lime known is the celebrated Chaux du 

 Teil whose composition according to Dr Michaelis is : 



Silica 22 . 588 



Alumina 2 . 629 



Iron oxid . 837 



Lime 65 . 624 



Magnesia 1 . 536 



Potash 124 



Soda 065 



Sulfuric acid . 523 ' 



Ignition 6 . 424 



Hydraulic limes have generally a yellow color. Their specific 

 gravity is about 2.9 and on ignition they lose about 8/^. They 

 harden slowly and their hardness by themselves is small. On 

 the other hand if mixed with sand they develop a high degree of 

 tensile strength. 



Natural or Rosendale cements 



Under this heading are included those made from limestones 

 which have over 20^ of aluminous impurities, and which, when 

 burned and finely ground, harden with water in a short time. This 

 includes the Roman cements used abroad, and also the dolomitic 

 cements made from magnesian limestones. They do not develop 

 much heat on mixing with water, and their strength does not equal 

 the artificial or Portland cement. They differ from hydraulic 

 limes in their quicker set, and lower percentage of lime. The 

 following anaylses given by U. Cummings^ indicate the wide 

 ran2:e of materials included under this class. Some authors in 



'to 



fact make two groups of the dolomitic and Roman cement ma- 

 1 American cements, p. 35. 



