78 KCONOMICAL MINERALOGY. 



The lime thus calcined is of a light drab colour ; and when reduced to powder, and mixed 

 with about one third its bulk of sand and made into a paste with water, soon becomes hard. 

 The grinding is performed in a mill, and the powdered cement is put up in barrels, which are 

 lined with paper, to prevent as much as possible the access of air. 



According to Mr. Mather, there were, during the year 1839, sixty kilns in this county for 

 the manufacture of cement, and it was estimated that 600,000 barrels were manufactured 

 during that year. It was used in the Croton Water- Works, shipped to various Atlantic ports, 

 and to the West Indies.* 



I analyzed a specimen of the hydraulic limestone from the vicinity of Rondout. It was of 

 a bluish colour, vdth occasional grey and reddish spots. The texture was close and compact, 

 and the fracture even and conchoidal. The following are the results : 



Carbonic acid, 34.20 



Lime, 25.50 



Magnesia, - 12.35 



SiUca, 15.37 



Alumina, .- 9.13 



Peroxide of iron, 2. 25 



Bituminous matter, moisture and loss, 1 . 20 



The calcined cement contained only five per cent, of carbonic acid, and its composition may 

 be thus expressed : 



Carbonic acid, ^ 5. 00 



Lime, - 37.60 



Magnesia, 16.65 



Silica, 22.75 



Alumina, 13.40 



Peroxide of iron, 3. 30 



Loss, 1.30 



By calcination, therefore, this limestone is reduced to a state approaching that of a double 

 silicate and aluminate of hme and magnesia ; and the theory of the hardening of this cement 

 is sufficiently in accordance with the views above suggested. The cement, moreover, forms 

 a jelly with muriatic acid, which still further proves that its chemical nature has been changed 

 during the calcination. 



An important practical inference from these experiments and observations is, that the more 

 completely the carbonic acid is driven ofi" from the limestone by calcination, without causing 

 its fusion, the more energetic will be the resulting product. It is probable that a long con- 

 tinued, rather than a very high heat, will best effect this object. 



It remains to be mentioned, that the limestone which overlies this cement has a blue colour, 



 New-York Geological Reports, 1840. 



