HYDRAULIC LIMESTONE. 77 



•As the proper selection and management of hydraulic limes is a matter of vast importance, 

 I trust I shall be excused for adverting to a few points, to which there is reason to fear that 

 sufficient attention has not been heretofore paid in the construction of our public works. 



1 . It is agreed on all hands, that there is a great difference in the hydraulic limes obtained 

 from different limestones ; and that the value of each, and the proper mode of using it, can 

 be determined only by experiment. 



2. It is admitted, that in all cases, the process of burning or calcining the limestone requires 

 great care. A limestone, very proper in other respects, gives, when the heat is urged too 

 high, what is called a dead lime, in consequence of the partial fusion of the mass ; whereas, 

 when the calcination is effected at too low a temperature, the resulting lime is meagre, and 

 not hydraulic. 



3. Hydraulic lime should be used as soon as possible after calcination ; and when kept for 

 any time, it should be carefully protected from the action of the air. It has been ascertained 

 that the hydraulic property of limes is much weakened by their being exposed to the air ; 

 and consequently, all other things being equal, recently prepared hydraulic lime is to be pre- 

 ferred, for important structures, to that which has been for some time manufactured. 



4. It is generally agreed that the rapidity with which hydraulic mortar hardens, and the 

 ultimate degree of hardness which it acquires, depend greatly upon the proper proportions of 

 lime and sand, their intimate incorporation, and the amount of water employed in their mix- 

 ture. All these are pyoints which must be settled by previous experiments. 



The leading principle involved in the hardening of these hydraulic compounds seems to be, 

 that certain earthy substances, and especially siUca, combine with the lime to which the car- 

 bonate of lime in these hydraulic compounds is brought by calcination, and that the silicate 

 thus formed is converted into a solid hydrate by combination with water. The setting of 

 boiled plaster, when made into a paste with water, affords the best illustration of the kind of 

 change which these hydraulic compounds undergo during the process of hardening in contact 

 with water. The selection of the raw material, and the different steps of its preparation, must 

 have in view the complete fulfilment of this necessary condition. 



I shall now proceed to notice some of the most important localities of hydraulic limestones 

 in this State, and give the results of the analyses made in reference to this subject. 



Ulster County. Hydraulic limestone is found in great quantities in this county, and the 

 manufacture of cement has for several years been quite extensively carried on. It is probably, 

 however, still only in its infancy. ^ 



The beds of hydraulic limestone that yield the cement in this county, are situated in the 

 vicinity of Kingston, Rosendale, Lawrenceville and High Falls. The mode of preparing the 

 cement is sufficiently simple. The limestone is first reduced to small fragments, which are 

 then thrown into a kiln, with layers of the screenings of anthracite intermixed. At an interval 

 of twelve hours, the lower layers of the kiln are removed, and fresh portions of the limestone 

 thrown into the upper part. These operations are so managed, that each layer is subjected 

 to heat for about three days. 



