26 
action df the air. It has been ascertained that the hydraulic property 
of limes is much weakened by their being exposed to the air, and con- 
sequently, all other things being equal, recently prepared hydraulic lime 
is to be preferred for important structures, to that which has been for 
some time manufactured. 
4. It is generally agreed that the rapidity with which hydraulic mor- 
tar hardens, and the ultimate degree of hardness which it acquires, de- 
pend greatly upon the proper proportions of lime and sand, their inti- 
mate incorporation, and the amount of water employed in their mix- 
ture. All these are points which must be settled by previous experi- 
ments. 
35. The leading principle involved in the hardening of these hydrau- 
lic compounds, seems to be, that certain earthy substance^, and especial- 
ly silica, combine with the lime to which the carbonate 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 com- 
pounds undergo during the process of hardening in contact with water. 
The selection of the raw material and the different steps of its prepara- 
tion, must have in view the complete fulfilment of this necessary condi- 
tion. 
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. 
36. Hydraulic limestone is found in great quantities, in Ulster county, 
and the manufacture of cement has for several years been there quite 
extensively carried on. Recently, however, it has increased in a sur- 
prising manner, though it is probably, still only in its infancy. 
The principal quarries of hydraulic limestone, at present opened in 
this county, are situated on the Rondout, and the Hudson and Delaware 
canal, but the strata are undoubtedly of great extent. The mode of 
preparing the cement is sufficiently simple. The limestone is first re- 
duced to small fragments, which are then thrown into a kiln, with lay- 
ers 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 ma- 
naged, that each layer is subjected to heat for about three days. 
