24 ( Assembly 
in volume, and without the evolution of much heat; and they moreover 
form with it a paste possessing little tenacity, and which when placed 
under water, hardens after the lapse of a few days. These are now 
known by the name of Hydraulic^ or Water Limes. They differ much 
in the rapidity with which they harden under water, and in the degree 
of solidity which they ultimately attain. 
There are still other limestones which afford limes, possessing the re- 
markable and very useful property of becoming hard almost instantly, 
like plaster of Paris, whether exposed to the air or in contact with wa- 
ter. These are sometimes called Roman Cements, It should be re- 
marked, however, that the French generally employ the term cimentj 
to designate fine pounded bricks or tiles; while in this country, the term 
cement is most commonly used in a generic sense, and includes the hy- 
draulic constituent of mortars, whatever that may be. 
It must always be considered as a most fortunate circumstance, that 
so soon after the commencement of the Erie canal, a material was found 
in its immediate vicinity, which yielded a lime well suited for the con- 
struction of locks and aqueducts. It is worthy of inquiry, however, 
whether in these and in similar works since constructed, sufficient atten- 
tion has been paid to the selection and preparation of the hydraulic limes 
thus employed. 
33. There is still some difference of opinion, as to which of the in- 
gredients of these impure limestones, the hydraulic property is to be as- 
cribed. Some of those who have examined the subject, have fixed upon 
the oxide of iron, as the important ingredient; while the claims of the 
oxide of manganese, silica, alumina, magnesia, and even soda, have each 
been urged by respectable authority. It is, after all, however, more 
than probable, that the hydraulic property is not due to a single ingredi- 
ent, but belongs rather to several substances, or to a class of compounds. 
It appears, from the experiments of Berthier and Vicat, the highest 
authorities upon this subject; — That no mixture, of which silica does 
not form a part, acquires hydraulic properties; — That limes containing 
only silica or alumina, or better those containing silica and magnesia, 
acquire a much greater degree of hardness than the silicates of pure lime; 
and that the oxides of iron and manganese contribute nothing to the 
hardening of these bodies. 
More recently, M. Vicat has asserted, that magnesia alone, when in 
sufficient quantity, will render pure lime hydraulic. The proportions 
of magnesia which he recommends, are from thirty to forty per cent of 
