or fmaller proportions. And, as no limeftone that can be calcined, contains fuch a large proportion of fand as is neceffary for making a perfect cement, we may naturally conclude, that every kind of lime is equally fit for becoming a firm cement, if it be first reduced to a proper degree of caufticity, and has afterwards a due proportion of fand properly mixed with it, before it be employed in work.

Different forts of lime, no doubt, vary very much from one another in the proportion of fand they naturally contain, and therefore muft require very different proportions of fand to be added to them before they can be made equally perfect as a cement. This is an oeconomical confideration of no small moment in fome cafes, as it may make one fort of lime vaftly cheaper than another on fome occafions, and therefore deserves to

be attended to by every builder.-Directions

fhall

shall be given in the Second Part of this Effay, by the help of which he may be enabled to discover the exact proportion of fand contained in any fort of lime he may wish to examine:

$ 31:

In the preceeding parts of this, Effay, Í have fpoken of fand as the only fubitance that is ever added to lime in forming ce ment; but, as others have, on fome occafions, been employed for this purpose, it will be proper here to point out their several excellencies and defects.

Almoft the only fubftances that I have known used as an addition to mortar, besides fand of various denominations, are powdered fand-ftone, brick-duft, and fea-fhells that have been broke into small fragments.

And, for forming plafter, where clofenefs father than hardness is required, the ufeful additions

A a a

additions are, lime that has been flaked and kept long in a dry place, till it has become nearly effete, powdered chalk, or whitening, and gypfum in various proportions; befides hair, and other fubftances of that fort.

Others that have been lately recommended by Monfieur Loriot, are balls of any fort of earth flightly burnt and pounded ;—the rubbifh of old buildings, (by which I underftand the old mortar after it has been separated from the ftones,) reduced to powder, and fifted, or almost any other thing that can be reduced to a moderately fine powder.

From what has incidentally occurred relating to this head, the reader will be able to judge in fome measure of the comparative value of these several additions: But, to render the fubject ftill more clear, the following obfervations may be of use.

It is fufficiently certain that none of these additions enter into the composition, so as to affect its qualities as a chemical mixt;-they

only

only operate in a manner purely mechanical. For, whatever the nature of the addition may, be, it poffeffes the fame qualities when fo united, as if by itself, and may be separated by mechanical means from the compound unaltered. Therefore we need give ourfelves no trouble about afcertaining their chemical qualities, but confider them merely as maffes of matter that may be more or lefs fitted for this purpose by their peculiar form, degrees of hardness, &c.

It has been already fhown, that fand ought to be preferred to chalky matters, chiefly on account of the hardness and firmnefs of the particles of which it confifts.And, as the pureft fand confifts of detached chrystals, which are fo hard as scarcely to admit of being broken into smaller parts, this kind of pure chryftalline transparent fand is, perhaps, on this account, the most proper addition that can poffibly be made to lime in forming mortar.

Sand

Sand-ftone confifts of an almost innumerable congeries of small particles of fand united to one another in a flight manner, by fome kind of natural cement.-But, as it is troublesome to reduce this kind of ftone to its fmalleft component parts, and as the particles of it, when not reduced to that ultimate degree of fineness, may be eafily broken into fmaller parts, it can never be looked upon as fuch a proper addition for a lime-cement as the pureft fand.

There are also many fubftances that are called fand, which are nothing else than fragments of decompofed granite, moorftone, fand-ftone, &c.—all of which may be easily reduced into smaller particles by moderate triture, and are liable to the fame objections as pounded fand-stone,

But almost any of thefe is preferable to brick-duft. Fine clay, when perfectly burnt in the fire, may be made to affume almost a ftony hardness. But common

brick