brick is fo imperfectly burnt, as to admit of being reduced, without much trouble, to a fine impalpable powder; infomuch that it is often used, when in this ftate, for fcouring polished iron or brafs, especially if the brick has had no fine fharp fand in its compofition.

As the rough particles of brick-dust may be fo easily reduced to a fine powder, the mortar formed with it can, in no cafe, be of the most perfect fort,

But brick-duft is ftill liable to a greater objection, when confidered as a component part of mortar.

Clay only lofes its quality of abforbing water, and in fome measure of diffolving in it, by a very perfect degree of burning, so that,

if

any part of it has escaped the violent action of the fire, that part, when mixed in mortar, will fill be apt to abforb water whenever it may reach it, and lofe its firmness,

and

and make the mafs, of which it is a part, crumble to duft.

It is exactly in this manner that all forts of marle are liable to fall into powder when drenched in water, and exposed to the air ; even although they fometimes appear, when dry, to be endowed with a ftony hardness.

On this account, brick-duft, which usually confifts of the imperfect burnt bricks, ought to be confidered as a very dangerous mixture for mortar, and fhould never be employed but in cafes of abfolute neceffity.

But the balls of other forts of earth flightly burned, as recommended by Monfieur Loriot, must be, on many accounts, far lefs proper; as many of these sorts of earth cannot, by the action of fire, be deprived of their quality of absorbing water, and of becoming foft with it. So that he who should be foolish enough to employ these fubftances, may be certain that his cement will not only be incapable of attaining any confiderable

rable degree of hardness at any time, but will alfo be liable to turn moift in a damp air, nor will be capable of retaining its firmness or cohefive quality in an expof.d situation.

Powdered lime-rubbish is liable to the fame objections with the fofteft fand-stone or brick-duft; as the particles of which it confifts never can be endowed with the adhefive firinnefs that is neceffary for forming a perfect cement.

Fine fhells are perhaps firmer than any other fubftance, next to pure fand, and may be employed where the other cannot be got, if this abounds. I have feen a cement that was as little affected by the weather as any other, and had ftood firm in the work a great many years, that had been originally formed with a fand confifting almost entirely of the fragments of fhells.-But it had not the rocky hardnefs of fome old mortar that we frequently meet with.

Pow

Powdered glafs, if fuch a thing could be got at a moderate expence, would form a most perfect fort of mortar ;-as it would not be liable to be affected with the weather, -would be fufficiently hard, and consist of very irregular fragments.

Thus, it appears, that, of all the fubftances that can be easily met with, fand forms the most proper addition to lime in making mortar; on which account, it has been justly preferred to all others for that purpose.

Pure, firm, chryftallized fand, is better than any other fort:-But all pure fands are not equally proper for this use.

It has been already fhown, that the principal advantages which refulted from the addition. of fand in making lime-mortar were, that it augmented the quantity of hard indiffoluble

matter,

matter, and put it in our power to employ a larger quantity of water in proportion to the lime, and thus forwarded the chrystallization of the calcareous matter, augmented the quantity of these chryftals, and rendered their quality more perfect. Those kinds of fand, therefore, which promote these purpofes in the highest degree, will be best adapted for mixing with mortar.

But, if fand confifts of irregular angular particles, a greater quantity of water will be retained in the vacuities formed between these angular pieces, than could have been, if the whole had confifted of round fmooth globules; and therefore it is natural to think, that rough, angular fand, will be more proper for this use than that which is fmoother.

Hence, if equally pure, fea-fand, which confilis of round globules, that have been worn perfely fmooth by the continued attrilion upon one another on the fhore, (like