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to affect the hardness of the mortar, yet it must still retainits white chalk-like appearance. As marble and limestoneare, however, always formed by those particles of lime thathave been wholly dissolved in water, and from which theyhave been gradually separated by a more slow and more per-fect mode of crystallization, they have nothing of that opaquecalx-like appearance, but assume other colours, and appearmore firm, uniform, and compact; the sand and other mattersthat may be enveloped in them being entirely surroundedwith a pure crystalline matter.

But to obtain the most perfect kind of mortar, it is not,however, enough that a large proportion of sand should beemployed, and that the sand should be intimately mixed withthe lime; it is also of the utmost importance that a largeproportion of water be added; for, without this, it is impos-sible that a large proportion of the lime can be crystallized:and the mortar, in that case, would consist only of a mixtureof chalky matter and sand, which could hardly be made tounite at all, and would be little more coherent than sand byitself, and less so than pure chalk. In that case, pure limealone must afford rather a firmer cement than lime with sand.It is also of very great importance that the water be retainedas long in the mortar as possible; for if it be suddenly eva-porated, it will not only be prevented from acting a secondtime upon the lime, after a part of what was first dissolvedhas been crystallized, but even the few crystals that wouldbe formed when the water was suddenly evaporating, wouldbe of themselves much more imperfect than they otherwisemost certainly would have been. In proof of which, instancesof the crystallization of common salt, lump-sugar, and sugar-candy, are adduced; after which it is noticed, that every oneknows what a difference there is between the firmness of thedifferent substances; and that as great must be the differencebetween the firmness of that cement which has been slowlydried, and that which has been hastily hardened by the power-ful action of a warm air.

It is contended, that it is owing to this circumstance thatthe lime, which remains all winter in a mortar-tub filled withwater, is always found to be much firmer and more coherentthan the mortar that was taken from the same tub and usedin any work of masonry, although in this case the materialswere exactly the same. From the same cause, any workcemented with lime under water, if it has been allowed toremain undisturbed and uninjured until it has once becomehard, is always much firmer than that which is above thesurface of the water.

In order to render the force of the above reasoning morestrong and convincing, lime-cement or mortar is compared toa mass of matter consisting of a congeries of stones closelycompacted together, and united by a strong cementing mat-ter, that had, while in a fluid state, pervaded all the inter-stices between the stones, and afterwards become a solidindissoluble substance. If the cementing matter be exceed-ingly hard and coherent, and if the stones bedded among itbe also very hard and firm, the whole mass will become likea solid rock, without fissures, that can hardly be brokento pieces by the power of man. But, although the cementshould be equally firm, if the stone, of which it consists, beof a soft and friable nature, suppose chalk or sand-stone, thewhole mass will never be capable of attaining such a degreeof firmness as in the former case; for, when any force isapplied to break it in pieces, although the cement shouldkeep its hold, the solid matter cemented by it would giveway, and the whole would be easily broken to pieces.Whereas, in mortar, the sand that is added to it representsthe stones of a solid matter, in the composition, the particlesof which are united together by the lime which had been

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exceedingly solid and indissoluble concretion ; -- ,

particles of sand are of themselves exceedingly "the cement by which they are united equally so, d ^ ^that the whole concretion must be extremely fir®* £ r olOrequire very great force to disunite any particle o or il/the whole mass. But if, instead of employing sa “.m » tte !solid body that is entangled among the cementm 0 g j S tsshould be chalk, (as in all cases where the mortar taB ce,of pure lime alone,) or any other slightly cohering s u solet the cementing particles of that composition b ^perfect, it is impossible that the whole can ever attain^ e£Lg iiydegree of firmness, as these chalky matters willbroken asunder. iectu re *

It is remarked, in addition, that a variety of C g n t e.10’have been made about the nature of the ]ime-cem poS*ployed by the ancients. It has been thought that s jjace

sessed an art of making mortar which has been ^ e e0

entirely lost; as the cement in the walls which 11built by them, appears to be, in many cases, mu M'h eI1

il, — _i__■_fJmPS. *

than that which had been made in modern times.

SnshT 686 ° Id buiIdi «gs is analyzed, it is & un ‘

t °'' of tbe same materials, and nearly in the same P'°K t

probable "f 1C A, th . ey are now use of. And it is tbo»,rials mr! mt f 16I f ° niy secret consisted in mixing the &build re P er f ect ly than the rapidity or avarice of nJ c ] )buddersw 111 permit; in employing their mortar in.«

fl i lld ) State , than is done ; and in allowing it ® *

their S °iT w ‘j Cb tbe immoderate thickness ot m c0 g-Jer ed d • d J naturall J’ Produce, without any P* dedZ,vni« 7 Sn ° n ‘ heir part - Tradition has even •„!down to the present times the memory of the most esse

mrfc nf 6 £ artlcu / ars > as the lower class of people, &bplfp-iro *1 e * n ? tI0n » at this moment, invariably supp' nr 30vp , . lat these old walls were composed of a ® ^ u jd,hotw ln / as to udmit of its being poured, hk e j th eetween the stones, after they were laid in the wall: a (gappearance of these old walls, when taken down, see ,avour this popular tradition. Nor is it doubtedtins may have been the ease. The stones in the o

e wall, it is thought, were probably bedded m ^eing” “f y , as t® Practised at present; and the heart, aft «f tic eSP< e well with irregular stones, might have the inbetween them entirely filled up with fluid mortar, ewould insinuate itself into every cranny, and, in tun*' { th eas firmly as the stones themselves, or even more so, ^stones were of a sandy friable nature. And that, a ofte nwalls were usually of very great thickness, it jvel/

happen, that the water in this mortar, by acting sue t h heupon different particles of caustic lime, would at ° toentirely absorbed by successive crystallizations, ^j. inbecome perfectly dry, without any evaporation ^which case, a very large proportion of the ° rl ° w 7*bly P el "have been regularly crystallized in a slow and toler** •fect manner, so as to attain a firmness little infersstone, or marble itself. . ■ e asJ 10

It is supposed that, upon these principles* ^ \vl ,eaaccount for the superior hardness of some old ce . t ; c e ^compared with that of modern times, in which a P todifferent is usually followed, without having recowonderful arcana whatever. pp3 th a*

. Ther e are likewise a few other circumstance ]j0eS toO

influence the quality of common lime-mortar. j S ture» * *be sufficiently calcined, it is deprived of all exP et>e °of

oi all its carbonic acid gas, or fixed air. ^ e ffusfjshows, that limestone will fall to powder on j c joed<water upon it, when it is much less perfectly

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