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and in complicated apparatus much endangersthe veffels by tile partial ft refs on the parts thatclofely fit.
It may be added, that the modern ufe ofWoulfe’s apparatus, or fimilar contrivances, focompletely takes away the preffure of expanfivevapours in all diftillations, that in many in-ftances very fimple lutes will anfwer now, whereformerly it was neceffary to render the joiningsof veflels as firm as the veffels themfelves. Butthe proper application of all thefe kinds ofcements is of great importance to the practicalchemift to prevent continual Ioffes and difap-pointments.
CEMENTS CALCAREOUS.
In this article it is propofed to give an ac-count of the various cements ufed in building,into which lime enters as an effential conftituentpart, and in order to treat the fubjedft with adegree of clearnefs in fome meafure correfpond-ing to its importance, it will be advifable toartan^G every kind of calcareous cement underone or other of the following three divifions,firft, fimple calcareous cement: fecondly, watercements: thirdly, maftichs or maltha.
§ X. Simple Calcareous Cements.
This feftion includes thofe kinds of mortarwhich are employed in buildings on land, -andgenerally confift of lime, fand, and freflx water.
The various kinds of marble, chalk, andlime ftone, as far as regards their ufe in cements,may be divided into two fpecies; the firft beingpure, or nearly pure carbonat of lime, the fecondcontaining befides from to T V of clay andoxyd of iron. Previous to burning or calcina-tion, there are no external characters by whichthe fimple limeftones can be diftinguifhed fromthe argillo-ferruginous ones; but the - former,whatever may have been their colour in thecrude ftate, become when calcined, of a whitecolour, while the latter poffefs more or lefs of alight ochery tinge. The brown lime is by farthe bell for all kinds of cements, but the whitevarieties being more abundant, and allowing ofa larger proportion of fand, are generally madeufe of. It was an opinion of the ancients, andis ftill commonly received among architedls,that the hardeft lime ftone, ceteris paribus, fur-nifhes the belt lime; hence marble was con-fidered as fuperior to common limeftone, andthis latter to chalk. The experiments of Mr.Smeaton / however, fhow that this is entirely amiltake, common chalk and the hardeft Ply-mouth marble, when fimilarly treated, affordingCements of equal firmnefs.
* Hiil, ofEddyftone Light Houfe. ‘ Ibid.
When Carbonated lime has been thoroughlyburnt it is deprived of its water, and of all, ornearly all of its carbonic acid ; if in this ftate itis plunged into water, and immediately takenout again, the water which it has abforbed willoccafion the mafs to crack and become excef-fively hot, and at length to fall into an impalp-able powder, much of the water being carriedoff in the form of fteam during the procefs.When lime has been thus flacked, if it is beatenup with a little water into a very ftiff pafte andallowed to dry, it will be found that the whitelimes, whether from chalk or marble, never ac-quire any degree of hardnefs, that the brownlimes become confiderably indurated, thoughnot fo much fo as when mixed with fand, andthat fhell lime (procured by calcining fea-lhells)concretes into a firm hard cement well qualifiedfor dry building, although it falls to pieces inwater. 1 *
A proper felection of fand is of great import-ance in the compofition of mortar ; the {harperand coarfer it is, the better, both becaufe it re-quires a fmaller proportion of lime, which isthe molt coftly ingredient, and becaufe themortar thus prepared is ftronger than when finegrained and round fand is made ufe of. Sea fandrequires to be well walhed in frelh water todiffolve out the fait with which it is mixed, other-wife the cement, into which it enters, neverbecomes thoroughly dry and hard. The Romanbuilders were accuftomed to allow four partsof coarfe fharp pit fand, and only three partsof river or fea fand to one of lime. c If, how-ever, the cement was required to be very com-pact, the proportion of lime was increafed: thusthe mortar ufed in conftrucling refervoirs forwater, confifted of two parts of the ftrongeftlime and five parts of pure and fharp fand. d
The weaknefs of modern mortar comparedto the ancient is a common fubjett of regret,and many ingenious men taking for grantedthat the procefs ufed by the Roman architectsin preparing their mortar is one of thofe artswhich are now loft, have employed themfelvesin making experiments to recover it inftead ofattending to the directions left us in Pliny andother authors, which when illuftrated by theactual practice of builders in various parts ofEurope , feem to leave little or no doubt on thefubject.
The charadteriftic of all modern artifts,builders among the reft, feems to be to fpare theirtime and labour as much as poffible, and to in-creafe the quantity of the articles they produce