fifteen months, he found that the mortar made with well burned non effervescent lime hardened fooner, and to a much greater degree, than that made with common lime, or with the lime above mentioned, prepared by the Author in a heat barely fufficient to melt copper. But the fpecimens made with the lime which had been leaft burned, were incomparably worse than any of the others :-' for they never acquired any confiderable hardnefs, and they mouldered in the winter; the fooner as they contained more of the lime, and cracked more in drying.'

Confidering that the heat, which he found neceffary to expel the laft portions of fixed air from lime-ftone, was much greater than that ufually employed in making lime in this country; Dr. Higgins fufpected that the lime commonly used in building is feldom or never fufficiently burned: and on examining feverai fpecimens of fuch lime, he found this fufpicion to be just; for they all effervefced, and yielded more or lefs fixed air, on the addition of an acid, and flaked flowly, in comparison with well burned lime.

The Author next relates fome experiments, which fhew how very quickly lime imbibes fixed air from the atmosphere; on its expofure to which, it by degrees foon lofes thofe characters which chiefly diftinguish it from mere lime-ftone or powdered chalk; by foon attracting from thence that very principle, to the abfence of which it owes its ufeful quality as a cement, and which had before been expelled from it in the burning. As this observation is of the utmost importance, the experiments on which it is founded delerve particular attention.

Two pounds, avoirdupois, of the beft non-effervefcent lime, confifting of pieces of the fize of a walnut, expofed on a board, in a paffage open to the air, acquired, in two days, an additional weight of fix ounces and one drachm. In fix days, the lime had increased in weight above twelve ounces: and, in three weeks, it had abforbed a quantity of fixed air, equal in weight to one pound two ounces, one drachm and a half. A fmall allowance, indeed, or deduction, is to be made from this quantity, on account of moisture abforbed, at the fame time, from the atmosphere. This however, according to the experiments made by the Author to afcertain this matter, amounts to fo fmall a proportion as only one-twenty-fourth of the acquired weight.

An obvious practical inference is deducible from thefe obfer. vations that as line owes its excellence to the expulfion of fixed air from it in the burning, it fhould be ufed as foon as poffible after it is made, and guarded from expofure to the air, as much as poffible, before it is used: as by such exposure it muft become more unfit for mortar every hour that it is kept, either in a heap, or in cafks pervious to air. In fhort, it must more or less resemble lime which had been imperfcctly burned,

and

and had confequently retained a part of its original or native fixed air. Mortar muft likewife fuffer injury from the fame caufe, though not perhaps in an equal degree; as not expofing fo much furface to the air as the dry and fpungy lumps of quickJime: though it is a prevailing opinion among the workmen, who generally make a large quantity at once, that it is not the worfe, but rather the better, for being kept fome time. The Author takes notice of a particular cafe or two, in plaistering and ftucco work, in which this obfervation of theirs holds good, and which has given origin to this error.

From a confideration of the circumftance above mentioned, Dr. Higgins very properly deduces a principal caufe of the imperfection of our common cements; even when the lime has been fufficiently burned. Being expofed,' fays he, a confiderable time before it is made into mortar, and drinking in acidulous gas all the while, the quicker as it is the better burned, it is incapable of acting like good lime, when it is made into mortar; and often approaches to the condition of whiting, which with fand and water makes a friable perishable mafs, however carefully it be dried. In London particularly, they ufe lime which is burned, at the distance of ten or twenty miles or more, in Kent and elsewhere, with an infufficient quantity of fuel. This lime remains in the kiln, to which the air has accefs, for many hours after it is burned. It is expofed for fome days in the tranfportation, and on the limewharfs; and it undergoes further expofure and carriage before the artist flakes it for mortar. It is no wonder that the London mortar is bad, if the imperfection of it depended folely on the badnefs of the lime; fince the lime employed in it is not only bad when it comes fresh from the kiln, but becomes worfe before it is ufed, and when flaked is as widely different from good lime, as it is from powdered chalk.'

For a fimilar reafon, every other caufe, which tends to reftore to the lime the fixed air of which it had been deprived in the burning, must deprave it. It must receive this kind of injury, for inftance, from the water, fo largely used, first in flaking the lime, and afterwards in making it into mortar; if that water contains fixed air, from which few waters are perfectly free; and which will greedily be attracted by the lime. The injury arifing from this caufe is prevented by the fubftitution of limewater, fo far as may be practicable or convenient.

The Author next recites various experiments made to afcertain the best relative proportions of lime, fand, and water, in the making of mortar; and afterwards authenticates the best proportions thus indicated in his trials, by an actual analysis of fome of the oldest and hardest cements, which he examined for this purpofe.

His fubfequent experiments fhew, that though the fetting of mortar (as it is called by the workmen) chiefly depends on the exficcation of it; yet its induration, or its acquiring a ftoney hardness, is not caufed by its drying, as has been fuppofed; but is principally owing to its abforption of fixed air from the atmofphere, and is promoted in proportion as it acquires this principle; the acceffion of which is indifpenfably neceffary to the induration of calcareous cements.

In the fubfequent fections, the Author treats of the fand or gravel ufed in mortar, and of the effects produced by bone-afhes, plaifter powder, charcoal, fulphur, and various other fubftances, occafionally introduced into it for different purpofes. His ob- . fervations and trials on this head are numerous; but will not admit of abridgment. We fhall only obferve, that he draws very, great advantages from the addition of bone-afhes, in various proportions, according to the different nature of the work for which the compofition is intended.

It is

An opinion has long been entertained, that the ancients ufed fomething which is unknown to us in their mortar; and that this long loft ingredient is the caufe of the hardness and duration of those cements, which we fo much admire in fome of their buildings. The Author however controverts the fuppofed skill of the ancients on this head; and alleges that we have the moft fatisfactory reasons to conclude that it did not exceed that of our modern builders, in the preparation of lime or mortar. certainly very improbable, as he obferves, that the remembrance of an ufeful ingredient, or any knowledge once acquired in an art practised in fo many countries, fhould have been loft. The excellent fpecimens of their cement, that remain in aqueducts and other ftructures, he confiders rather as monuments of the good luck, than of the fkill, of the builders. From the perufal of the following enumeration of the circumftances which might fortuitoufly concur, fo as to render fome of their cements uncommonly hard and durable; our Readers will collect fome of the circumftances requifite to the formation of good and lasting mortar, which we have not yet noticed.

When,' fays he, the neighbouring quarries afforded good lime-ftone, free from gypfum, and fuch as required to be well burned, before it could flake freely; when the preparation of the lime, at the public expence, afforded no temptation for parfimony in fuel; and when the vicinity of the lime-stone, and the quick confumption of the lime in great maffive works, prevented thofe injuries which it fuftains in long transportation and expofure, in the flaking of great quantities of it at once, or in the keeping of mortar made with it, the ignorance of the artifts could not produce any defects dependent on bad lime;

6

because

becaufe neceffity or chance inforced all that could have been fought by choice, in this inftance.

When the vicinity afforded fand, clean, quartofe, sharp, well fized, and refembling our mixture of the coarse and fine; chance furnished all that kill could aim at, in the choice and preparation of this article.

When walls of immenfe thickness were constructed chiefly with fmall ftones, in the way of boulder-work, the great confumption of mortar made every practicable faving of lime an object of great importance; and as the mortar must be made ftiff for fuch work, it was neither convenient nor neceffary to mix much lime in it, or to ufe fine fand in it, or to exclude the rubble from it: and thus, by motives of economy and convenience, rather than by any others, they were led to the measures which infured, to the cement of fuch ftructures, every perfection dependent on the goodness of lime and fand, and on good, if not the beft, proportions of them.

When the ftones ufed in building were recently dug, or collected from the beds of rivers, the artifts needed no precautions against the bad effects of dry, bibulous, and dusty stones or bricks; and their works had, of neceffity, every good quality attainable by the practice, which I commend, of foaking thele materials. When their water was good, the cement, abounding in lime, was not much the worfe for their ignorance of the ufe of lime water.

When the structure was intended to ftand by its own ftrength, rather than to depend on timbers; and was, by the folidity of its bearings and the diameter of its ftoney fubftance, fecured from agitation; when the thickness of the walls prevented the cement from being haftily dried, and afterwards fee cured it from being thoroughly wetted; and when the enormous weight contributed to the approximation and cohefion of the parts of the cement to each other, and to the ftones: every defect of cementitious buildings, of a contrary defcription, was obviated by the nature of the ftructure; which rendered it as perfect, in the hands of any artists, as the most confummate fkill could make our modern, flender, tremulous, bibulous walls.' It will naturally be expected that Dr. Higgins fhould take notice of the reputed improvement lately made in the preparation of mortar, by M. Loriot, and published by order of the King of France. With refpect to this difcovery (made public by order of his Majefty in 1774), we need fay no more to the Reader who is acquainted with the rationale of the process, as above laid down, than that M. Loriot ufes old flaked lime, that is, lime which has regained a part of the fixed air that had been expelled from it; and then, at the time of ufing the mortar,

adds

adds fresh powdered quicklime to the mafs. In fhort, not being acquainted with the true principles of the art, he corrects the bad quality of the old and effete lime, which conftitutes the bafis of his mortar, by the addition of fresh and non-effervescent lime, haftily added to it, at the time of ufing the compofition, and which muft undoubtedly improve the imperfect mafs. On this fubject, the Author, very properly in our opinion, obferves, that when an ignorant artift makes mortar with whiting inftead of lime, he can mend it confiderably by adding lime to it: but his mortar will still be defective, in comparison with the beft that may be made, by reafon of the old flaked lime or whiting. For on repeated trials I found this to be the true ftate of the cafe *.'

་

Before we conclude this article, we fhould add, that the Author's obfervations and precepts do not merely reft on the fmall trials above mentioned, made with mortar fpread on tiles, &c.; but that he has realifed them in practice, or on the great fcale, particularly in incrustations, external and internal. Towards the end of this performance is given A Specification made in confequence of letters patent' granted to him; in which are minutely delivered the most useful practical inftructions, deduced from the foregoing experiments and obfervations. We here learn too that the Author has fecured to Meffrs. James and Samuel Wyatt, architects, an exclufive right in his cement: intending to confine this privilege to them, till the public convenience requires its extenfion to others equally well qualified and difpofed to adopt his improvements in practice. With respect to his new cements employed in incrustations or flucco, exceeding Portland ftone in hardness,' he refers the Reader to feveral houfes in London and its neighbourhood, which he fpe'cifies; in which public trials have been made of his compofition, towards the end of the year 1778, and laft year; and adds that Meffrs. Wyatt are engaged to ftucco a great number of capital houses with it this fummer.- Thefe,' fays the Author, will be done in the highest perfection, becaufe the workmen are now compliant, and experienced.'

We cannot conclude this Article without obferving, that the public are much obliged to the ingenious Author for his meritorious attempts to improve an art in which they are so much interested; and for communicating to them the refults of his numerous experiments, and the principles deducible from them. Thefe principles, or at leaft a rational practice founded upon them, will foon, we hope, become known and familiar to the inferior artists, so as to conftitute a part of their routine.

For our account of M. Loriot's publication relative to this fubject, fee Rev. vol. li. p. 184.

ART,