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elaborate investigation of the principles of government; but the fubject has been fo completely exhaufted in the American controverfy, that we find no new light here thrown upon it.-Mr. Burke fomewhere fays, that "it is not eafy to make a monopoly of theorems and corollaries:" The Irish are too quickfighted not to perceive that the arguments employed by Ame rica are ready-made to their hands; and if they do not apply them to their own cafe, it certainly is not the fault of their writers. Among the able advocates that have ftarted up in their cause, what precife degree of rank and estimation the Author of this pamphlet may have obtained, we are not fufficiently informed to pronounce +. Whether he be the firft in point of popularity or not, he has a claim on the national gratitude, as firft in the order of time; having appeared in print before the letters figned Guatimozin, and other popular publications, forced themselves into notice. We lament the mistake which has fo long poftponed our account of his performance..

We have heard this publication afcribed to Chs F Sh-d-n, Efq.

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ART. VII. Experiments and Observations made with the View of im proving the Art of compofing and applying calcareous Cements, and of preparing Quicklime: Theory of theje Arts; and Specification of the Authors cheap and durable Cement for building, Sc. By Bry. Higgins, M. D. 8vo. 5 s. unbound. Cadell. 1780.

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HE Author of this performance availing himself of the lights thrown on the nature of lime-ftone and calcareous earths, and their change into quicklime in the process of calci nation, by Dr. Black, and thofe who have fucceeded him in this inquiry, was led to apply these discoveries to the useful purpofe of improving the mortar ufed in our buildings; on the goodness of which their ftrength and durability chiefly depend: -a matter, as he obferves, of great importance, in this country particularly; where the weather is fo variable and trying, and the mortar commonly used is fo bad, that the timbers of houfes laft longer than the walls; unless the mouldering cement be frequently replaced by pointing.'

Seeing however, the Author adds, that many years are requifite for the greatest degree of induration, which cementitious mixtures like mortar can acquire, or for our discovering the imperfections of them; and that the life of man is too short to allow any confiderable improvements of them to be derived from fuch experiments as had hitherto been made; I refolved in the beginning of the year 1775 to investigate more closely than I had hitherto done, the principles on which the induration and ftrength of calcareous cements depend; not doubting that this would lead me by an untried path to recover or to excel the

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Roman cement, which in aqueducts and the most exposed ftructures has withftood every trial of fifteen hundred or two thousand years.'

Dr. Higgins accordingly first made feveral experiments, with a view to ascertain the changes which lime-ftones or calcareous earths undergo in the feveral stages of calcination; particularly with refpect to the effential part of that procefs-the expulfion of the fixed air combined with them; the prefence or absence of which conftitutes the principal difference between lime-ftone or chalk and quick-lime.

Among many other obfervations deduced from thefe experiments, we learn that lime-ftone or chalk exposed only to a red heat, how long foever continued, ftill retains fo large a portion of its fixed air*, (or ‘acidulous gas,' as the Author denominates it) as to effervefce ftrongly with acids; and to lofe no more. than one-fourth of its original weight; and that, in this state,

flakes flowly and partially, and with little heat:-and that expofed to a heat barely fufficient to melt copper, it lofes about one-third of its weight in twelve hours, and very little more in any longer time; that this lime effervefces but flightly in acids, heats much fooner and more ftrongly than the foregoing, when water is fprinkled on it, and flakes more equably, and to a whiter powder. In a variety of trials, this lime appeared to be in the fame ftate with the best pieces of lime, prepared in the common lime-kilns.'

From fucceeding experiments we learn further, that a white heat, fuficient to melt fteel, preceded by a long continued red beat, is requifite to expel all the fixed air from pure lime-ftone and calcareous earths; fo as to render them perfectly non-effervefcent, and to give them the properties of flaking inftantly on the addition of water, growing hiffing hot, and falling into a fine white powder. In the procefs thus conducted, 48 ounces of lime-ftone, after the total expulfion of its fixed air, lofes 21 ounces of its weight.

The Author next fhews, that the perfection of lime, prepared for the purpose of making mortar, confifts chiefly in its being totally deprived of its fixed air. He afcertained the truth of this propofition, by making feveral parcels of mortar with Jime which had been more or lefs ftrongly calcined; and had accordingly retained a lefs or greater quantity of its fixed air. He fpread each fpecimen, as foon as it was made, to the thicknefs of half an inch, on a plain tile previously foaked in water; and expofed them equally in an open place to the influence of the fun and rain. Comparing them at the end of fourteen or

For reasons that we have formerly aligned, we chufe to retain the old term, till custom has given a complete fanction to a better. fifteen

fifteen months, he found that the mortar made with well burned non effervefcent 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 specimens made with the lime which had been leaft burned, were incomparably worfe 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 ufed in building is feldom or never fufficiently burned: and on examining feveral 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 obfervation is of the utmost importance, the experiments on which it is founded deferve particular attention.

Two pounds, avoirdupois, of the best 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 increafed 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 absorbed, 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 obfervations that as lime 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 ufed: as by fuch expofure it muft become more unfit for mortar every hour that it is kept, either in a heap, or in casks pervious to air. In fhort, it must more or lefs refemble lime which had been imperfe&ly burned,

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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 quicklime 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 plaiftering 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 perifhable mafs, however carefully it be dried. In London particularly, they use 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 exposed for fome days in the transportation, and on the limewharfs; and it undergoes further exposure 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 worse before it is used, and when flaked is as widely different from good lime, as it is from powdered chalk.'

For a fimilar reason, every other cause, which tends to restore to the lime the fixed air of which it had been deprived in the burning, muft deprave it. It must receive this kind of injury, for inftance, from the water, so 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 ariling from this caufe is prevented by the fubftitution of lime, water, 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 oldeft and hardest cements, which he examined for this purpose.

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His fubfequent experiments fhew, that though the setting 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 caused by its drying, as has been fupposed; but is principally owing to its abforption of fixed air from the atmofphere, and is promoted in proportion as it acquires this prin ciple; 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 used in mortar, and of the effects produced by bone-ashes, plaifter powder, charcoal, fulphur, and various other fubftances, occafionally introduced into it for different purposes. His obfervations 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.

An opinion has long been entertained, that the ancients used fomething which is unknown to us in their mortar; and that this long loft ingredient is the cause of the hardness and duration of those cements, which we fo much admire in some of their buildings. The Author however controverts the supposed skill of the ancients on this head; and alleges that we have the most fatisfactory reasons to conclude that it did not exceed that of our modern builders, in the preparation of lime or mortar. It is certainly very improbable, as he obferves, that the remembrance of an useful ingredient, or any knowledge once acquired in an art practifed in fo many countries, fhould have been loft. The excellent fpecimens of their cement, that remain in aqueducts and other structures, 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 circumstances requifite to the formation of good and lasting mortar, which we have not yet noticed.

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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-ftone, 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; because

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