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elaborate investigation of the principles of government; but the subject has been so completely exhausted in the American controversy, that we find no new light here thrown upon it. -Mr. Burke somewhere says, that “it is not easy to make a mono. poly of theorems and corollaries :" The Irish are too quicksighted not to perceive that the arguments employed by America are ready-made to their hands; and if they do not apply them to their own case, it certainly is not the fault of their writers.--Among the able advocates that have ftarted up in their caure, what precise degree of rank and estimation the Author of this pamphlet may have obtained, we are not suficiently informed to pronounce t. Whether he be the first in point of popularity or not, he has a claim on the national gratitude, as first 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 so long postponed our account of his performance..
+ We have heard this publication ascribed to Ch-F Sh-d-n, Esq.
Art. VII. Experiments and Obfervations made with the View of in
proving the Art of composing and applying calcareous Cements, and of preparing Quicklıme : Theory of theje Arts; and Specification of the Authors cheap and durable Cement for building, &c By Bry. Higgins, M. D. 8vo. 5 s. unbound. Cadell. 1780.
HE Author of this performance availing himself of the
earths, and their change into quicklime in the process of calcio nation, by Dr. Black, and those who have succeeded him in this inquiry, was led to apply these discoveries to the useful purpose of improving the mortar used in our buildings; on the goodness of which their strength and durability chieħy depend : -a matter, as he observes, of great importance, in this country, particularly ; where the weather is lo variable and trying, and the mortar commonly used is fo bad, that the timbers of houses last longer than the walls; unless the mouldering cement be frequently replaced by pointing.'
Seeing however, the Author adds, that many years are requisite for the greatest degree of induration, which cementitious mixtures like mortar can acquire, or for our discovering the imperfe&tions of them; and that the life of man is too short to allow any confiderable improvements of them to be derived from such 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 uncried path to recover or to excel the
Roman cement, which in aqueducts and the most exposed structures has withstood every trial of fifteen hundred or two thousand years.
Dr. Higgins accordingly first made several experiments, with a view to ascertain the changes which lime-Itones or calcareous earths undergo in the several stages of calcination ; particularly with respect to the effential part of that process--the expulsion of the fixed air combined with them; the presence or absence of which conftitutes the principal difference between lime-ftone or chalk and quick-lime.
Among many other observations deduced from these experiments, we learn that lime-stone or chalk exposed only to a red heat, how long foever continued, still retains so large a portion of its fixed air *, (or acidulous gas,' as the Author denominates it) as to effervefce strongly with acids; and to lose no more. than one-fourth of its original weight; and that, in this state, ityflakes flowly and partially, and with little heat:--and that 'exposed to a heat barely sufficient to melt copper, it loses about one-third of its weight in twelve hours, and very little more in any longer time; that this lime effervesces but fightly in acids, heats much sooner and more strongly than the foregoing, when water is sprinkled on it, and Nakes more equably, and to a whiter powder. In a variety of trials, this lime appeared to be in the same state with the best pieces of lime, prepared in the common lime-kilos.'
From succeeding experiments we learn further, that a white heat, suficient to melt steel, preceded by a long continued red beat, is requisite to expel all the fixed air from pure lime-stone and calcareous earths; so as to render them perfectly non-effervefcent, and to give them the properties of Naking instantly on the addition of water, growing hilling hot, and falling into a fine white powder. In the process thus conducted, 48 ounces of lime-stone, after the total expulfion of its fixed air, loses 21 ounces of its weight.
The Author next shews, that the perfection of lime, pre. pared for the purpose of making mortar, consists chiefly in its being totally deprived of its fixed air. He ascertained the truth of this propofition, by making several parcels of mortar with Jime which had been more or less strongly calcined ; and had accordingly retained a less or greater quantity of its fixed air. He spread each fpecimen, as soon as it was made, to the thick. ness of half an inch, on a plain tile previously soaked in water ; and exposed 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 chuse to retain the old term, till custom has given a comple;e fanction to a better.
fifteen months, he found that the mortar made with well burned non effervescent lime hardened sooner, 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 sufficient to melt copper. But the specimens made with the lime which had been leaft burned, were incomparably worse than any of the others :-' for they never acquired any confiderable hardness, and they mouldered in the winter; the sooner as they contained more of the lime, and cracked more in drying.'
Confidering that the heat, which he found necessary to expel the last portions of fixed air from lime-ftone, was much greater than that usually employed in making live in this country; Dr. Higgins suspected that the lime commonly used in building is seldom or never sufficiently burned: and on examining fevesal specimens of such lime, he found this fufpicion to be just; for they all effervesced, and yielded more or less fixed air, on the addition of an acid, and Naked Nowly, in comparison with well burned lime.
The Author next relates Come experiments, which few how very quickly lime imbibes fixed air from the atmosphere; on its exposure to which, it by degrees soon lotes those characters which chiefly distinguish it from mere lime-stone or powdered chalk; by foon attracting from thence that very principle, to the ab. sence of which it owes its useful 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 deserve particular attention.
Two pounds, avoirdupois, of the best non-effervescent lime, consisting of pieces of the size of a walnut, exposed on a board, in a passage open to the air, acquired, in two days, an additional weight of fix ounces and one drachm. In six days, the lime had increased in weight above twelve ounces : and, in three weeks, it had absorbed a quantity of fixed air, equal in weight to one pound two ounces, one drachm and a half. A small al. lowance, indeed, or deduction, is to be made from this quantity, on account of moisture absorbed, at the same time, from the ac. mosphere. This however, according to the experiments made by the Author to ascertain this matter, amounts to so small a proportion as only one-twenty-fourth of the acquired weight.
An obvious practical inference is deducible from these observations :-- that as time owes its excellence to the expulsion of fixed air from it in the burning, it should be used as foon as possible after it is made, and guarded from exposure to the air, as much as possible, before it is used : as by such exposure it must become more unfit for mortar every hour that it is kept, either in a heap, or in casks pervious to air. In short, it muit more or less resemble lime which had been imperfeâly burned,
and had consequently retained a part of its original or native fixed air. Mortar must likewise suffer injury from the same caufe, though not perhaps in an equal degree; as not expofing fo much furface to the air as the dry and spungy lumps of quickTime: 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 case or two, in plaiftering and stucco work, in which this observation of theirs holds good, and which has given origin to this error.
From a consideration of the circumstance above mentioned, Dr. Higgins very properly deduces a principal cause of the imperfection of our common cements; even when the lime has been sufficiently burned.-Being exposed,' says 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 sand and water makes a friable perishable mass, 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 insufficient quantity of fuel. This lime remains in the kiln, to which the air has access, for many hours after it is burned. It is exposed for some 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 solely on the badness 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 Naked is as widely different from good lime, as it is from powdered chalk.'
For a similar 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 instance, from the water, fo largely used, first in Naking the Jime, 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 arising from this cause is prevented by the substitution of lime. water, so far as may be practicable or convenient.
The Author next recites various experiments made to ascertain 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 some of the oldest and hardeft cements, which he examined for this purpose.
His fubsequent 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 stoney hardness, is not caused by its drying, as has been supposed; but is principally owing to its absorption of fixed air from the atmo. sphere, and is promoted in proportion as it acquires this pring ciple; the accession of which is indispensably necessary to the induration of calcareous cements.
In the subsequent sections, the Author treats of the sand or gravel used in mortar, and of the effects produced by bone-alhes, plaifter powder, charcoal, fulphur, and various other fubftances, occafionally introduced into it for different purposes. His oba fervations and trials on this head are numerous ; but will not admit of abridgment. We fall only observe, that he draws very great advantages from the addition of bone-ahes, in various proportions, according to the different nature of the work for which the composition 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 so 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 satisfactory 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 observes, that the remembrance of an useful ingredient, or any knowledge once acquired in an art practised in so many countries, should have been loft. The excellent specimens of their cement, that remain in aqueducts and other structures, he considers rather as monuments of the good luck, than of the skill, of the builders. From the perusal of the following enumeration of the circumstances which might fortuitously concur, so as to render some of their cements uncommonly hard and durable ; our Readers will collect some of the circumstances requisite to the formation of good and lafting mortar, which we have not yet noticed.
When,' says he, the neighbouring quarries afforded good lime-stone, free from gypsum, and such 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 par. fimony in fuel; and when the vicinity of the lime-ftone, and the quick consumption of the lime in great massive works, prevented those injuries which it fuftains in long transportation and exposure, in the faking of great quantities of it at once, or in the keeping of mortar made with it, the ignorance of the artists could not produce any defects dependent on bad lime ; 6