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130

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by him, is as follows, by measure: Lime, newly slacked, onepart; fine sand, three parts; coarse sand, four parts. Healso found that an addition of one-fourth part of the quantityof lime, of burnt bone-ashes, improved the mortar, bygiving it tenacity, and rendering it less liable to crack indrying.

The mortar should be made under ground, then coveredup, and kept there for a considerable length of time, thelonger the better; and when it is to be used, it should bebeat up afresh. This makes it set sooner, renders it lessliable to crack, and harder when dry.

The stony consistence which it acquires in drying, isowing to the absorption of carbonic acid, and a combinationof part of the water with the lime: and hence it is that limethat has been long kept after burning is unfit for the purposeof mortar, for in the course of keeping, so much carbonicacid has been imbibed as to have little better effect, ina composition of sand and water, than chalk or lime-stonei reduced to a powder from the crude state, would have inplace of it.

Grout is a cement, containing a larger proportion of waterthan is employed in common mortar, so as to make it suffi-ciently fluid to penetrate the narrow irregular intersticesof rough stone walls. Grout should be made of mortarthat has been long kept and thoroughly beat, as it will thenconcrete in the space of a day: whereas, if this precautionbe neglected, it will be a long time before it sets, and mayeven refuse setting for ever. See Grout.

Mortar made of pure lime, sand, and water, may beemployed in the linings of reservoirs and aqueducts, pro-vided it have sufficient time to dry; but if the water beput in while it is wet, it will fall to pieces in a short time;and, consequently, if the circumstances of the building besuch as render it impracticable to keep out the water, itshould not be used: there are, however, certain ingredientsput into common mortar, by which it is made to set imme-diately under water, or if the quick-lime contain in itself acertain portion of burnt clay, it will possess this property.

From the friable and crumbling nature of our mortar,a notion has been entertained by many persons, that theancients possessed a process in its fabrication, which has beenlost at the present day; but the experiments of Mr. Smeaton ,Dr. Higgins, and others, have shown this notion to be un-founded, and that nothing more is wanting than that thechalk, lime-stone, or marble, be well burned, and thoroughlyslacked immediately, and to mix it up with a certain propor-tion of clean large-grain sharp sand, and as small a quantityof water as will be sufficient for working it; to keep ita considerable time from the external air, and to beat it overagain before it is used: the cement thus made will be suffi-ciently hard.

The practice of our modern builders, is to spare theirlabour, and to increase the quantity of materials they pro-duce, without any regard to its goodness : the badness of ourmodern mortar is to be attributed both to the faulty natureof the materials, and to the slovenly and hasty methods ofusing it. This is remarkably instanced in London , wherethe lime employed is chalk lime, indifferently burnt, con-veyed from Essex or Kent, a distance of ten or twentymiles, then kept many days without any precaution to pre-vent the access of external air. Now, in the course of thistime it has absorbed so much carbonic acid as nearly to loseits cementing properties, and though chalk lime is equallygood with the hardest liine-stone, when thoroughly burned,yet, by this treatment, when it is slacked, it falls into a thinpowder, and the core or unburned lumps are ground down,and mixed up in the mortar, and not rejected, as it ought to be.

The sand is equally defective, consisting of small globulagrains, containing a large proportion of clay, which provenit from drying, and attaining the necessary degree of barness. These materials being compounded in the most has ;manner, and beat up with water in this imperfect state,not fail of producing a crumbling and bad mortar; andcomplete the miserable composition, screened rubbish, a0 _the scraping of roads, are thrown in, as substitutespure sand.

for

How very different was the practice of the Romans!

The

d

lime which they employed was perfectly burnt, the sa °sharp, clean, and large-grained: when these ingredientsmixed in due proportion, with a small quantity of water, ‘mass was put into a wooden mortar, and beaten with a hea ;wooden or iron pestle, till the composition adhered to ^mortar; being thus far prepared, they kept it till it ' vaS o9 tleast three years old. The beating of mortar is of the utn 1consequence to its durability, and it would appear that ■effect produced by it, is owing to something more than a 111mechanical mixture. See Mortar. , g y

Water cements are such as are impervious to water: 1 jare generally made of common mortar, or of pure lin 1 ® pwater, with the addition of some other ingredient to g lVthe property of hardening under water.

For this purpose there are several kinds of ingredient’ ^puzzolana, cellular basalt, or wakke, compact basalt, .ashes, coal-cinders, wood-ashes, pumice-stone, brick- 0powder of quick-lime, forge-scales, roasted iron-ore, ( j 0l iThe cement employed by Mr. Smeaton , in the construe^of the Eddystone lighthouse , was composed of equal P‘by measure, of slacked Aberthaw lime and puzzolana > gproportion was thought advisable, as the building was exp ^; cto the utmost violence of the sea: but for other ‘ 0 fworks, as locks, basins, canals, &c., a composition ina rt |oBilime, puzzolana, sand, and water, in the following prop r

,ort -°of

lid

viz., two bushels of slacked Aberthaw lime, one bus*

er/

puzzolana, and three of clean sand, has been foundeffectual. It is well known, that sand and lime, mix e °tlier with care, will incorporate and form a mortar > 0 pSvious to water, and sufficient even for the linings ofand reservoirs ; but then the mortar must be hardened joit is exposed to the water, or otherwise it will crUlD .. u ir 0pieces; and therefore, if the situation be such as to ^ pii9the mortar to be dried in a certain time, the usecement must be abandoned.

Among the ancient nations, the Romans appear . a n/been the only people who practised building in wateigreat extent, particularly in the sea. The discovery of P uis attributed to the following circumstance, among t 1 jpg'people. The Bay of Baite, like our fashionableplaces, was the summer resort of all the wealthy 10 ^i° lthe inhabitants of this place did not content themsel' pie/erecting their houses as near the shore as possible, tb e -veven constructed moles and small islands, onerected their summer-houses in the morethe bay. By the fortunate discovery of an eaithy s upd *at the neighbouring town of Puteoli , they were j tl°build both expeditiously and securely in water. | ^ pult 1circumstance, the earth thus discovered was cal » o <Puteolanus, “ powder of Puteoli, ” “ Puteolean P 0 "j ne r^as it is now denominated, puzzolana, which is a in gll pp °f e }la light porous, friable nature, and of a red co l° u ''v eg u'’ji 9to be formed by concretion of the volcanic ashes ot ^ T^near to which mountain the town of Puteoli is sltu ‘ v/b tC j

original material seems to be a ferruginous

clay’

, 'V

baked and calcined by the force of volcanic fire> &awith common mortar, not only enables it to acquire