Cl I At’. V.

MORTAR AND CEMENTS.

7id

Leicestershire , are all on the same bed, and Smeaton found, upon a careful analysis, thatthe proportions of iron and clay in each were the same, or about 11 or 12 per cent.

For the Eddystone lighthouse he used equal measures of Aberthaw lime, in the state ofhydrate, and of fine powdered puzzolana, proportions which when reduced to weight, andallowing 24 per cent, for water, agree with those stated by Vitruvius to be in use amongthe Romans. The Dorking lime is used for land and water cements, as is the Merstliam,both of which are obtained from the chalk marl, as is the Hailing on the Medway.

In the cements made from lias the oxide of iron they contain appears to combine withthe lime during the process of burning, and sets without difficulty when mixed with a largeproportion of sand.

The Tournay mortar is a lias cement, and thus formed : after the lumps of lias arewithdrawn from the kiln, what remains with the ashes of the slaty coal, in the propor-tion of three of ashes and one of lime, is sprinkled with water, just sufficient to slack thelime. It is then beaten by an iron pestle for half an hour ; this is repeated three or fourtimes, until it attains the consistence of mortar. It is then placed under cover, and in a fewdays beaten again, either with stone or brick ; it acquires in a short time great hardness.The coal ash in this instance is burnt clay in a state of fine division, and therefore ready tocombine rapidly with the argillo-ferruginous lime, which, being in a state of hydrate, andallowed to be some time in contact with the ash, a combination takes place, which isfurther increased by beating, the lime parting with its water, and combining with theash.

As it is of the utmost importance to understand the nature of the hydraulic limestones,M. Berthier recommends the following method of analysis : after the specimen has beenreduced to a fine powder, it is passed through a hair-sieve, and a given quantity is put intoa vessel; muriatic or nitric acid, diluted with a small quantity of water, is poured upon it,taking care to stir the whole with a glass or wooden rod. When the effervescence hasbegun, the solution is to be evaporated by a gentle heat, until the whole becomes a thickpaste ; this is put again into a small quantity of water, and filtered; all the clay containedin the paste will remain in the filter, and the substance which has passed through must bedried in the sun or by the fire, and afterwards weighed : or the specimen to be examinedmay be calcined to redness in an earthenware or metal crucible, and very clear water beingpoured into the solution, a precipitate will be formed, which is magnesia ; this, washed inpure water, and dried speedily, is to be then weighed. The clay may be thus estimated, aswell as any fine sand that it may contain.

The only apparent difference between lime obtained from limestones and chalk is that ofthe greater retention or expulsion of the carbonic acid gas, and both Smeaton and Higginshave proved that when chalk or stone lime is equally fresh from the kiln, their qualities ascements are nearly equal ; but as chalk lime absorbs carbonic acid more rapidly from itsspongy texture, it loses much of its cementing quality, and does not make so good mortaras the lime from stone.

Limestones are sometimes found wholly composed of lime and carbonic acid : the besthydraulic limestones contain silica, alumina, magnesia, iron, and manganese; the silicabeing the most abundant.

The rich or fat Limes are those which in slaking double their volume, and after havingbeen immersed for a length of time retain their consistency, and in pure water will dis-solve to the last particle; they are derived from the pure limestones, which contain from0*1 to 06 of silica, alumina, magnesia, and iron; they absorb in slacking nearly 300 percent, of their weight of water.

The poor Limes do not much augment their volume, and only partly dissolve, leaving aresidue, which has little or no consistency ; they are produced from the limestone in whichsilica is present in the state of sand, magnesia, the oxides of iron and manganese, and absorbabout 200 per cent, of water.

The moderately hydraulic Limes set in 15 or 20 days after they are immersed, and thencontinue to become harder in quality; at the end of a twelvemonth they acquire the con-sistence of soap, in pure water dissolve with difficulty, and expand in slaking.

From Limestone , united with Clay, Magnesia , Iron , and Manganese , in the proportion ofnot more than 15 or 18 parts out of 100 of the whole, are obtained the hydraulic limes ; theseset after six or seven days’ immersion, and continue to acquire hardness; such lime absorbs,on slacking, 250 per cent, of its weight in water. Some of the best hydraulic limes areobtained from limestones which have, in addition, a greater amount of silica, or where itoccupies nearly half the whole quantity of the other substances ; these kinds set on thesecond or third day after immersion, and in a month become hard and perfectly insoluble.I-itne is said to set when it will bear without depression a rod the twentieth part of an inchm diameter, loaded with a weight of 10 or II ounces avoirdupois, or when it will resistany indentation made by the finger moderately pressed.

By these observations it would seem that there arc no definite proportions betweenthe quantities of silica and alumina or of magnesia which unites with the calcareousmatter; but it is well ascertained that no good hydraulic mortar can be made without