"Rich" or "fat" limes are those calcined from pure, or very nearly pure, lime carbonate, not containing sufficient foreign constituents to have any appreciable effect upon either the slaking or setting actions. The solubility and want of setting power of fat lime render it unsuitable for making mortar, except for the walls of out-houses and for other similar positions. It is nevertheless frequently used for the mortar in structures of a much more imposing character. It is, however, better than hydraulic limes for sanitary purposes (being purer), and is very useful for plastering and for whitewashing. It is also extensively employed in the manufacture of artificial hydraulic limes and cements. Fat lime requires to be mixed with a great deal of sand to prevent excessive shrinkage, but this addition does not materially injure it, as it attains no strength worth mentioning under any circumstances. The only setting that takes place in it is the formation of a thin surface crust, bearing a small proportion to the whole bulk; mortar made from such lime may therefore be left and re-worked repeatedly without injury.

Some of the lime which finds its way into the London market, under the assumed names of Dorking, Hailing, and Merstham, is merely fat lime tinged with iron sufficiently to give it the buff" colour characteristic of the hydraulic lime made out of the grey chalk from these localities. Of course, this stained lime makes mortar of the same inferior description as would be obtained from a common fat white lime, and! has no hydraulic properties whatever.

" Poor " limes are those containing 60-90 per cent. of lime carbonate, together with useless inert impurities, such as sand, which have no chemical action whatever upon the lime, and therefore do not impart to it any degree of hydraulicity. These limes slake sluggishly and imperfectly, the action only commences after an interval of a few minutes to more than an hour after they are wetted, less water is required for the process, and it is attended with less heat and increase of volume than in the case of the fat limes. If they contain a large proportion of impurities, or if they are over-burnt, they cannot be depended upon to slake perfectly unless first reduced to powder. The resulting slaked lime is seldom completely pulverized - is only partially soluble in water, leaving a residue composed of the useless impurities, and without consistence. The paste formed from the slaked lime is more incoherent, and shrinks less in drying, but behaves in other respects like that made from fat lime - in fact, it is like a fat lime mortar containing a certain proportion of sand.

Mortar made from poor lime is less economical than that from fat lime, owing to the former increasing less in slaking, bearing less sand (as the lime already contains some in the form of purities), and requiring a more troublesome manipulation than the latter. It is in a way superior as regards setting, and should therefore only be used when no better than be had.

"Hydraulic" limes are those containing, after calcination, enough quicklime to develop more or less the slaking action, together with sufficient of such foreign con-tituents as combine chemically with lime and water to confer an appreciable power ' setting without drying or access of air. Their powers of setting vary considerably. he best of the class set and attain their full strength when kept immersed in water. hey are produced by the moderate calcination of stones containing 73-92 per cent. of lcium carbonate, combined with a mixture of foreign constituents of a nature to produce hydraulicity. Different substances have this effect, but in the great majority of natural hydraulic limes commonly used for making mortar, the constituent which confers hydraulicity is clay. In some varieties, a portion of the lime carbonate is replaced by magnesia carbonate, which increases the rapidity of setting, and adds to the ultimate strength of the mortar. The phenomena connected with the slaking of limes vary reatly according to their composition.

With none is it so violent as with the pure sone carbonate, and the more clay the limes contain the less energy do they display, until we arrive at those containing as much as 30 per cent. of clay, when hardly effect at all is produced by wetting the calcined lime, unless it is first ground to wder. The setting properties of hydraulic lime also differ very considerably in oportion to the amount they contain of the clay or other constituent, which gives lime its power of setting without drying or the access of air.

Artificial hydraulic lime may be made by moderately calcining an intimate mixture of fat lime with as much clay as will give the mixture a composition like that a good natural hydraulic limestone, of which the product should be a successful Iritation. A soft material like chalk may be ground and mixed with the clay in the w state. Compact limestone, on the other hand, is more commonly burnt and slaked the first instance (as being the most economical way of reducing it to powder), then mixed with the clay and burnt a second time. Lime so treated is called " twice kilned" me. The mixture may be made by violently agitating the materials together in water by machinery, or by grinding them together in a dry state, afterwards adding water to form em into a paste. The paste in either case is moulded into bricks, which are dried, lcined, and otherwise treated like ordinary lime. Artificial hydraulic limes are not much manufactured or used in this country.