The common chalk lime of the neighbourhood of London may be considered to represent the type of the rich limes ; some of the impure oolitic stones yield the decidedly poor limes ; and the blue Has lime may be taken to represent the type' of the hydraulic limes ; the so called gray stone low of London is simply a moderately hydraulic lime ; whilst strictly speaking, the Portland cement ought to be con sidered an eminently hydraulic artificial lime [PORTLAND CEMENT]. The gray stone lime is obtained from the basement beds of the chalk ; and the blue lias lime from the formation of that name. The Romans seem to have used pounded brick dust for the purpose of making artificial hydraulic limes, whenever they could not obtain puzzuolanos, or similar volcanic silicates of alumina; and the modern engineers liars lara- r empl I td the latter clam of materials, such as the Italian or the tram of NVestphalia, In their marine works. Of Lie rear., however, the use of the Roman and of the Portland cements has tamest entirely superseded that of the mechanical mixtures of LUDO and the silicate of alumina.
The proportions of sand It may be desirable to mix with a lime for the purpose of making mortars, must vary with the richness of the bums; and it may briefly be said, that it ought to be equal to the amount of expansion of the lime itself in slaking ; that is to say, that a lime expanding to 3 or 4 times its original volume may be mixed with three or four times its said volume of sand, and so on. In practice, thee proportions of sand may often be exceeded without inconvenience; and it would appear that the chemical nature of the sand has a marked influence upon the character of the resulting mortar; for in proportion as the sand participates of the character of the soluble silicate of alumina, or in proportion, in fact, as it con tributes to the formation of a double silicate of lime and alumina, will the compound between it and the lime acquire the property of hardening under water.
It may be added, that when the rich, or the poor limes, are im mersed in water, In vessels open at the top, as is done in trials upon the quality of this class of materials, they never sot. The hydraulic limes act after 6 or 8 dais' inunersion, and at the end of about 6 mouths they become as hard as the softer kinds of chalk ; and the eminently hydraulic limes set within 3 or 4 days after their immersion, becoming within 6 months as hard as ordinary limestones. The pure lime itself is soluble, to the very last grain, in water, in proportions varying with the temperature of the latter. Dr. Dalton considered that at moderate temperatures (about 60'), water could take up of its own weight of lime; and at 212' Fahr., it is able to take up about of that weight. The colouring material used in building operations under the name of lime-white, is merely water containing an excess of lime, to which a little size should be added, in order to counteract the tendency of the lime to detach itself from the surfaces to which it has been applied when the water has evaporated. The rich lime is used in some trade operations, as in tanning ; and also to a great extent as a disinfectant in the form of lime-white upon walls, and as a soluble hydrate in the case of waters containing impurities in suspension. In farming operations it is also largely employed, as a dressing or even as a manure; and latterly, the lime-light may be considered to indicate a further remarkable application of this important material. This light is obtained by igniting the united jets of oxygen and hydrogen gas upon a piece of caustic lime, which thus becomes heated to a most intensely luminous degree. The practical applications of lime in building operations will be further referred to under MORTAR; Ptes