that the mortar fhall confift almost entirely of fand enveloped in chryftalline matter, and become in due time as hard as stone itfelf; whereas mortar, confifting of pure lime without fand, can hardly ever be much harder than chalk.

§ 19.

It is not, however, to be expected, that, in any cafe, this dried mortar will affume that transparent chryftalline form, or the compact firmnefs of marble or lime-stone.In mortar, in fpite of the utmost care that can ever be taken, a very confiderable quantity of the lime must remain undiffolved; which undiffolved lime, although it may be fo much feparated by the fand and chryftallized lime-ftone, as not much to affect the hardness of the mortar, yet it must still retain its white chalky-like appearance.

But, as marble and lime-ftone are always formed by thofe particles of lime that have been wholly diffolved in water, and from which they have been gradually feparated by a more flow and more perfect mode of chryftallization, they have nothing of that opaque calx like appearance, but affume other colours, and appear more firm, uniform, and compact; the fand and other matters that may be enveloped in them being entirely furrounded with a pure chry ftallized matter.

$ 20.

To obtain the most perfect kind of mortar, however, it is not enough that a large proportion of fand fhould be employed, and that that fand fhould be intimately mixed with it.It is alfo of the utmost importance that a large proportion of water be added. For, without this, it is impoffible that

a

a large proportion of the lime can be chryftallized, and the mortar, in that cafe, would confift only of a mixture of chalky matter and fand, which could hardly be made to unite at all-would be little more coherent than fand by itself, and lefs fo than pure chalk. In that cafe, pure lime alone would afford rather a firmer cement than lime with fand,

$ 21.

It is alfo of very great importance that the water be retained as long in the mortar as poffible. For, if it be fuddenly evaporated, it will not only be prevented from acting a fecond time upon the lime, after a part of what was firft diffolved has been chryftallized, but even the few chryftals that would be formed when the water was fuddenly evaporating, would be

of

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of themselves much more imperfect than they otherwise most certainly would have been.

Common falt confifts of chryftals haftily formed by a fudden evaporation of sea-water by means of fire, has the appearance of a dry whitish calx, that may be easily broke in pieces :-The fame falt, when flowly chryftallized by a gentle evaporation in the shade, confifts of large cubical chrystals, as tranfparent, and little fofter, than crown glass.

Ordinary lump-fugar, it has been faid, likewise consists of another fubftance haftily concreted by a fudden evaporation of the fluid in which it was diffolved :-Sugarcandy is the fame fubftance, flowly chryftallized by a more moderate evaporation. -Every one knows what a difference there is between the firmness of these two fubftances. As great must be the difference between the firmnefs of that cement which has been flowly dried, and that which has

been haftily hardened by the powerful action of a warm air.

It is owing to this circumftance that the lime which remains all winter in a mortartub, filled with water, is always found to be much firmer, and more coherent than the mortar that was taken from the fame tub, and used in any work of masonry; although, in this cafe, the materials were exactly the fame.-From the fame cause, any work cemented with lime, under water, if it has been allowed to remain unhurt till it once become hard, is always much firmer than that which is above the furface of the water.

$ 22.

To make the reader comprehend the full force of the foregoing reafoning, I would compare lime cement, or mortar, to a mass

of