But it still remains a defideratum, to afcertain the due proportion of fand; as authors, as well as practical masons, differ very much from one another as to this particular.

They likewife differ very much in their directions about the mode of mixing the materials, and of applying the cement ;-fome modern authors, especially, attributing amazing effects to a small variation in these particulars, while others deny that these circumftances have any fenfible effect on the durability or firmness of the cement.

Thefe different and contradictory opinions seem to arise from an imperfect knowledge of the nature of quick-lime, and the variations it may admit of: For these variations are so very great, as to render it impoffible to give any general rules that can poffibly apply in all cafes. It, therefore, behoves those who wish. to attain any consistency of knowledge on this fubject, to endeavour, first to ascertain the circumftances that render cal

M m

calcareous substances capable of becoming a cement at all, and then to trace the feveral changes that may be produced upon it by

other extraneous causes.

This I fhall endeavour briefly to do.

§ 5.

Lime-ftone and marble are nothing else than a calcareous matter chryftallized*, and affume different appearances according to accidental circumftances that have occurred at the time of their original formation.

ift, The

*Saline fubftances, when diffolved in water, and put into proper circumft.nces for that purpose, separate from the water, and shoot into regular figures, which affume different forms, and are more or less transparent according to the different nature of the falt, as nitre, alum, &c. These regular tranfparent bodies are properly called chryftals.-Hence every body in nature that affumes a form and appearance fimilar to thefe, and is produced in the fame manner, is faid to be chryftallized.

ift, The more perfect these chrystals are, the harder and more compact will the stone be that confifts of them.

2d, The smaller the proportion of extraneous matter that is entangled among these chryftals, the purer and finer will the lime be that is made from the ftone of which they confift.

From a variation in one or other of these two particulars, arife all the varieties of calcareous matter, that can be converted into lime; which varieties may be diftinguished from one another by the following particulars.

ift, When the calcareous matter is pure, and perfectly chryftallized; when it affumes a clear and fomewhat tranfparent appearance, and is found in regular ftrata, without many fiffures, it is then called marble.

2d, When the calcareous matter is pure, but the chrystallization lefs perfect, tho' in regular ftrata, it still obtains the name of marble; but, as it is more opaque, and lefs compact

than

than the former, it is reckoned lefs valuable, and coarfer.

3d, When the calcareous matter is ftill pretty pure, but haftily concreted into an uniform mafs, without having been in a state that permitted it to chryftallize, or to subside into regular flrata, it is called chalk; which, when reduced to a powder, without calcination, is called whitening.

4th, When the chryftals are tolerably perfect, but have had a confiderable proportion of fand entangled among them, it is no longer called marble, but lime-ftone. And this is more or lefs pure, or affords a richer or poorer lime, as it contains a greater or smaller proportion of calcareous matter; and is more or lefs hard according to the degree of perfection of the chrystals.

Even the pureft calcareous matter perfectly chryftallized, is called lime-ftone, and not marble, when it confifts of small pieces that have not been concreted into regular ftrata. 5th, When

gth, When the calcareous matter is perfectly pure, and fhot into smaller chrystals of a tranfparent whitenefs, it is called Sparr, -and, in other circumftances, ftalactites.

6th, When the calcareous matter has been formed by nature as a covering for animals, it is called hell; in which class may be included corals and corallines.

These are all the fubftances that have hitherto been employed for making lime. The other varieties of calcareous matter, that I may bring them all under one view, and point out their effential diftinctions,) are as follow:

7th, When the calcareous matter, while in its fluid ftate, has been abforbed into a bed of clayey matter, and with it concreted into an uniform, compact, unchrystallized mass, it has been denominated marle,-which is more or less pure, according to the proportion of calcareous matter it contains, and more or

lefs