Page images
PDF
EPUB

fed of the same chemical qualities in every respect.~Indeed, nothing is more easy than to form artificial chalk from pure lime-stone, as I have more ihan once experienced, which the reader may also do, by following the directions in the margin *, if his curiosity prompts him to it.

And

* In flaking lime composed of pure lime-stone, it will be observed, that the pieces fall into powder much more slowly than when the lime stone has contained r.ny sand in it.

If a great quantity of water be successively poured upon a large heap of these pure lime-shells, without stirring them, and if it be allowed to lie some time afterwards, it will be found, on opening the heap, that some pieces of lime-shells have only expanded considerably in bulk by the operation of slaking, without being crumbled down to a powder.— These pieces, if allowed to remain in a elofe place, where they are not exposed to the vicissitudes of the weather, will still retain their form, and, as they gradually absorb their air, they acquire a sort of firmness of consistence, and in time become chalk, in

And the practice of the southern provinces of Britain sufficiently confirms the justness of these observations. For, to the south of the Humber, on the east coast, almost all the lime they use is made of chalk; yet there are many buildings in these counties, in which the cement is as firm as in any part

of

every fense of the word ;-~-haying the same degree of firmness, of softness, and every other quality of chalk.

This is the most perfect resemblance of chalk that can be made; but, if any quantity os that pure lime be reduced to a very thin paste at the time of slaking, and be hastily dried to a certain degree,—it acquires a fort of consistency, so as to be capable of retaining its form. And, if this be kept in a place not exposed to the vicissitudes of the weather till if has attained its whole air, it will resemble chalk almost as much as the former, and might be employed for every pur* pose that the chalk is used for.

The reader will please to observe, that this can pnly be done with lime that is perfectly free of sand \ for, if it has the least particle of sand among it, noart can give it the softness of chalk.

of the island :—Nor does the ordinary mode of building in these places indicate any deficiency in the quality of their mortar ;—for their houses are almost all coated on the outside with a crust of lime, stuck full of small pebbles, which remain in it very firmly for many years.—We know well that this is the most trying manner of employing mortar.

There is, however, greater danger that lime made of chalk should form, on some occasions, a weak cement, than that from lime-stone.

For, as chalk never contains any sand, its lime will always form a very soft cement, unless care be taken to mix a large proportion of sand with it in beating up the mortar; which is not so indispensibly necessary in forming mortar from lime-stone ;—as it sometimes contains so much sand as to form

a pretty a pretty firm cement, without any additional sand at all.

Even if the lime-stone should be equally pure calcareous matter as the chalk, the lime of the first has a chance of becoming a firmer cement than that of the last.

For, as it is impossible to reduce the pure lime-stone to a- powdery calx^ without subjecting it to the action of a very strong fire, which, by diflipating the water, and fully drying the chrystals, carries, off the whole of its fixed air ; so that the calx is almost enr tirely caustic.

But chalk may be reduced to calx by such a moderate heat as is scarce sufficient to dissipate any ot its air; so that what assumes the appearance of lime made from it may be nothing else than a powdered effete calcareous earth, which never can become a cement of any fort.—But, as there is no danger of vitri.fying chalk by over-burning, this inconvq.'' nience nience may be entirely obviated, by a care-* ful and perfect calcination.

In those countries, therefore, where chalk-? lime is common, care ought to be taken to choose only that kind of it for mortar, that has been calcined by a very strong fire, and to reject that which has been burnt by furze or brakes', as unfit for that purpose.

But, it is obvious, that, as this defect arises entirely from the unsldlfulness of the c*» perator, which may be easily avoided, it ought not to be considered as any objection to the quality of the lime considered in it-» self.

§ 30.

It is unnecessary to extend our observations to all the other kinds of lime-stone that may be met with; as these general observations on the two extremes, marble and chalk, may be easily applied to all the intermediate

% kinds.

« PreviousContinue »