the marvellous which runs through the whole, and the many hyperbolic compliments paid to Mr Loriot with which it abounds, are but ill calculated for giving the reader a favourable opinion of the intention of the compiler. Future experiments will bring to light thofe circunftances which seem to be at prefent doubtful and myfterious. I now proceed to point out fome other circumstances that may influence the quality of lime-mortar.

§ 25.

If lime-ftone be fufficiently calcined, it is deprived of all its moisture, and of all its fixed air. But, experience fhows, that limestone will fall to a powder on the effufion of water upon it when it is much less perfectly calcined, and while it ftill retains almoft the whole of its fixed air. And, as we have hardly any other rule for judging whether lime-stone be fufficiently calcined, except this fingle circumftance of its falling to a pow

der

der when water is poured upon it, 'we may thus easily perceive, that the fame lime may be more or lefs fitted for making good mortar, according to a circumftance that in a great measure eludes the observation of operative masons. For, if it should happen that all the lime-fhells drawn from a kiln at one time were juft fufficiently calcined to make it fall to a powder with water, and no more, that powder would be altogether unfit for making mortar of any kind.

This is a cafe that can feldom happen.But, as there are a great many intermediate degrees between that state and perfect calcination, it must often happen that the stone will approach much nearer to one of these extremes at one time than at another, so that mortar may be much more perfect at one time than at another, owing to a variation as to this particular.

$26.

$ 26.

Every author who has written on the subject of lime as a cement, has endeavoured to ascertain what is the due proportion of fand for making the most perfect cement.-But a little attention to the fubject will show, that all rules that could be prescribed as to this particular, must be fo vague and uncertain, as to be of little use to the practical mason. For,

Befides the variation that may arise from a more or less perfect degree of calcination, and which has been just now taken notice of, it is a certain fact, that some kinds of limeftone are much more pure, and contain a must smaller proportion of fand than others do.

I have examined lime-ftone that was fo perfectly free of any mixture of fand whatever as to diffolve in acids as entirely as fu

gar

gar does among water;-I have also tried another kind that contained eleven twelfths of its whole weight of fand, and only one twelfth part of lime ;-and have met with other forts in all the intermediate proportions between these two extremes.

Now, it would furely be abfurd to say, that the pure lime would require as small a proportion of fand when made into mortar, as that which originally contained a much larger proportion of fand than any writer on the subject has ever ventured to prescribe. What adds to this uncertainty is the variation that may arise from the circumstance last mentioned, viz. the greater or lefs perfect degree of calcination that the stone may have undergone. For it ought to be remarked, that, although lime-ftone, when pure, requires a very intenfe degree of heat to convert it into a vitrified * mass; yet, when fand

*A vitrified fubftance is one that has been melted by heat, and affumes fomewhat of a glafs-like appearance.

fand is mixed with the lime-ftone in certain proportions, a very moderate heat is fufficient to convert the whole mafs into vitrified fcoriae, or, in the common language of the country, make it run into danders.

From this circumftance, it happens, that those who are poffeffed of a very impure lime-stone, are obliged to be extremely eautious not to give it an over-proportion of feuel, left it should vitrify the stone; and they are very happy if they can get their ftone calcined just enough to make it fall with water, and no more; fo that, in general, it may be prefumed that a very large proportion of fuch lime is never so sufficiently burnt as to be reduced to a perfectly cauftic flate, or to be capable of forming a ce

ment.

But, as there is no danger, at any time, of giving pure lime-ftone too much fire, those who poffefs it are not under the neceffity of being afraid of crring on that

fide.