This fcorching fun is highly beneficial to the production of nitre, at the falt-works near Granada; where, with about one hundred men, employed during the fummer, and twenty-fix in winter, government obtains three thousand quintals annually. But then the lixiviating water is not carried by men, as in Madrid, but conducted by pipes to every filter.

Το procure the proper kind of earth, they obferve the fpots which, early in the morning, appear black, and towards noon become white round the edges; this they collect, and find that land, on which has been laid much dung, is the most productive. Once collected, it will endure for ages; and, having been expofed to the influence of the fun and of the air, yields the fame quantity as at firft on every subsequent filtration. In the places where they find this earth, there is neither limestone, chalk, nor gypfum; and, afhes being extremely scarce, none are used at the bottom of the filters.

I have already confidered this wonderful production as a merchant; and happy G 4 fhould

fhould I be, were I qualified to difcufs it with any fatisfaction as a chemist.

Here a thoufand questions crowd in upon the mind. From whence does this earth collect the vegetable alkali, whence the nitrous acid? Suppofing the former to be originally the result of putrefaction, yet, after the earth has been lixiviated, and all the vegetable alkali has been carried off by water, how is it impregnated afresh, merely by exposure to the fun and air; and where does it obtain this inexhauftible fupply both of the alkali and its combining acid? But, if we reflect, that, with the nitrous falt, there is conftantly found muria, or sea-falt, in confiderable quantities, whence does it derive the foffil alkali, and whence the muriatic acid, not once, but upon every fubsequent exposure ?

It is well known that old mortar produces fix kinds of falt; for, befide the two just mentioned, both the nitrous and the muriatic acid are found combined with magnefia and with calcareous earth. But, if we recollect, at Añover and Aranjuez we faw Epfom and Glauber's falt, with the

muria

muria and the nitre, and both thofe falts contain vitriolic acid.

Here, then, new questions will arife. What is the relation between these various substances, of chalk, magnesia, the foffil and the vegetable alkalis? What connection can we trace between the muriatic, nitrous, and vitriolic acids? and, Is there one common principle of acidity?

This question will be both more natural and more interesting, when we confider, as far as relates to England, France, and Spain, the only countries which have come under my observation, that, in proportion to the quantity of fun, the chalk is found impregnated with vitriolic acid, and forms felenite or gypfum. At least it may be obferved, that in our island we have much chalk, and little gypfum; that in France both these substances abound; whilft in Spain, there is very little chalk, and a profusion of gypsum, more especially in Arragon, and in the fouthern provinces. Indeed a learned naturalift, who refided many years in Spain, and traverfed it in all directions, with a view to minerals, affures us, that he had never difcovered there the

leaft

leaft veftige of chalk. (v. Bowles, p. 13.) But I have already noticed it in one place; and in the neighbourhood of Granada it is likewise found, although I was not able to identify the spot.

His obfervation is however ingenious, and worthy of attention. He never met

with it, I saw it only twice.

The connection between chalk and gypfum became evident to me from the moment that I difcovered flinty gravel in the latter, precifely fuch as we always meet with in the former. Hence it feems to be plain, either that chalk was gypfum, and has loft its vitriolic acid, or that gypfum was once chalk, and has made this acquifition. I am inclined to adopt the latter hypothefis; and, if this be the true one, we must enquire whence has it derived the acid?

Should we be inclined to feek the principle of acidity in the folar ray, we may perhaps be confirmed in this idea by the confideration, that, by means of green vegetables and water expofed to its meridian influence, all modern chemifts have produced vital, that is dephlogisticated air, in great abundance, always in proportion to

the

the quantity of light, or, in other words, to the greater or leffer influence of the folar rays: and that faltpetre, by diftillation, produces the fame kind of air, in the proportion of twelve thousand inches to a pound, leaving behind the vegetable alkali uncombined with acid.

Should we be inclined to grant, agreeable to the experiments of Dr. Ingenhousz, that vegetables by day emit vital and by night mephitic air; confidering that Mr. Cavendish produced nitrous acid by the combination of vital air with atmospheric mephitis, in the proportion of seven to three, we should not be at a lofs for a never-failing fource, from whence this acid may arise.

Thefe fpeculations might be pursued, and, obferving that one pound of nitrous acid, distilled on mercury, yields one thoufand eight hundred and eight cubic inches. of nitrous and one thoufand nine hundred and four of vital air, we fhould be con-firmed in our opinion that we have discovered the origin of the fought-for acid.

Nitrous air is obtained from animal fubftances fimply by putrefaction, or it may be had by the combination of inflammable and

vital