the ufual manner, and the refult will be a very good aqua fortis, which will readily diffolve filver, and have no effect upon gold. The filversmiths of Madrid use no other. To clear up more fully this fingular phenomenon, and fee whether the chemists are mistaken or not, nothing is left but to afcertain whether this fal gem contains a vitriolic acid, because in such case it would not be the marine acid that was more powerful than the nitrous, but the vitriolic: however as it has not been demonftrated or known, that any fuch vitriolic acid exists in common falt, the difficulty still remains. This fingularity of the Spanish fal gem merits the attention of chemifts and repeated experiments; as this system seems to militate against the received opinions concerning the nature of the three acids, the mafter-key of chemistry, and overturns the theories now in vogue.§

What rhapsodies have been published with respect to the physical caufes of the faltness of the sea, fome faying that immenfe beds of falt exifted at its bottom; others, finding this argument destroyed itself, had recourse to the idea of rivers bring

ing

§ Mr. Bowles feems to have drawn his conclufions rather too haftily, without reflecting that nitre when divided to a certain degree and kept divided, may by the force of heat alone be deprived of its acid; whilft common falt can fuftain a much greater degree of heat without any decompo

tion.

ing down falt enough to the fea to impregnate its waters, which fuppofition is as falfe as the former, as we are positive that fea water is at present as briny as it was in ancient times in proportion to its fituation, temperature, evaporation, or quantity of fresh water running into it; besides this, I have made several experiments, but never found falt at the mouth of rivers, where they difembogue into the fea. It is true, that sometimes after diftillation and evaporation I have found a thoufandth part of common falt, and I once difcovered as a refidu um, a little nitre; but this proves nothing, and with refpect to the nitre, I confider it to be a reduum of common falt, being perfuaded that this may change its acid and alkaline basis, and become nitre with motion and ebullition, and reciprocally nitrous, and the alkaline bafis change into common falt.f

Ee 2

LETTER

It would have been acceptable to the chemical readers, if Mr. Bowles had favoured us with any fufficient reason to Support this belief.

LETTER XX.

Obfervations made by Don Guillermo Bowles, on the roundness of pebbles in the beds of rivers.

THE pebbles of which I am now going to speak,

are those which are commonly found in moft. places without angles, or points, and though not perfectly round, have yet more or less that form, with a smooth surface. They are composed of various matter, and the first idea which occurs of their having acquired this smoothness, is from their rubbing against each other, or some harder body; this being the method we use in order to give fuch a polish to any fubftance, and as these ftones are frequently found in large quantities in the beds of great rivers, it is very easy to say they have been brought down by the waters, and become smooth by their conftant friction. For my part, I was always of this opinion till my arrival at Aranjuez, where I discovered the fallacy of this reasoning, for I clearly perceived that the stones in the bed of the Tagus never moved from their places. This of course staggered my former belief; I was confirmed in my new principles by a variety of observations, but to avoid being prolix I fhall only offer

a few

a few of them, which have all the appearance of being decifive in favour of my fyftem.§

There are no pebbles more fingular than those *crystalline ones found in the bed of the river Henares, near San Fernando; now if these pebbles had any progreffive motion, let it be ever fo fmall, they ought certainly by this time, after so many ages, to have reached the bed of the Tagus fo near them, yet none are to be found there.

The Tagus is full of calcareous ftone at Sacedon; a little lower, at Aranjuez, not one of these is to be found in its bed.

The kingdom of Jaen near Linares, there is a hill chiefly compofed of round fmooth ftones, about the size of an egg; the smooth polish of these and their roundness cannot be attributed to rains, because they are not exposed to them, nor dispersed on the furface of the hill, but buried underneath at a confiderable diftance: nor much lefs can it be attributed to any river, for I do not know from what system, or by what chronology, it may be conjectured that any river ever run over that emi

nence.

In

Mr, Bowles tells us, he looks upon this as one of the happiest discoveries he ever made, as striking him in a forcible light, and ferving as a key tending to illuftrate the true physical theory of the earth; but in what manner it had this effect he has not informed us.

In the village of Maria, three leagues above Zaragoffa, there is a broad gully full of quartz, fandstone, and limestone, and gypfum of a perfect white, yet at Zaragoffa the Ebro contains. none of these forts.

I believe no body has ever feen in the bed of the Ebro large or small round granite ftone, nor blueish ftone veined with white, but the Cinca before it empties itself into that river is full of them: in fo much that it carries no other fand but these ftones reduced very fmall, near San Juan in the valley of Giftau.

The river Naxera is full of fmall fandy ftone, and of white quartz refembling little almonds, mixed with others of a red colour. This river runs into the Ebro, in whofe bed when it paffes by Zaragoffa, none of thefe forts of ftones are to be feen.

The bed of the Guadiana has in its different parts the fame kind of pebbles as are found on its banks, and on the adjacent hills, without being mixed with thofe that are found a league higher or lower; and at Badajoz, where the country is without any ftone, none are to be found in the bed of the river.

This holds good not only in Spain, for I have taken notice of the fame in other countries; not

to