will be less than the numbers aforegoing, in a ratio as radius to the cofine of the latitude, from which refults this THEOREM: "From the double cofine of the latitude, fubtract this conftant logarithm 2.4608978, the remainder is the logarithm tangent of the elevation of the apparent horizon towards the weft, and of the depreffion of the apparent horizon towards the caft."

Hence this deviation at the tropics becomes 100", at the Obfervatory at Paris 5' 10", at the Obfervatory at Greenwich 438, and at the polar circles 158".

From these few extracts the reader may in fome measure form a judgment of the work now before us, with regard to its utility and the manner in which Mr. Dunn has executed it, As to ourselves, as far as we are capable of judging of the more exalted parts of mathematical philofophy, we apprehend the whole to be founded upon principles that are at best but very defective, if not abfolutely abfurd. However this may be, as we do not pretend to a final determination in matters of fuch confequence, we fhall only add, that if Mr. Dunn has really made any ufeful difcovery with regard to the longitude, which the work itself, and the application of his corrections to future obfervations can only evince, he certainly deferves, and we fincerely with him, all the encouragement due to his merit.

VIII Commercium Philofophico-Technicum; or, The Philofophical Commerce of Arts: Defigned as an Attempt to improve Artsy Trades, and Manufactures. By W. Lewis, M.B. and F. R. S. Parts II. III. IV. Pr. 1l. 4s. in boards. Wilcox.

TH

HE reader will pleafe to remember, that the first partof this performance was analyfed in our Review for February. How the remaining Parts came to be fo long delayed, requires (we hope) no other explanation, but that the gentle. man who executed the first is now no more, and the article has been hitherto overlooked in the hurry of publication. hope the delay thus occafioned will not be imputed to any difrefpe&t for Dr. Lewis, whom we esteem in a very particular manner, not only as a man of genius and learning, but as our mafter, from whofe operations, lectures, and writings, we have learned the beft part of what we know in the art of chyniftry. We fhall begin where the former article left off, at the ninth fection, which treats of the refining of gold, and the feparation of fmall portions of it from other metals; a subject very interefting to all thofe artifts who work upon this preciousmetal, The firft feparation of it from bafe metals is by tefting with lead. The teft is a large cupel made of bone ashes, commonly fixed in an iron hoop. It is fometimes made of calcined fpath, and vegetable afhes but nothing endures the fire fo well

as

as the ashes of bones; which are fold very reasonably by the barrel in feveral parts of London, particularly in Clerkenwell and that neighbourhood. There are people who collect bones, and after extracting the oil, of which they make grease for carriage-wheels, what remains is calcined to afhes, for the purpose of smelting. He next gives the process of feparating gold from filver by aqua-fortis; and on this occafion describes the manner of preparing, from a folution of copper, the blue pigment called verditer. He proceeds to the purification of gold from filver and bafe metals, by cementation, where the acid refolved into fumes, is applied to the metal at the fame time strongly heated, and corrodes a part of the filver, though its proportion be very minute. He then defcribes the method of refining gold from filver and bafe metals by antimony; as alfo its purification from platina, filver, and base metals, by aqua-regia. What follows is the extraction of a small portion of gold from a large quantity of filver: the extraction of gold from copper; and the feparation of gold from gilt work.Section 10. contains the method of tinging glass and enamel by preparations of gold; which is extremely curious.

In the next fection, we find the mineral hiftory of gold, which mentions all the parts of the world where it is now found, including even the kingdom of Great-Britain, many parts of which afford fmall quantities of it to this day.

There are two noble Scotch families which have pieces of pure gold weighing above an ounce each, found upon their own lands near the Lead Hills in North-Britain. We ourfelves have feen feveral pieces of the fame metal collected in the fame place upon the fide of a rivulet after torrents and heavy rains; and we have been very well informed that about fixty years ago, an Englishman made a very comfortable fortune by gathering thefe grains, during a course of twenty years that he lived in this country.

Dr. Lewis obferving that the negroes on the coast of Guiney frequently adulterate their gold duft with filings of brass, propofes the hydrostatical ballance, a mixture of aqua-fortis, and other methods for diftinguishing the cheat: but the truth is, the African company retains in every fort a gold-taker, who feparates the brafs from the gold in the most dexterous manner, by means of a common blow-pipe and the touch-stone, and >that with fuch expedition and exactnefs as are altogether incredible. Any chemical procefs for this purpose would be impracticable, confidering the nature of the trade.-We wish the doctor had been more full in his account of the feparation of gold from earthy and ftony bodies by water, and by mercury, the methods practised in the West Indies; as well as of its extraction when intimately combined in the composition of sands,

and with the ores of other metals. If we are not mistaken, Frazer and D'Ulloa are more particular and fatisfactory on this fubject.- -The twelfth fection treats of the alchemical history of gold, including the various methods by which a set of wrong-headed philofophers pretended to attain the art of tranfmutation.

The alchemifts fuppofed that nature, in all her works aiming at perfection, in producing metals aimed at gold: that the imperfect or bafe metals failed of being gold, either from a redundancy or deficiency of fome particular element in their compofition, or for want of fufficient coction, maturation, or depuration of their principles; and that art could correct or remove these impediments, fo as to complete the work which nature had begun.

They fuppofd the general principles of metals to be chiefly two fubftances, to which they gave the names of mercury and fulphur; and that of both these there were different kinds, particularly of the latter, which they admitted as many varieties of as there are metals; and which, in gold, they held to be pure, red, fixt and incombuftible, but of different qua lities in the other metals. In these points there is no perfect uniformity among the different alchemical philofophers, which indeed could not be expected in hypotheses on so abftruse a fubject, where experience had afforded fo little light: fome have added a faline, some an earthy, and others an arsenical principle.

They supposed that the pure mercurial, fulphureous, or other principles of which they imagined gold to be compofed, were contained, feparately, in certain other bodies. These principles therefore they endeavoured to collect, and to concoct and incorporate by long digeftions. In the many volumes written profeffedly to teach the process at full length, the subjects, from which the golden feeds are to be obtained, are wrapt in impenetrable obfcurity: thus much is plain, that the Juppofed adepts in this myfterious fcience do not all make choice of the fame fubjects, or work upon them in the fame manner, their practice being probably adapted to their particular hypotheses."

After all, fon e very ingenious alchemists, and at the fame timė very honest men, and among the reft Boerhaave himself, continued to the laft in the fame opinion, that the transmutation of metals was not unattainable. The alchemists aspired alfo at the elixir, which was a product of a higher order still, called likewife the medicine for metals, the tincture, the philofophers ftone; which by being projected on a large quantity of any of the interior metals in fufion, fhould change them into fine gold; which being laid on a plate of filver, copper, or iron, and moderately heated, fhould fink into the metal, and change

into gold all the parts it was applied on; which, on being properly heated with pure gold, fhould change the gold into a substance of the fame nature and virtue with itself, so as thus to be fufceptible of perpetual multiplication; and which by continued coction, should have its power more and more exalted, so as to be able to tranfmmute greater and greater quantities of the inferior metals, &c. &c. Alchemifts have moreover endeavoured to find a method for deftroying gold, which they affirm, is more difficult than its production: this view they have eagerly profecuted on the fuppofition that its deftruction or decomposition would afford fome grounds for its artificial production. Mr. Boyle himself was of opinion this was prac ticable, and even gives a process, by which he imagined part of the fubftance of gold was transmuted to filver: but he certainly was mistaken.

In fection 13. we are made acquainted with the different imitations of gold, called Pinchbeck, Prince's metal, &c. Then, be describes gold coloured pigments, gold-coloured varnifh, or Jacker; and in his addition to the history of gold, he describes the method of embellishing linen with flowers and other ornaments of gold leaf: but we must own the machine is described in fuch a manner as is not easily comprehended. We are afterwards favoured with many curious experiments on the converfion of glass veffels into porcelain, and for establishing the principles of the art, containing the successful changes produced in green glass by baking; a comparison of the effects of different kinds of materials on green glafs by baking; together with experiments of the baking of different forts of glass, and of bodies approaching to a vitreous nature. As these experiments, however, did not fucceed, it were to be wished our au, thor had tried others upon mixtures of two different earths femivitrefied, which he seems to think are the basis of the true porcelain. We have good reafon to believe that the porcelain made at Chelsea was a compofition of flint and pipe clay le vigated together.-After this difcuffion, our author expatiates on the expansion and contraction of certain bodies at the time of their paffing from a fluid to a solid state,

It has been frequently observed, that when thermometers prepared with different fluids, as quickfilver, fpirit of wine, warer, and oil, have two distant points of heat marked equally on them all, and the spaces between divided into an equal number of parts; the heat, which makes the fluid in one expand to any of thefe intermediate points, fhall raise that in another above the corresponding division, and in another not fo high. It was probably this irregularity in the expansion of the fluids, that prevented the agreement of the mercurial

and fpirit thermometers which Boerhaave fays he had made for him by Fahrenheit: the different expanfions of different kinds of glass, to which the ingenious artist has recourse in order to account for the variation, appears to be infufficient for producing it; fince, if the expanfion of the two tubes be always uniform, or in the fame proportions to one another, the quantity of this expanfion cannot influence the apparent proportional expanfions of the fluids. I have feen a mercurial and spirit thermometer very nearly correfpond, at different divifions, from the freezing point to the heat of melted wax the divifions of the mercurial one were all equal, thofe of the other widened upwards; as if heated spirit either expanded more, or heated mercury lefs, by a certain additional heat, than the fame fluids do by an equal addition of heat made to them in a colder ftate. Reaumur fays, that water from freez. ing to temperate expands only one tenth part as much as fpirit does, but that from freezing to boiling it expands half as much as fpirit in the fame interval. Though the difference in the proportion at different periods of the heat is doubtlefs very confiderable, I apprehend it does not amount to quite fo much as this, and that the mistake arofe from fuppofing the full heat of boiling water to have been communicated to the fpirit thermometer immerfed in it for a little time; whereas Spirit cannot bear fo great à heat as that in which water boils, and confequently, in this part of the experiment, the spirit was lefs heated than the water it was compared with. Thefe variations in the proportional expantions of different fluids feem to have been little confidered by thofe who have given comparisons of different thermometers, by reducing the divifions of one to thofe of the other from only two correfponding points on each.'

He proceeds to confider the nature of ice, wax, refins, animal fat, pure clay, and gypfum, or plafter of Paris, as alfo the contraction and dilatation of filver, lead, tin, copper, iron, and other metals.

His next inquiry relates to the blowing of air into furnaces by a fall of water. After having defcribed the different kinds of bellows, he obferves there is another method of applying water, fo as to produce a strong blast, by means more simple than any of the foregoing, and a little expence. A ftream of water, falling through a pipe, in certain circumftances, carries air down with it, and this air, afterwards difengaged from the water at the bottom, may be fo collected as to have no other vent than a pipe which fhall carry it to the furnace. He fpecifies thefe machines, beginning with à fimple pipe, fuch as is used at Tivoli, near Rome: the next is a pipe with air holes,

inferted