coloured rays being thereby dif pofed to be tranfmitted through the plate, and confequently rays of different colours being difpofed to be reflected at the fame place, fo as to prefent the appearance of different colours to the eye. A variation in the denfity of the plate, he fhews, will occafion a variation in the colour; but ftill a medium of any denfity would exhibit all the colours according to the thickness of the different parts of it. Thefe obfervations he confirmed by experiments on plates of He alfo air, water, and glafs. mentions the colours which arife on polished steel, by heating it: as likewise on bell-metal, and fome other metalline fubftances, when melted and poured on the ground, where they may cool in the open air and he afcribes thefe colours to the fcoria, or vitrified parts of the metal, which, he says, most metals, when heated, or melted, do continually protrude, and fend out to their furface, covering them in the form of a thin glaffy skin. This capital difcovery, concerning the colours of bodies depending upon the thickness of the fine plates, which compofe their furfaces, of whatever denfity thofe plates be (and which may be of fuch admirable ufe to explain the colours, and perhaps, in due time, the conftituent parts and internal ftructure of natural bodies) I have been fo happy as to hit upon a method of illuftrating and confirming, by means of electrical explofions. Thefe, being received upon the furfaces of all the metals, change the colour of them, to a confiderable diftance round the fpot on which they are difcharged, fo that the whole fpace is divided into a number of concentric circular It was not by any reafoning a Pleafed with this experiment, I afterwards purfued and diverfified it in a great variety of ways, the refult of which I fhall comprise in the following obfervations. 1. When a pointed piece of me. tal is fixed oppofite to a plain furface, the nearer it is placed to the furface, the fooner do the colours appear, the clofer do the rings fucceed one another, and the lefs fpace they occupy; as, on the other hand, the farther it is placed from the furface, the later do the colours appear; but the rings then occupy proportionably greater fpace, and felves: have more room to expand them felves. No 1. on the fteel*, was made by the explosions paffing from the point of a needle, fixed at the distance of of an inch from the steel; and N° 2. was made at the fame time, when the needle was placed at the distance of of an inch. It feems, however, that when the point is placed at fuch a distance, as that the electric matter has room to dilate, and form as large a circular fpot as the battery will admit, the rings are as large as they are capable of being made; but that fill the colours appear later, in proportion to the difiance beyond that. When the point is fixed exceeding near, or is made to touch the furface, the colours appear at the very firft explosion, but they fpread irregularly, and make not diftinct rings, as N° 1. upon the tin. 2. The more acutely pointed is the wire, or needle, from which the electric matter iffues, or at which it enters, the greater number of rings appear. A blunt point makes the rings larger, but fewer; and in that circumftance it is likewife `much later before the colours make their appearance at a given diftance. N° 3. upon the fteel, was made by a blunt wire, and No 2. upon the tin by a brafs knob fixed oppofite to it. 3. In making thefe rings, the first appearance is a dufky red, about the edges of the circular fpot; prefently after which (generally after four or five ftrokes) there appears a circular space, vifible only in a pofition oblique to the light, and looking like a fhade on the metal. This pace expands very little du ring the whole courfe of the explo fions, and it feems to be, as it were, an attempt at the first and faintest red: for by degrees, as the other colours fill the bulk of that space, the edges of this fhape deepen into a kind of brown; as may be feen particularly in No 4. upon the fteel, where it is fomething more than half an inch in diameter, and in N° 1. where it is near of an inch. 4. After a few more explosions, a fecond circular fpace is marked out by another fhade, beyond the firft, generally about or of an inch in diameter, which I have never obferved to change its appearance, after ever fo many explosions. This fecond fhade, by fucceeding the firft; which, as I obferved, becomes gradually of a brown, or a light red, feems to be an attempt at the fainter colours, which intervene between the reds. 5. All the ftronger colours make their first appearance at the edges of the circular spot; and more explofions make them continually expand towards the extremity of the fpace firft marked out, while others fucceed in their place; till, after about thirty or forty explofions, three diftinct rings generally ap pear, as in No 4. upon the feel. If the explosions be continued farther, the circle becomes lefs beautiful, and lefs diftinct; the red commonly prevailing, and fuffufing all the other colours, as in N° 1. upon the fteel; though I attribute the confufion of the colours in that circle, in part, to the needle having been feveral times accidentally broken from the cement which fup. Forted All the coloured rings mentioned in this paper were fhewn to the Royal Society, but could not be well reprefented by a print. ported it, and to its not having been replaced exactly as before. 6. The laft formed colours are always the most vivid, as appears very diftinctly in the reds of N° 1. the steel. Alfo the last formed upon rings lie clofer to one another than the first. 7. Thefe rings may be brushed with a feather, and even wetted, or a finger may be drawn over them, without their receiving any injury; but they eafily peel off, when fcratched with one's nail, or any thing that is fharp, the innermoft rings being the most difficult to erase. An 8. The first circles are fometimes covered with a quantity of black duft; part of which, however, may be wiped off with a feather, fo as to show the colours under it. attempt to wipe off more, on the rough fide of the fteel, took off the colours along with it; but inore than half yet remains, with the duft upon it, as it was first formed. 9. It makes no difference whether the electric matter iffue from the pointed body upon the plate, or from the plate upon the pointed body; the plate oppofed to the point being marked exactly alike in both cafes. Alfo the points theinfelves, from which the fire iffues, or at which it enters, are coloured to a confiderable diftance, often about half an inch, but not very diftinctly. The colours likewife return here, in concentric rings, as upon the plate. 10. I think that the more circles are made at the fame time, the more delicate will the colours be; whereas the furface is, as it were, torn, or corroded by more violent explofions; which makes the colours appear rough and courfe, N° 4. is I think on this account, as well as fome others, marked in a more de- 11. A polished furface is not ne- 12. These coloured rings appear I have not tried any of the femimetals; but I have no doubt of their anfwering as well as the proper metals. 13. When the pointed wire was made to incline to the plane on which the colours were exhibited, the circular fpot was quite round, the centre of it being in the perpendicular let fall from the point; but the colours were projected oppofite to the point, in an oblong figure. Upon fhewing these coloured rings to Mr. Canton, I was agreeably furprifed to find, that he had, likewife, produced all the prifmatic colours from all the metals, but by His a different electrical procefs. method had been to extend fine wires over the furface of pieces of glafs; and when the wire was exploded, he obferved that the glafs remained tinged with all the colours from all the metals. They are not indeed difpofed in fo regular and beautiful a manner as in the rings I produced; but they equally demonftrate, that none of the metals difcovers the leaft preanother. ference to any one colour more than another. A variety of other very extraordinary appearances occured in the courfe of Mr. Canton's experiments in melting wires. In what manner these colours are formed, it may not be eafy to conjecture. In Mr. Canton's method of producing them, the metal feems to be difperfed in all directions from the place of explofion, in the form of spheres, of a very great variety of fizes, tinged with all the variety of colours, fome of them too fmall to be diftinctly vifible by any magnifier. In my method, it fhould rather feen that they are produced in a manner fimilar to the production of colours on fteel, &c. by heat, i. e. the furface is affected, without the parts of it being removed from their places, certain plates only, or lamina, being formed, of a thickness proper to exhibit the refpective colours at certain diftances; and that the thickness of thefe plates is continually changing by the repetition of the explosions. N. B. The battery made ufe of in the above-mentioned experiments was of twenty-one Square feet of coated glafs. A short account of the manner of inoculating the fmall-pox on the conft of Barbary, and at Bengal, in the Eaft-Indies, extrañied from a memoir written in Dutch, by the Rev. Mr. Chais, at the Hague; by M. Maty, M.D. S.R.S. [Read April 14, 1768.] AVING long thought that the Arabs, who, about the middle of the fixth century, were the firft who wrote upon the fmall-pox,. were likewife the firft inventors of the method to prevent the fatal confequences of that cruel diforder, I was very defirous to get what in formation I could concerning the introduction of inoculation in Africa, and in the Eaft-Indies. About twenty years ago, Caffen Aga, a Tripolitan ambaffador at London, informed the people about him, that inoculation was univer. fally practifed, as well at his court, as at Tunis and Algiers; but that no certain account could be given, either of the introducers of the method, or of the place from whence it took its rife. One of the chief minifters of ftate in Holland was fo good, on this information, and at my defire, to fend a few queries on that fubject, drawn up by myself, to a gen. tleman, who, for feveral years, has refided with a public character at Algiers. The following is a fummary of his anfwers to my queries; The fmall-pox is, as well as "in Holland, à contagious dif "temper at Algiers. Tunis and " Tripoli, and fully as deftruc "tive. In order to avoid the bad "confequences of the natural difor"der, many people have recourfe "to inoculation, which there is per"formed in a very different manner "from what is ufed in our coun"try. The perfon, who intends "to be inoculated, having found "out a house, where the fmall pox is, and is of a good fort, "goes to the bed of the fick per"fon, if he is old enough, or, if a child, to one of his relations; and fpeaks to him in the fol"lowing manner: I am come here "to buy the small-pox: the answer "is, buy if you please. A fum of "money is accordingly given, and "6 one, three or five puitules (for "the * the number must always be an "odd one, not exceeding five), ex"tracted whole, and full of mat"ter. Thefe are immediately rub"bed upon the fkin of the hand, "between the thumb and fore"finger. This is fufficient to com"municate the infection; and as take ef "foon as it begins to "fect, the inoculated patient is "put to bed, carefully covered "with red blankets; and heating "medicines are given him with "fome honey of rofes. He is al lowed goat's broth for his nou"rishment, and for his drink an "infufion of fome herbs; not"withstanding this treatment, it "feldom happens that the fmall"pox procured in this manner "has any bad confequences; and "almoft never that any body dies "of it; but hitherto the propor"tion of the mortality in the na"tural, to that in the artificial "way, has not been afcertained. Laftly, though the time when this practice was introduced in Africa "be unknown, yet it is there very "old, and the Arabs are generally "thought to have been the inventors " of it." From this account it plainly ap1. That in Africa the opepears; it is in ration is performed as Wales, by the rubbing in of the matter, and that this is done to prevent the fatal confequences too often following the natural infection; 2. that this inoculation is nerally fuccefsful, notwithstanding the heat of the climate, and the bad management of the patients; and 3. that the origin of it is very ancient, and afcribed to the Arabs. ge Before I had received thefe informations from Algiers, I had engaged fome friends fettled in three VOL. XII. different parts of the Eaft-Indies, "I have fent for feveral phy"ficians, to be informed of the "things you feem defirous to know "about inoculation; the practice "is hitherto not ufed in this pro"vince: but having met with a "Bengalian doctor, he gave me "the following account. "Though the first introduction "of the operation at Bengal is now unknown, it has been in "ufe in that country for a very "long time, and is performed in "two different ways. "For the firft, fome of the va"riolus matter of a good kind "having been gathered, is kept "for ufe. When a child is to be between "inoculated, the fkin fome of the fingers is pricked "by means of two fmall needles "joined to one another. After having rubbed in a little of the "matter upon the fpot, a circle "is made by means of feveral "punctures, of the bigness of a "common puftule, and matter is The "again rubbed over it. "wound is then dreffed with lint; 66 a fever enfues, and after fome "days, the eruption, which, if the "fever has been strong, is obferved To ex"not to be very copious. "cite the fever, the patient is made "to bathe in a tub of water. "As this way of managing the "operation is very painful, a more "eafy one has been invented for "people of quality and fubftance. "A little of the matter is mixed "with fugar, and fwallowed by G <<< the |