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one of thefe images is moveable, but the other is all the while fixed. This is the natural and obvious method in the examining the properties of the (tone : but there is a way to render the moveable object fixed, and the fixed one moveable ; or to ren- der both the one and the other moveable at the fame time. The moveable image, in the common way of obferving, docs not move about at random, but always in a regular manner about the fixed object, and while it turns about this, it never dcfcribes a perfect circle, but only in one cafe. The common laws of dioptricks, teach us, that a diaphanous body having only one furface, fends from one object but one image re- fracted to the eye, and having more furfaces than one, it re- prefents one image of the object in each ; and, as in this jfland cryftal, when we look only upon one furface we have two images, it is evident, that there muft be fomething pe- culiar in the nature and internal ftructure of the body itfelf to occafion it. The whole mafles of this ftone are wonderfully compofed of plates, or laminas, extreamly thin, laid evenly and regularly on one another; and forming, in a manner, lcarce conceivable, both the horizontal and the perpendicular fur- faces. Each of thefe plates is compofed of a vaft number of fmall parallellopiped figures, arranged in a neat and orderly manner together; and each of thefe minute parallellopipeds is ao-ain compofed of plates compofed of other parallellopipeds, and fo on as far as microfcopes can trace them. There is therefore fomething lingular in the ftructure of this body, which may very well give it the property of refracting diffe- rently from the other pellucid ftones ; and, it is obfervable, that all the (pars of tins ma fly kind, as well asfome of the pyrami- dal ones, have the fame property; and that, in the fame de- gree, in proportion to their bignefs, whenever they are fuf- ficiently pellucid. The vaft mafles of white fpar which are found in the lead mines of Derbyshire, though they are not externally of the parallelepiped figure of the ifland cryftal, nor have any thing of its brightnefs or tranfparence in the general lump ; yet, when they are broken, they feparate into rhom- boidal fragments, and fome of thefe are found to be tolerably pellucid : all thofe which are fo have the property of the ifland cryftal ; and being laid upon paper, where a black line is drawn, they all fhew that line double in the fame manner as the real ifland cryftal does. Bartholine has two hypothefes for the explication of the various refractions of this ftone, not fo much founded on reafonings as on experience : the one is, that there are fome lines, by which the rays pafs through the body of the ftone unrefracted, which lines, though they have been held to be perpendicular ir. the diaphanous bodies, commonly known, yet it does not appear neceflary, that they fhould always be fup- pofed fuch ; fince they may, perhaps, in fome cafes, be much otherwife ; the other is, that it may be fuppofed, that half the light or appearance diffufed from the object, is refracted, ac- cording to the ufual laws of refraction ; but the other half, according to the unufual refraction ; or which is all one, that the ufual and unufual refraction, have the fame power to re- fract the rays of the object. Barthol. de Cryftal Ifland. See the article Parallellopipedium. DISEASE (Cycl.)— The colder the country, in general, the fewer and the lefs violent are the Difeafes. SchefFer tells us, that the Laplanders know no fuch thing as the plague or fe- vers of the burning kind, nor are fubject to half the di- jiempers we are. They are robuft and ftrong, and live to eighty, ninety, and many of them to more than a hundred years ; and, at this great age, they are not feeble and decrepid, as with us ; but a man of ninety is able to work or travel as well as a man of fixty with us. They are fubject, however, to fome difeafes more than other nations : thus they have often difiempers of the eyes, which is owing to their living in fmoak, or being blinded by the fnow. Pleurifies and inflammations of the lungs are alfo very frequent among them ; and the fmall-pox often rages with great violence. They have one general re- medy againft thefe, and all other internal difeafes : this is the root of that fort of mofs, as Scheffer exprefles it, which they call jerth. They make a decoction of this root in the whey of rain-deer milk, and drink very large dofes of it warm, to jceep up a breathing f'weat; if they cannot get this, they ufe the ftalks of angelica boiled in the fame manner; they have not fo great an opinion of this as of the other remedy ; but the keeping in a fweat, and drinking plentifully of diluting liquors, may go a great way in the cure of their difeafes, whether either the one, or the other of the drugs, have any virtue or not. They cure pleurifies, by this method, in a very few days; and get fb well through the fmall-pox with it, that very few die of it.
It has been always obferved, that people of particular places were peculiarly fubject to particular Difeafes, which are owing to their manner of living, or to the air, and effluvia of the earth and waters. Hoffman has made fome curious ob- fervations on difeafes of this kind. He obferves, that fwel- lings of the throat have always been common to the inha- bitants of mountainous countries : and the old Roman authors fay, Who wonders at a fwclled throat in the Alps ? The peo^ pie of Swifferland, Carinthia, Styria, the Hartz-foreft, Trail- fylvania, and the inhabitants of Cronftradt, he obferves are all
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fubject to this difeafe from the fame caufe ; and, it feems, that thefe ftrumous fweliings are owing to the water which they drink, and which in mountainous places is ufually very highly- impregnated with (parry or ftony particles. The French arc peculiarly troubled with fevers, with worms, and with hydro- celes and farcoceles ; and all thefe diforders feem to be owing originally to their eating very large quantities of chefnuts. The people of our own nation are peculiarly afflicted with hoarfenefi'es, catarrhs, coughs, dyfenteries, confumptions ; and the women with the fluor albus, or whites ; and children, with a Difeafe fcarce known elfewhere, which we call the rickets. In different parts of Italy different Difeafes reign. At Naples the venerea! Difeafe is more common than in any- other part of the world. At Venice people are peculiarly fub- ject to the bleeding piles. At Rome tertian agues and lethar- gic diftempers are moft common. In Tufcany, the epilepfy, or falling ficknefs. And in Apulia they are moft fubject to burning fevers, pleurifies, and to that fort of madnefs which is attributed to the bite of the tarantula, and which, it is faid, is only to be cured by mufic.
In Spain apoplexies are common, as alfo melancholy hypocon- driacal complaints and bleeding piles. The Dutch-are pecu- liarly fubject to the fcurvy, and to the ftone in the kidneys. Denmark, Norway, Sweden, Pomerania, and Livonia, are all terribly afflicted with the fcurvy ; and, it is remarkable, that in Denmark, Sweden, and Norway, fevers are very com- mon ; but in Ifland, Lapland and Finland, there is fcarce ever fuch a Difeafe met with ; tho' peripneumonies be very com- mon in thefe places, as alfo difeafes of the eyes and violent pains of the head. The Ruffians and Tartars are afflicted with ulcers, made by the cold, of the nature of what we call chilblains, but greatly worfe ; and in Poland and Lithuania there reigns a peculiar Difeafe, called the plica polonica ; Co terribly painful, and otfenfive, that fcarce any thing can be thought of worfe.
The people of Hungary are very fubject to the gout and rheumatifm : they are more infefted alfo with lice and fleas than any other people in the world, and they have a peculiar Difeafe which they call cremor.
The Germans, in different parts of the Empire, are fubject to different reigning Difeafes. In Weftphalia they are peculiarly troubled with peripneumonies and the itch. In Silefia, Fran- conia, Auftria, and other places thereabout, they are very liable to fevers of the burning kind, to bleedings at the nofe, and other haemorrhages ; and to the gout, inflammations, and confumptions. In Mifnia they have purple fevers ; and the children are peculiarly infefted with worms. In Greece, Macedonia, and Thrace, there are very few Difeafes ; but what they have, are principally burning fevers and phrenfies. At Conftantinople the plague always rages ; and, in the Weft- Indian ifhnds, malignant fevers, and the moft terrible co- lics. Diseases feigned— The cheat of feigning Difeafes, for idlenefs, or profit fake, or for other occafional purpofes, has been too common in all ages, and is too difficult to be difcovered in all cafes ; though, in many, there are ways of coming at the truth. We have had a very late inftance of a young lady, who puzzled feveral of our beft phyficians, to account for the nature of a Difeafe, which fhowed itfelf in a Angular kind of excoriations ; and the fecret perhaps had never been difcovered, had not a bottle of aquafortis been accidentally found in her chamber ; the touching the arms, &c. with which had occa- fioned all the appearances, which fome ftrange fault in the juices had been before accufed of.
Diffimulation, in regard to Difeafes, may be reduced to three kinds: in the firft, the ficknefs is pretended by words only, and really is not ; of this nature are the pretended head-ach, colic, and the like ; which, as the patient can only know of, the phyiician may always be deceived by his words. In the fecond, there are not only words for tcftimonies of the fick- nefs, but there appear ligns and marks of it on the body, which yet are only counterfeited to ferve certain purpofes. Scabbynefs, leprofy, and even the exulceration of the lungs may be thus counterfeited ; the laft by a pretended fpitting of blood. Gravidation is often pretended alfo. Our beggars often contrive the means of thefe to excite compaffion. In the third kind of diffimulation of Difeafes, we are to include the con- cealing a Difeafe, which really exifts, inftead of the owning one which does not.
The great occafion of this kind of diffimulation is the pox, and the concealing of this often proves the ruin of the patient's conftitution, or his reafon ; a miferable death or madnefs being too often the confequences.
In the difcovery of thefe deceits, on which the credit of the phyflcian often depends, it is not mere medical knowledge that will fucceed, but there muft be added to it a fagacity in exploring all circumftances and outward accidents, and efpe- cially thofe things which relate to the genius, temper and ftate of the perfon fufpected. The Dasmoniacs, of former ages, exorcifed by the priefts, who wanted to propagate the credit of their religion, or their own fanctity, have generally been cheats of this kind, and might have been difcovered by any
who
