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nefs, or diffimilitude, fome authors of the late ages have arranged them into genera. This general figure in fifties confifts in the (hape of the head and body, and the fhape, fize and propor- tion of the fins and tail, and tho' very obvious, is very indif- tinct ; it being in many cafes where two fifhes have the fame general external face, yet hard to fay on a clofe examination, in what it is, that the likenefs coniifts. The authors who founded the diilinction of the genera of fifhes on thefe exter- nal refemblances, ran into very great errors; for it is not enough in a generical diftinction for the character to be obvious, but it muft alfo be precife and determinate. What this Fades externa had in the firft of thefe requifites, it often wanted in the ]aft ; and in general, it has been found to be no true ba- fis of diilinction.
Many fifh have the Fades externa, or general appearance fo alike, as to be eafily coupled at fight into the fame genus, and that juftly,as they really belong to the fame, when more precifely referred to it, according to their natural and more effential cha- racter. Of this kind, are the gadi, the clupeas, the falmons, the petromyzas, the coregones, the pleuronectse, the rays, and many of the cyprini. In thefe the Fades externa is of real ufe, as it is an obvious mark, and leads to the road of truth ; but there are befide thefe many other fifhes, which though they are tru- ly of the fame genus, yet differ extremely in their feveral ex- ternal appearances, fo that- any method founded on the Fades externa muft feparate them, though nature had really joined them in their real characters. The tench and the trutta la- cuftris, or lake trout, are in regard to their Fades externa, ex- tremely alike one to the other, yet here this obvious character deceives us; for the tench is a fpecies of the cyprinus, and the other a true and genuine falmon, two genera of fifties effentially and very widely different, tho' this method of judging by the external appearance, would have coupled to- gether fifh belonging to them both. In the fame manner, the fcorpjena and the cattus are very like one to the other in their external appearance, but when nicely examined accord- ing to the rules of Ichthyology, they are found to belong to two very different genera, and to have very little real likenefs. From thefe and numerous other examples of a like kind, it evidently appears, that as the Fades externa cannot be depend- ed on for the eftablifliing the genera of fifties, fome more ef- fential characters muft be enquired after, for the regular and natural completing this neceffary bufmefs. Therefore the ge- nerical characters of fifh are to be fought after, in their ex- ternal and invariable parts, and they are to be arranged into families and genera, according to the agreement of thefe in number, fituation, figure and proportion. Among all thefe, the characters taken from the number of the parts, where that is certain and invariable, are moft valuable, as they moft readily offer themfelves to the eye, and are leaft of all liable to errors. Artedi Ichthyol. FACETS, (Cyd.) among jewellers, a name given to the fm all triangular faces, or planes, both in brilliant and rofe diamonds. In brilliants there are two forts, skew or skill facets, and Jlar facets. Skill facets are divided into upper and under. Upper skill Facets, are wrought on the lower part of the
bezil, and terminate in the girdle. Under skill Facets, are wrought on the pavilions, and termi- nate in the girdle. Star Facets, are wrought on the upper part of the bezil, and terminate in the table. Jeffries on diamonds. See Dia- monds. FACET ANUS lacertus, in zoology, the name of a peculiar fpecies of lizard, called at Rome and Naples, the tarantola. See Tarantola. FACK, in a fhip, is any one round of a cable, when it is quoil-
ed up out of the way. F/ECES (Cyd.) — There have been many opinions of the great power of an oil to be drawn from the human feces, one of which is the effect it is fuppofed to have on crude mercury, in fixing it at once into pure filver. Mr. Homberg was applied to, with great afiurances of fuccefs on this occafioti, and heartily engaged in it. The neceffary characters of the ex- pected oil were, that it fhould be colourlefs as water, and void of fmell; he at length found the way of procuring fuch an oil as this, but when he had got it, it had not the properties expected in it. As the method of working on this difagreea- ble matter, however at length furnifhed him with a very fine phofphorus, it may be worth the attention of the curious. An obfervation which he made very early in thefe refearches was, that ten or twelve ounces of this fubftance, when the humi- dity was evaporated in a balneum marine, did not leave more than an ounce or thereabout of dry matter ; yet all it loft in this great evaporation, proved to be only its water, which when the receiver was examined, proved to be very clear and pel- lucid, and infipid to the tafte, but fmelling confiderably ftrong- ly of the matter it was made from. The fait, oil and earth of the fubftance, all remained behind, and it is remarkable, that all thefe do not make up above one eighth at the utmoft, often but a tenth or twelfth of the whole. When this was afterwards more nicely examined, the fait in it was found to be ■about equal in quantity to the earth, and both thefe to-
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gether, to be nearly equal to the oil. The oil of the faces fimply diftilled, always appears black and ftinking. Mr. Hom- berg to prevent thefe ill qualities, diflblved it in warm water allowing a pint to every ounce of the matter. This liquor was then filtered and evaporated to a pellicle, after which it yielded regular cryftallizations of falts, adhering to the fides of the veilels ; thefe may properly be called the effential fait of the faces. This fait has fome refemblance of nitre, and in the manner of that fait, melts in the fire, but the flame of the faecal fait is red and dull, whereas that of the fait petre is white and very lively; the reafon of this difference evidently is, that in the one of thefe falts there is a large quantity of an oily matter, and in the other there is fcarce any at all. This fait diftilled in a glafs retort, affords toward the end of the operation, a reddifh foetid oil; this is preceeded by an aque- ous liquor of an acrid and acid tafte. As this oil was not white or limpid, nor fcentlefs, Mr. Homberg after feveral re- petitions of the diftillation, gave over the operation, and went to work again upon the matter fimply dry'd in the balneum marias, and mixed with different intermediate fubftances as quick lime, lime extinguiihed by the air, alum, colcothar, powder of bricks and the like ; but all thefe operations only produced an oil more thin and fluid than the former, which ever fo often repeated rectifications, only rendered clearer, not limpid, nor without its ftinking fmell, which it retained to the laft.
In the diftilling the fecal fait alone, the matter in the retort always took fire at the time of the rifing of the oil ; and in thefe diftillations of the dried matter with the mixtures of alum or colcothar, when the whole was over, and the veffels cold, on the opening them, the caput mortuum always in a few minutes took fire on being expofed to the air. This flame often burft the retort, and fometimes when it did not do that, there came out a great body of flame at its mouth; and after that, the remainder at the bottom of the veffel appeared for fome minutes ignited in the manner of a burning charcoal. Still however, the defired limpid oil was not found ; and as it had not been obtained by violent fires, the next trial was made by the gentler means of fermentation, by which the feveral principles of a mixed body are ufualy feparated one from, another, by flow and gentle means. The matter was diftill- ed to a drynefs in a balneum maris, and afterwards the dry refiduum being powdered, was mixed with fix times its weight of its own diftilled water, and put into a large cucur- bit which being flopped by another glafs and fo made a double veffel, was kept three months in a gentle balneum marine, the water of it being never made more hot than fo as that one's hand might bear it ; after this, a head being luted on to the cucurbit, the diftillation was performed in the fame balneum with a gentle fire; the aqueous humidity which came over was fomewhat turbid, but had loft its ftink, having only a faint weak fmell. This water on trial, proved an excellent cofmetick, by degrees recovering the skin to a delicate white- nefs and foftnefs, tho' before ever i'o much impaired by wea- ther. The remainder in the bottom of the cucurbit was found to have loft juft one twentieth part of its weight, and had wholly loft its difagreeable fmell, being become fomewhat fragrant and agreeable, with an aromatick flavour ; the vef- fel in which this procefs had been performed being fet by open in a corner of the laboratory, acquired after fome time fo ftrong a fcent of ambergrife, that it was not to be endured, and was forced to be removed out of the laboratory. Any perfon would have thought it a veffel in which ambergrile had been made into effence. What can be more wonderful than that a fimple digeftion fhould be thus of power to change one of the moft difagreeable fmells in nature, into the fcent of one of the fined perfumes. Mem. Acad. Par. 171 1. The dry matter taken out of the cucurbit was powdered and put by two ounces at a time into glafs retorts that held about a pint and half of water, and diftilled in a fand heat ; there firft arofe a little water, but after this came an oil perfectly colourlefs and limpid as water. The fame degree of fire being continued, there afterwards arofe an oil as red as blood : the receiver was changed, and thefe two oils kept afunder; the firft had fcarce any fmell, and what it had was aromatick, and the latter was very fxtid and empyreumatick. The white or limpid oil being rectified, there was found to be produced of it, one ounce from every nineteen ounces of the dried mat- ter ; hut even the fineft and moft colourlefs of this oil, will after about a year's keeping become red and change its grate- ful odour into an empyreumatick fmell. The change of co- lour always begins at the bottom of the vial, and it gradually reddens all the way up to the furface of the oil. The reafon appears to be this, that there was originally contained in this pure limpid oil, fome fmall quantity of the red oil in diffemi- nated particles, and unperceived there, but that in long ftand- ing thefe particles being different from the others, collected themfelves together, and by their weight fubfided to the bot- tom of the veffel, where by degrees they contaminated the oil next above them, and by degrees the whole quantity. Thus was the defired oil obtained, but it had not the properties re- quired of it, nor could by any means be made to effect any
change
