PUR
the chryfalis as well as under the nymph ftate, with proper at- tention in the examination, and that therefore the diftincticn , is founded on a very vague and uncertain bafis. The far greater part of the infect kingdom are hatched from the egg in the worm ftate, and become, after a time, changed into thefe chryfalifes and aurelias; but fome are hatched perfect, and in their own form, from the egg. Among thofe which undergo the change into the chryfalis ftate, the principal diftinction is into worms that have legs, and worms that have not any. The breaft of thofe worms which have no feet is never changed at all, and among thofe worms which have many feet, the fix fore feet are never changed, but ferve the animal as well in its flying ftate as in its creeping one. The wings, the horns, the feet, £3V, grow up under the flrin by degrees.
In all worms, of whatever kind, thefe parts may be diftin- gu lined more or lefs plainly under the (kin, at the different times of their growth, in different degrees of forwardnefs. Even the frog may be thus traced, by proper care, in the tad- pole, and all its changing from the tad-pole ftate to that of the perfect animal, is no other than as in the butterfly, the developing and unfolding its parts, as they require a proper age, confidence, and maturity.
Mouffet, and other writers on infects, call the aurelia a fepa- rate and peculiar thing, and exprefly neither an egg nor an animal. Harvey a;fo miftakes the matter as grofly, in calling the aurelia an egg, and faying, that the bloodlefs animalcules are produced by transformation; but thefe are expreifions much to be lamented in the works of fuch great men, and have been the means of darkening and perplexing the know- ledge of the aurelia ftate in infects, and muft be overthrown before any true judgment can be formed of that ftate. All the change happening in the puppets, or nymphtr, which was fuppofed by Harvey to be the production of one animal from another, is in reality no more than the exfudation of a certain abundant moifture which leaves the animal dry, and all its parts firm and capable of expanding and extending them- felves, and bearing the weather.
Gocdartius has alfo propagated another groundlefs error, which, fcy its confequences, tends to overthrow all the true know- ledge of thefe changes. This is, that the caterpillar may change before its time, and that in this cafe it becomes a very different animal in the winged ftate. This tends wholly to overthrow the conftant and uniform law of nature, according to which, all the lineaments of the perfect animal are formed even in the egg, and have only to grow and enlarge in th worm and chryfalis ftate ; and can therefore be fubject to no change in either.
What has led Geodartius into this error, was real obfervation, only not carried fo far as it mould have been. In fome of the infects which fuffbr the chryfalis ftate, the males are winged and the females not. In thefe the courfe and order of nature is as invariable as in the reft, all the males being of the winged, and all the females of the reptile kind ; but this au thor, not knowing this great difference in the fexes, when he had been ufed to fee a winged creature come out of the chry- falis of a worm be knew very well, and found inftead of it a creeping animal without wings, fuppofed the creature, which was only of a different fex, to be of a different fpecies. Some difference alfo is found in the fize of the fly produced from the chryfalis of the fame fpecies of caterpillar, accord ing to its changing fooner or later into that ftate. All the difference, however, is, that if the caterpillar eats its fill firft, the fly is the larger and the fairer; and if it goes into the chryfalis earlier, then the fly is the lefs perfect: and of what- ever fize the fly is when produced out of the chryfalis, in that it remains ; for it can grow no more after it has palled this latter change.
From the knowledge of the propagation of this fort of ani- mal, this author goes up to that of larger creatures, and firm in the opinion, that there is properly no fuch thing as generation at all; that all that our bfindnefs calls fo, is only a fenfible production by the growing of parts at firft very mi- nute. All the parts of the butterfly may be feeii, by a proper infpection, in the caterpillar; and as to the imaginary tranf- formation of the caterpillar into the chryfalis, it mav be re- tarded and made to come on fo eafily, as to become the ob- ject of the fenfes, and finally flopped in the progrefs, fo as to appear half caterpillar, half chryfalis. Swammerdams Hift Infect.
PUPPIS OS, a name given by fome authors to the os frontis. The future in this bone is aifo called by many anatomifts the futura puppls.
POPULAR a name ufed by fome to exprefs the extremities of the fingers.
PUR autre vie, in our law-books, is ufed where lands are held for the life of another.
PURCHASE-/^, among traders, is the name given to a book which is a kind of journal, containing an account of all the purchafes made, or things bought in the day.
rurchafc, n\ the fea-language, is the fame as draw in : thus when they fay, the cajjfan purchafes apace, they only mean it draws in the cable apace; alfo when they cannot draw or Supp. Vol. II.
PUR
hale in any thing with the tackle, they fay, the tackle will not pwchafe.
PURETTA, a name given by fome writers to the common firining black fand, ufed to flrew over writing, and erroneoufly called by famejieel-duft.
It is a natural mineral fubftance, found on the fhores near Ge- noa and in other places.
PURGATIVE (Cycl.)— We have had attempts of adjufting the dofes of purgative medicines fcientifically. Dr. Cockburn en- deavoured at the folution of this problem ; but, it is faid, on wrong principles. Dr. Balguy ha- alfo given us an eilay on this fubje£t in the Med. Eff. Edinb. Vol. IV. art. 5. He affumes, that part of the medicine is fpent on the firft pafiages, where it acts as a ftimulus ; and that the other part, is carried into the blood, and has its effect there by thinning and rarifying it. This being premifed, ift. If the medicine acts only in the firft paffage?, the dofe will be as the fize of the perfon into the conftitution. 2d. If the whole medicine pafles into the blood, the dofe will be as the fize into the fquare of the conftitution ; and therefore, 3d. You are to dofe fo much of the medicine as is fpent on the ftomach and inteftines di- rectly, as the confutation ; and fo much as is carried into the blood, as the fquare of the conftitution, and the fum into the pcrfon's fize, is the quantity required.
The fame rules hold in vomits. How far in either cafe the practice of phyfic may be thereby improved, we leave to the judgment of the learned. The folution of the problem fup- pofesa great poftu latum, no lefs than the art of meafuring a perfon's conftitution !
PURGATION, pwgat'w, In rhetoric, is ufed for that kind of defence which takes place when the accufed perfon owns the fact, but denies that he did it with defign, or with any bad in- tention. fcJf.Rhet. 1. 1. p, 148.
PURIFICATION, in pharmacy. See Trying.
PURPURA, the purp!e-fijh 9 in natural hiftory, the name of a genus of fhell-fifh, the characters of which are thefe: it is a univalve fhell, jagged and befet from head to tail with fpines, - tubercles, umbos, or Arise. The mouth is fmall and round ifh ; the tail is fliort, and ufually the bafe runs out into a long beak. It has been ufual with moft authors to con 'bund together the genera of the murex and purpura, and to ufe the words as fyn- onymous : but tho' there is fome external refemblance be- tween many of the fhells of the two genera, yet they are eafily diftinguifhed by this, that the mouth of the purpura is lefs long, and is lefs dentated and alated than that of the mu- rex. The body and the head of the fhells of this genus are not fo elevated as thofe of the murex kind, and are not cover- ed with points or buttons at the mouth. If a fhell is there- fore found to have a fmall, fmooth, and round mouth, and a body covered with undulated leaves, as it were, like thofe of favory or endive, and fometimes with long points, and its tail, whether long or fliort, be hollowed and fomewhat bent, this may be called a purpura and not a murex. The antients diftinguifhed three kinds of purpura, one which had a long and crooked tail, made hollow like a tube or pipe; a fecond which had either no tail at all, or at the moft a very fhort one; and a third which had no fpiral head, or, as we fhould exprefs it, no clavicle.
On examining the whole family of the purpura, we may dif- tinguifh four remarkable fpecific differences among them. The firft of thefe comprehends thofe purpura which have the body of the fhell garnifhed with a fort of undulated foliage in clouded ridges, and have a fhort and crooked tail. The fecond comprehends thofe which have the body of the fhell covered with acute points, and have a long tail. The third comprehends thofe which have as long a tail as the former, but have a fmooth body, or at the utmoft have only a few flight protuberances and wrinkles on it. And the fourth takes in thofe which are fmall, and have an elevated clavicle, a fliort crooked tail, and the body of which is covered either with flen- der fpines or hairs. See Tab. of Shells, N°. 14. This fpecies of fifti, as well as the murex, ferved among the antients to dye the fine purple colour they were fo fond of, and fome of the buccina have been of late found to have the fame juice. The purpura and murex are both fifhed up in great plenty in the gulph of Tarentum ; but the fmall quan- tity of the coloured juice which each fifti contains, and the neceflity of ufing it before the animal dies, makes it im- poflible to bring it to any regular article of trafKck. The antients ufed this colour only on cotton and woollen fluffs, whereas our cochineal, which was unknown to the antients, ftrikes equally well on filks and fluffs.
The purpura lives on other fifh. ' It ufually hides itfelf at a fmall depth in the fand, fometimes even in frefh-water rivers, and as it lies hid, it thrufts up a pointed tongue, which wounds and kills any thing that comes over it. We frequently find fea-fhells with round holes bored through them, as regularly as if made with a boring inftrument: thefe are generally al- lowed to be made by the tongue of the purpura, in order to its feeding on the fifh within.
Trie purpura has two horns like that of a fnail ; and Fabius Columna fays, that they have eyes in thefe, not placed at the ends, as in the fnail, but in the middle of each horn.
7. H h Of
