Affam furnishes us with the greatest quantity of lac in ufe; and it may not be generally known, that the tree on which they produce the beft and largeft quantity of lac is not uncommon in Bengal, and might be employed in propagating the fly, and cultivating the lac, to great advantage. The fmall quantity of lac collected in these provinces affords a precarious and uncertain crop, because not attended to. Some attention at particular seasons is neceffary to invite the fly to the tree; and collecting the whole of the lac with too great an avidity, where the infect is not very generally to be met with, may annihilate the breed. "The best method of cultivating the tree, and preferving the infect, being properly under ftood in Bengal, would fecure to the Cofs poffeflions the benefit arifing from the fale of a lucrative article, in great demand and of extenfive ufe.' Dr. GRAY'S INFERENCES deducible from an Attention to the EXTERNAL CHARACTERS of SERPENTS. [From the fame Work.] "Shall make no further reShall make no further remarks on the external characters of ferpents; the principal inferences to be deduced from thofe I have already made, are the following. 66 "ift, That a broad head, covered with fmall fcales, though it be not a certain criterion of venomous ferpents, is, with fome few exceptions, a general character of them. "2dly, That a tail under onefifth of the wh dength, is alfo a general character of venomous ferpents; but, fince many of thofe which are not venomous have tails as fhort, little dependance can be placed upon that circumfience alone. On the other hand, a tail exceeding that proportion, is a pretty certain mark that the fpecies, to which it belongs, is not venomous. are, in fome measure, characteristic of venomous ferpents, fince ia them they are more common than smooth ones, in the proportion of nearly 4 to; whereas, smooth scales are, in thofe ferpents which are not venomous, more common, in the proportion of nearly 3 to 1. "Upon the whole therefore it appears, that though a pretty certain conjecture may, in many inftances, be made, from the external characters; yet, in order to determine, with certainty, whether a ferpent be venomous or not, it becomes neceffary to have recourfe to fome certain diagnoftic. This can only be fought for in the mouth; I fhall therefore next confider, how the fangs, with which the mouths of venomous terpents are furnished, are to be difiguifhed from common teeth. << adly, That a thin and acute tail is by no means to be confidered." To thofe who form their ideas as peculiar to venomous ferpents; of the fangs of venomous ferpents, though a thick and obtufe one is from thofe of the rattle-fnake, only to be found among those which or even from thofe of the English vier, it will appear ftrange, that there fhould be any difficulty in diftinguish are not venomous. 4thly, That carinated fcales diftinguishing thofe weapons from common teeth; and indeed the diftinction would really be very eafy, were all venomous ferpents furnithed with fangs as large as thofe of the fore-mentioned fpecies. But the fact is, that in many fpecies the fangs are full as fmall as common teeth, and confequently cannot, by their fize, be known from them; this is the cafe with the coluber laticaudatus, lacteus, and feveral others. I cannot, however, better demonftrate that the diftinction, between the venomous fangs and common teeth, is not very obvious, than by fhewing how very vague, and erroneous Linnæus's ideas about them were; nor can I better prove the want of information on this fubject than by obferving that, erroneous as the ideas of Linnæus were, no one, that I know of, has yet attempted to furnish more correct ones. "Linnæus thought the fangs might be diftinguifhed by their mobility; this, at leaft, may be fairly inferred, from his never mentioning them in the Mufeum Regis, without adding the epithet mobilia, except in one inftance (the coluber aulicus); and, in that very instance, the want of mobility in the fuppofed fangs appears evidently to raise doubts in his mind, whether they are really fangs or not. His words are, 66 Dentes, five tela, duo, rigida, parva, non mobilia." Thefe doubts, refpecting the above mentioned fpecies, I am not able to remove, as I am not fure that I have ever feen it. But with regard to mobility, confidered in general as a character of venomous fangs, I must affert, not only that I have never found it fo, but also, that I have never been able to discover in them any thing which I thought could properly be called mobility. I have, indeed, fometimes found fome of them loofe in their fockets; but then I have found others, in the fame fpecimen, quite fixed. The fame thing was obferved both by Dr. Nicholls, and by the Abbé Fontana, in the common viper, even during life. The loofe fangs may be fuch as have not yet been firmly fixed in their focket, or they may have been loofened by fome accident: for I fufpect that the fangs may be at any time loofened, and even difplaced, by a small degree of violence; and that, perhaps, may be one reafon why there is always a certain number of fmall fangs, near the bafe of the full grown ones, ready to enlarge and take their place, if they should be, by any accident, torn out. "Linnæuis feems alfo to have thought that the fangs might be known by their fituation. In the introduction to the clafs amphibia in the Syftema Naturæ, he fays they are," Dentibus fimillima fed extra maxillam fuperiorem collocata ;" and in the defcription of the Crotalus Dryinas, in the Amœnitates Academicæ, he fays, "Dentes ejus duo canini uti in reliquis venenatis ferpentibus non in maxiliis hærent, iis enim vulnerando, non autem ictus infligendo utitur." "Thefe two quotations fhew, that Linnæus thought the fituation of the fangs different from that of the common teeth; the laft alfo fhews that he thought their mode of action influenced by it. What difference in fituation may be found by accurate diffection, it is foreign from my prefent purpose to enquire; I am, however, very certain that common examination will not difcover any difference, in that refpect, between the fangs of venemous ferpents and the teeth of others. "But the moft fingular opinion of Linnæus, respecting the venomous fangs, was, that they were fometimes fixed in the bafe of the jaw. Of this he has given two inftances in the Museum Regis. One in the defcription of the coluber feverus, of which he fays, "Hafta mobiles folitariæ verfus bafin maxillarum interius adhærent." The other in that of the coluber ftolatus. His words there are," Tela mobilia ad bafin maxillarum affixa, ut vix vulnerare valeat hoftes, folum cibos, veneno inficere." "Linnæus's opinion refpecting the ufe of the fangs, in the laft mentioned fpecies, appears to me not very clearly exprefled. But I have quoted both defcriptions, merely to fhew that Linnæus thought the fangs were fometimes placed in the bafe of the jaw; an idea for which I have never been able to difcover any foundation. The first of the two fpecies in queftion I have never feen; of the ftolatus I have examined several fpecimens, and am convinced it is not venomous. "I fhall not dwell any longer on the falfe notions which have been entertained, refpecting the fangs of venomous ferpents, but fhall proceed to fhew how, in my opinion, they may be most easily and most certainly, diftinguifhed from com mon teeth. "With refpect to their fize, I have already obferved that it is very various, confequently no certain judgment can, in all cafes, be made from that circumftance. In fone fpecies they are fo large, that their fize alone fufficiently diftinguishes them from common teeth; but in others they are fo fmall, that it is very difficult to difcover them. The fize of the common teeth alfo varies very much, in different fpecies. In the Coluber myfterizans they are remarkably large, efpccially thofe which are fituated near the apex of the upper jaw; which circumftance probably helped to lead Linnæus into the erroneous opinion he entertained, that this ferpent was venomous But in many fpecies the teeth are so small, that it is impoffible to difcover, merely by looking into the mouth, that the animal has any. Yet in that cafe they may be very easily detected, by drawing a pin (or any other hard fubftance) with a moderate degree of preffure, along the edge of the jaw, from the apex to the angle of the mouth, when they will be felt to grate against the pin, like the teeth of a faw. "Although the size of the venomous fangs is very various, their fituation is, I believe, always the fame; namely, in the anterior and exterior part of the upper jaw, which fituation I confider as the only one, in which venomous fangs are ever found. But as in those ferpents which are not venomous, common teeth are found in that part of the jaw, it is plain that we cannot, by fituation alone, diftinguish one from the other. They may however, be diftinguifhed with great eafe, and I believe with great certainty, by the following fimple operation. When it is difcovered that there is fomething like teeth in the forementioned part of the upper jaw, let a pin be drawn, in the manner already defcribed, from that part of the jaw to the angle of the mouth (which operation may, for greater certainty, be tried on each fide). If no more teeth are felt in that line, it may I believe be certainly concluded, that those first difcovered are what I have diftinguished by the name of fangs, and confequently, that the ferpent is a venomous. one. If, on the contrary, the teeth firft difcovered are found not to ftand alone, but to be only a part of a complete row, it may as cer tainly tainly be concluded, that the ferpent is not venomous. "In the upper jaw, both of venomous ferpents and others, befides the teeth already fpoken of, there are two interior rows; confequently, the distinction I have endeavoured to establish might be expreffed in other words, by faying, that all venomous ferpents have only two rows of teeth, in the upper jaw, and all others have four. I think it better, however, to leave the interior rows out of the queftion, as, in many fpecies, the teeth of which they are compofed are fo fmall, as to make it very difficult to difcover them. Indeed, in two fpecies of anguis, I can hardly be sure that I have difcovered them; but as, in every other species, I have never failed to do fo, I prefume I may, with very little risk of error, affert, that all ferpents whatever are furnifhed with them; and that thofe only, which are not venomous, have the exterior rows. "What I have faid fufficiently fhews that Linnæus's ideas refpećting venomous ferpents, were fuch as did not permit him to feparate them from the others; if the method I have propofed fhall be found to render the diftinction of them fufficiently clear and eafy, it natu rally follows, that they fhould be made generically diftinct. Some other reforms might alfo be made in Linnæus's clafs of amphibia, the confideration of which I do not mean, at prefent, to enter further into. But, before I conclude, I think it neceffary to notice an inaccuracy of Linnæus, of a different kind from those I have already pointed out. "In the preface to the Mufcum Regis, and in the Introduction to the clafs amphibia, in the Syftema Naturæ, Linnæus fays, that the proportion of venomous ferpents to others, is one in ten; yet, in the Syf tema Naturæ, of which the fum total in fpecies is one hundred and thirty-one, he has marked twentythree as venomous, which is fomewhat more than one in fix. How he came to be fo much at variance with himfelf, I know not; but the last mentioned proportion feems to me to be not far from the truth; as I find that I have examined one hundred and fifty-four fpecies of ferpents, of which number twenty-fix appear to be venomous. "I have already mentioned, that the coluber ftolatus and the myêterizans, though marked by Linnæus as venomous ferpents, certainly are not fo; and that I fufpect the fame may be faid of the leberis, and dipfas. I have also observed, that the boa contortrix, coluber ceraftes, and laticaudatus, none of which are marked in the Systema Naturæ, are all of them venomous; to thefe laft may be added the coluber fulvus. "If Linnæus's fpecies were all accurately examined, I have no doubt but more errors, of both kinds, would be found; for it must be obferved, that though I have ex. amined a greater number of species than Linnæus, not above half that number are of those described by him; confequently there remains more than one-third of his fpecies which I have never seen. The number I have examined, however, feems to me fufficiently great to warrant the inferences I have drawn from that examination. That fome exceptions to them might be found, by the examination of a greater number, is very poffible; but, if these obfervations fhall tend to rectify the falfe notions which have been entertained respecting venomous ferpents, and to render the diftinction between them and others more clear, I trust they will be thought not totally ufclefs." EXPE EXPERIMENTS on the CONGELATION of QUICKSILVER in England, by Mr. RICHARD WALKER, in a Letter to HENRY CAVEN DISH, Efq. F. R. S. I [From the fame Work.] Now beg leave to trouble you with the particulars of my experiments relative to the congelation of mercury; to which I fhall add an account of a few experiments, relating to the production of artificial cold, made fince my last paper was written. "Exp. 1. On December 28th laft, a favourable opportunity of fered of beginning fome experiments on the congelation of mercury, which I was defirous of effecting completely; how far I have fucceeded will appear in the fequel. "For this purpose I prepared a mixture of diluted vitriolic acid (reduced by water till its fpecific gravity was to that of water as 1,5596 to 1) and ftrong fuming nitrous acid, of each equal parts. I preferred this mixture of acid because it has been found by Mr. M'Nab, in Hudfon's Bay, to be capable of producing much greater cold, when the temperature of the materials at mixing is very low, than the nitrous acid alone; the former finking a fpirit thermometer to -54, the latter never lower than-46°. "The glafs tube of a mercurial thermometer, with its bulb half filled with mercury, was provided, this occurring to me as a convenient method of ascertaining when the mercury was congealed; for if, after being fubjected to the cold of a frigorific mixture, the thermometer glafs fhould be taken out and inverted, and the mercury found to remain completely fufpended in that half of the bulb now uppermoft, no doubt can remain of the fuccefs 1789. of the experiment; an hydrometer, with its lower bulb half an inch in diameter, and three-fourths full of mercury, was likewife provided, in cafe any accident should happen to the other. "It may be proper to premise here, that in all experiments of this kind I remove each veffel, · when the liquor it contains is fuffi ciently cooled, out of the mixture in which it is immerfed for that purpose, immediately previous to adding the fnow or falts with intention to generate a ftill further increafe of cold; and likewife prefer adding the fnow or powdered falts to the liquor, inftead of pouring the liquor upon thefe: it is neceffary alfo to ftir about the fnow or falts, whilft cooling in a frigorific mixture, from time to time, otherwife it will freeze into a hard mass, and frustrate the experiment. "A half-pint glafs tumbler, containing two ounces and a half of the above-mentioned diluted mixture of acids, being immerfed in mixtures of nitrous acid and fuow, untill the liquor it contained was cooled to30, was removed out of the mixture and placed upon a table; fnow, likewife previously cooled in a frigorific mixture to-15°, was added by degrees to the liquor in the tumbler, and the mixture kept stirring, until a mercurial thermometer funk to-60°, where it remained stationary; the hydrometer was then immerfed in the mixture (the thermometer glafs having been broken in the course of the experiment), and ftirred about in it for a fhort time, |