of Polypodium. When the artificial character is not fo determinate as to affociate in every inftance with its deftined family, tafte and addrefs are required to reconcile the jarring difference. But here the partial botanist must ever be at a lofs, for he cannot be aware of the various difficulties which the more general obferver has to deal with. The Agaricus quercinus muft appear, to a young ftudent, rather a Boletus than an Agaricus; yet the beft botanists clafs it with Linnæus. So alfo the Acroftichum feptentrionale has its fructification fometimes in fhort lines, but with them dots are obfervable, and in the end, all are confluent. Which then fhall determine the character? Shall the lines fay that it is an Afplenium, or the dots a Polypodium, or their confluency an Acroftichum? It is magnifying difficulties to start fuch objections; therefore we muft caution the reader when he reads the following fentence: See page viii. It must offend the tafte of the judicious reader to find the characteristics of the genera fixed on foundations fo unfteady, when he finds plants very diffimilar in their appearance united; as alfo when he fees a feparation take place between Ofmunda lunaria and Ophiogloffum vulgatum, between Acroftichum feptentrionale and Afplenium ruta-muraria.' We hope that every reader will not have the exquifite feelings of Mr. Bolton. The union of plants apparently different muft conftantly occur in an artificial fyftem; whether in one clafs, as in the Didynamia, or in the graffes, or in fingle families, as in Trifolium, and various other Diadelphia plants. Why should the feparation of Ophiogloffum and Ofmunda be lamented, when the fimplicity of the one, and the ramification of the other, keep them wide apart? Neither do we fee the neceflary union between the Acroftichum feptentrionale, and the Afplenium ruta-muraria. The dots of the one, and the lines of the other, are a fufficient diftinction. To speak plainly, this dabbling in amendments is an actual accumulation of difficulty and diftrefs. Why is every author to be inventing new arrangements, who may not be of confequence to be allowed fuch indulgence? We grant that Mr. Bolton's remarks on the Linnæan claffification are tokens of his clofe obfervation, and we hope his end is anfwered in their being thus noticed. Being a provincial publication, it is not to be fuppofed that it was able to reach to the highest authority; otherwife, fome of Mr. Hudson's, as well as his own, errors about the English 1pecies The Polypodium fragrans and Rhæticum are not English species. Here alfo we may remark, that Mr. B. is deceived about the Trichomanes pyxidiferum. His plant is in all probability what he imagines it to be, a variety of T. tunbrigenfe; the writer of this article K 2 has fpecies would not have been detailed. Perhaps, the time may come, when a British FLORA, of real authority, the authority of the Linnæan herbarium, may put away all doubt concerning the species which are actually natives of this ifle. Still the labours of a Bolton will be always ufeful and valuable; but does it, in any degree, take from his merit to fay, that the laft hand must be put to the work by directions ex cathedra? Gud-h. ART. VI. Experiments and Obfervations relating to various Branches of Natural Philofophy; with a Continuation of the Obfervations on Air. By Jofeph Priestley, LL. D. F. R. S. &c. &c. Vol. III. Svo. 6s. Boards. Johnfon. 1786. TH HOUGH the title-page calls this the third volume, the preface begins with mentioning five preceding ones, which, to a reader not very converfant in Dr. Priestley's experimental writings, may carry an appearance of fome inaccuracy. The cafe is, that three volumes were published under the title of Experiments on AIR; but the field of enquiry becoming then more enlarged, the Author, it feems, thought it neceffary to enlarge the titles alfo, and begin a new feries of volumes, fo that what is called the fourth volume of Experiments on air is the firft of the volumes under the prefent title, and the prefent is the fixth of the whole work. This laft mode of numeration appears to us the most eligible, as the other may occafion ambiguities, both in purchafing and making references. The preface ends with a real inaccuracy refpecting weights: A pennyweight (we are told) is twenty-four grains, or a twelfth part of a pound troy.' This explication was written for the fake of foreigners, and to them it may occafion fome difficulty; but the English cannot be at a lofs for fupplying a few words, which by fome accident have been omitted: a pennyweight is the twentieth part of an ounce troy, and this ounce is the twelfth part of a pound *. The Doctor avails himself of the opportunity which this preface affords him, to apologife to his philofophical friends, who complain of his throwing away fo much of his time in religious writings: he affures them, that the whole fix volumes of the Hiftory of the Corruptions of Chriflianity, and of The Opinions conhas feen fpecimens of T. pyxidiferum in complete fructification, which were gathered near the fpot mentioned by Dr. Richardfon. But Mr. B. fpeaking of his plant, fays, Seed veffels, none ever discovered upon this phænomenon of a plant.' Page 57. *The above account was written foon after the publication of the work. In fome copies, which we have lately feen, the error is corrected, by striking out the words or a twelfth part of a pound troy ; but as many copies are in the hands of the Public without correction, we have thought proper ftill to take this notice of it. cerning cerning Chrift, with all the controverfial pieces written in defence of the former, did not take up near fo much of his time as the experiments, of which an account is given in this fingle volume: -that during the compofition of thofe works, the greatest part of every day was spent in his elaboratory, and only the evenings and mornings in reading or writing;-and that the different ftudies relieve one another, fo that he can do more in each of them, than if he gave his whole attention to one only. But he refts his principal defence upon the dignity and importance of theological ftudies, fuperior to every other, inafmuch as it is of infinitely more confequence to fecure a happy lot in a future endless life, than to make the beft poffible provifion for the prefent. This leads him to ftate the grounds on which his belief of a future life is built, and to expoftulate with thofe philofophers, who reject the miracles and refurrection of Christ, in order to believe what is infinitely lefs credible, that Christianity could have been propagated, as we know it was, and that things could be as we now find them, without the most convincing evidence of the truth of the gospel history. The three firft fections of this work contain the Author's late communications to the Royal Society, on phlogiston; -on the feeming converfion of water into air;-and on air and water; which have been published in the Philofophical Tranfactions, and of which an account has already been given in our Review *. These make somewhat more than a fourth part of the volume. The reft confifts chiefly of experiments and obfervations fupplemental to the preceding, and to different articles in his former publications; fome of them tending to confirm the doctrines already advanced, and others creating new difficulties. Of these experiments, to ufe his own words, fome are of more, and others of lefs importance, and fome perhaps of little confequence. The volume clofes with obfervations relating to a general theory of the experiments in which the different kinds of air are concerned. In this, we think, the Author has not been very happy; he does not seem to have communicated much new light, nor to have availed himself fully of what had already been ftruck out by others. He fuppofes air to confift of nitrous acid and phlogifton, or of dephlogisticated nitrous vapour and phlogifton (p. 407.), fo that nitrous acid is one of the component parts of nitrous air (p. 296.); and this he calls the established hypothefts. We believe the established and most probable hypothefis to be, that the nitrous and other analogous airs are component parts of the refpective acids; that nitrous acid, for inftance, confifts of nitrous air and pure air; that into these two See Monthly Review, vol. lxxi. p. 112. and vol. lxxiv. (for May laft) p. 323. principles the acid is refoluble; and that from the re-union of thefe two it is reproducible. This hypothefis, first advanced by Lavoifier, affords an obvious and fatisfactory interpretation of many of the Doctor's experiments, both in this and the preceding volumes; and we think a little attention to it might have produced not only an improvement in the theoretical, but fome valuable additions in the experimental parts of the prefent publication. Thus, when we find a large quantity of pure air to be forced out, by fire, from nitre, or from allum, or from vitriolic acid and clay (which produce allum in the process), or from turbith mineral (which is a compound of vitriolic acid and mercury), we fhould naturally fay, that the acids had fuffered a decompofition, that the pure air being thus difengaged from them and collected by itself, the other component principle remained united with the fixed alcaline bafis of the nitre, the earthy one of the allum, and the metallic of the turbith; and if we can thus obtain combinations of the pure characteristic principle of the feveral acids with different fixed bodies, a chemical experimenter cannot be at a lofs how to avail himself of fuch compounds. It is known that the vitriolic acid principle, remaining in fome compounds of this kind, and poffeffing no fenfible character of acidity, will imbibe from the atmosphere the pure air which it wants, and there with produce vitriolic acid again: it is furely worth trial whether the nitrous refiduum will do the fame; and if fo, an examination of that refiduum may lead to difcoveries of real importance. We fhall juft mention a circumftance which occurs to us at the moment, that the common alcaline falts, when a bit of coal falls into the crucible during their fufion, become fimilar in appearance to the nitrous refiduum in queftion. Though we cannot much approve of the Doctor's theory, particularly refpe&ting airs and acids, we give him due praise for the account of phlogifton, which concludes the section on theory. He ftates, clearly and juftly, the opinions of Kirwan and Lavoifier, and establishes the former, that metals contain phlogifton; that in a certain degree of heat they imbibe pure air; that this air unites with their phlogifton, and forms fixed air (fo that the calx is a compound of the metal with fixed air), and that in a greater heat this fixed air is decompofed, the pure air being fet loofe, and leaving the metal, with its phlogifton, revived. This last circunftance indeed takes place only in a few metals, the others requiring an additional fubftance for their revival; which additional fubftance may be communicated by any combuftible body, and correfponds exactly to Stahl's phlogifton, the doctrine of which is confirmed by thefe experiments; that is, we muft ftill fay, that in all combuftible fubftances there is a principle capable of being transferred to other fubЯances, fubftances, which, when united to the calces of metals, makes them to be metals, and which, united to oil of vitriol (deprived of its water), makes it to be fulphur.' It will occur to the reader that this laft circumftance is rather inaccurately expreffed; for it does not appear to be vitriolic acid itself, but the principle above mentioned, which forms the acid when combined with pure air, that exifts in fulphur. Our limits will not admit of our entering into a detail even of the fubjects treated, much. lefs of the particular experiments; but we fhall mention fome of thofe, of which we can exprefs the general refults in the fmalleft compafs. Inflammable air was obtained in large quantity, from fpirit of wine, ether, and oils, by paffing them in fteam, through redhot earthen tubes; and from dry inflammable substances, by tranfmitting fteam of water over them when heated. The fteam of fpirit of wine, paffed over copper juft melted, yields inflammable air as it does in other circumftances, and converts the copper into a fubftance called metallic charcoal, weighing near twenty times as much as the metal itself did. The minute divifion and volatility of this charcoal are remarkable; for though a large glafs balloon with feveral adopters, were used for collecting it, part efcaped in fmoke, and all the veffels were lined with an uniform footy black. By the heat of a burning-glafs, in common ait, it melted, without undergoing any further change; but in dephlogisticated air, it burned rapidly, and converted almoft the whole of that air into fixed air. By paffing fteam of water over it, in red-hot earthen tubes, it yielded inflammable air as common charcoal does, was diminished one half in weight, and became lighter coloured. Silver underwent a change nearly fimilar to copper, but gold was not affected by the procefs. It was therefore natural to try, whether copper could not on this principle be feparated from gold? the trial was not indeed fuccefstul, the copper being protected by the gold from the action of the fteam; but this was probably owing to the copper being in too fmall proportion, only one tenth of the gold; whereas, in order to feparate filver from gold, by aqua fortis, it is found necefiary to put two or three parts of filver to one of gold. There are many experiments of the quantities of air extracted from mineral bodies by means of heat, with a view to ascertain the productions that are truly volcanic, for thefe may be prefumed to have parted with their air in the fufion they underwent from fubterraneous fire. The known lavas yielded lefs air than other mineral fubftances, and on this ground, bafaltes can hardly be claffed among volcanic productions, as it yields more air than any other known lava. We muft obferve, however, that on this K 4 |