peared, and by its own evidence it triumphed over both. These are facts worthy of the attention of philofophers, as fuch. To quit this fubject for that with which I began this preface, and in which I fhall be more attended to by philofophers in general, I would caution my reader not to be too fanguine in his expectations from the happy train which this branch of philofophy feems to be in. Confidering the unexampled rapidity with which difcoveries have hitherto been made in it, the number of persons in many and diftant countries now engaged in these pursuits, and the emulation that is neceffarily excited in fuch circumftances; and confidering, at the fame time, how nearly this fubject is allied to the most general and comprehensive laws of nature with which we are acquainted; fome may be apt to imagine, that every year must produce difcoveries equal to all that were made by a Newton or a Boyle; and I am far from faying that this may not be the cafe, or that it is very improbable. But, But, though I have little doubt, from the train that things are visibly in, that philofophical discoveries in general will go on with an accelerated progress (as indeed they have done ever fince the revival of letters in Europe) it would be too rafh to infer, from any prefent flattering appearances, that any particular expedition into the undiscovered regions of fcience will be crowned with more diftinguished fuccefs than another. Nothing is more common, in the hiftory of all the branches of experimental philofophy, than the most unexpected revolutions of good or bad fuccefs. In general, indeed, when numbers of ingenious men apply themselves to one fubject, that has been well opened, the investigation proceeds happily and equably. But, as in the hiftory of electricity, and now in the discoveries relating to air, light has burft out from the moft unexpected quarters, in confequence of which the greatest masters of science have been obliged to recommence their studies, from new and fimpler elements; fo it is alfo not uncommon for a branch of fci ence to receive a check, even in the most rapid and promising state of its growth. Judging, however, from my prefent views of the fubject, I am willing to hope that the beginning of this preface will not contribute to raise too high expectations. The incomplete experiments, indicated in the courfe of both thefe volumes, and efpecially in the fecond, will, alone, furnish matter for, at least, as much experimental investigation as all that I have yet gone through; and I need not tell the real philofopher, that many of them are of fuch a nature, as promise to reward the fagacious experimenter with the most important discoveries, as they evidently border upon, and may lead to, much greater things than any that I have hitherto in.veftigated; and my hints for other new experiments, which I have not thought it worth while to trouble the reader with at this time, are more than I have ever had before me fince I began these inquiries. From this I think I may reasonably infer, that the fubject is fo far from being exhaufted, that the moft that can be faid of it is, that it is pretty well opened, fo as to exhibit an inviting prospect to future investigation. To accelerate this investigation, I have hitherto made the most early publication of my obfervations, and have concealed from no perfon whatever any thing that has occurred to me; and though this conduct has expofed me to fome inconvenience, I am not yet difcouraged; but, whoever may avail themselves of it, I fhall, for fome time longer, at least, and I hope through life, perfift in the fame habit of the most open and unreserved communication, private and public. I have not in this volume, as in the former, a section of conjectures, fpeculations, and hints, because I have not yet fufficiently reflected upon the facts that fuggeft them. The facts, however, will furnish abundant matter to those who are difpofed to fpeculate, and efpecially on the fubject of the mutual convertibility, and ultimate identity, of all the acids when combined with fubftances in the form of |