_To give the Reader an account, Why the following Treatise is suffer'd to pass abroad so maim'd and imperfect, I must inform him that 'tis now long since, that to gratify an ingenious Gentleman, I set down some of the Reasons that kept me from fully acquiescing either in the Peripatetical, or in the Chymical Doctrine, of the Material Principles of mixt Bodies. This Discourse some years after falling into the hands of some Learned men, had the good luck to be so favourably receiv'd, and advantageously spoken of by them, that having had more then ordinary Invitations given me to make it publick, I thought fit to review it, that I might retrench some things that seem'd not so fit to be shewn to every Reader, And substitute some of those other things that occurr'd to me of the trials and observations I had since made. What became of my papers, I elsewhere mention in a Preface where I complain of it: But since I writ That, I found many sheets that belong'd to the subjects I am now about to discourse of. Wherefore seeing that I had then in my hands as much of the first Dialogue as was requisite to state the Case, and serve for an Introduction as well to the conference betwixt_ Carneades _and_ Eleutherius, _as to some other Dialogues, which for certain reasons are not now herewith publish'd, I resolv'd to supply, as well as I could, the Contents of a Paper belonging to the second of the following Discourses, which I could not possibly retrive, though it were the chief of them all. And having once more try'd the Opinion of Friends, but not of the same, about this imperfect work, I found it such, that I was content in complyance with their Desires; that not only it should be publish'd, but that it should be publish'd as soon as conveniently might be. I had indeed all along the Dialogues spoken of my self, as of a third Person; For, they containing Discourses which were among the first Treatises that I ventur'd long ago to write of matters Philosophical, I had reason to desire, with the Painter, to_ latere pone tabulam, _and hear what men would say of them, before I own'd my self to be their Author. But besides that now I find, 'tis not unknown to many who it is that writ them, I am made to believe that 'tis not inexpedient, they should be known to come from a Person not altogether a stranger to Chymical Affairs. And I made the lesse scruple to let them come abroad uncompleated, partly, because my affairs and Prae-ingagements to publish divers other Treatises allow'd me small hopes of being able in a great while to compleat these Dialogues. And partly, because I am not unapt to think, that they may come abroad seasonably enough, though not for the Authors reputation, yet for other purposes. For I observe, that of late Chymistry begins, as indeed it deserves, to be cultivated by Learned Men who before despis'd it; and to be pretended to by many who never cultivated it, that they may be thought not to ignore it: Whence it is come to passe,