As concerning this, of which, though I made no mention in the Preface; for it was not my Resolution to mention it in the last Part, because I was purposed only to treat of the Instruments, as well earthen, as those of glass, and also of the other necessary things belonging to those four parts premised; yet I am willing now in this Part, (which I have judged to be the most convenient place for it, for which I did before design another) to discover the wonderful Nature thereof, as far as I may for the Studious Artist’s sake. And although I know that more in this part, than in all my other writings, especially the ignorant and unskilful, will be offended; yet I will not therefore pass it by, perswading my self, that by this means I shall do a work, that will be most acceptable to the searchers of Art, and Nature. For I do devoutly affirm, That this is the choicest of all my secrets that I confide in, in which I have already seen wonderful things, hoping that the Divine Benediction will some time or other be obtained upon the practice thereof. And as for the structure of it, much cannot be said thereof, because it is not built as other Furnaces are, but it is every where found extructed by Nature, being ordained for no other works, than those of Nature, _viz._ for the making of any _menstruum_ that shall dissolve gold, silver, and all other metals, and minerals without any noise, as also pretious, and common stones, and also glasses: the original of which, is the original of the _Menstruum_. Now what, and what manner of Furnace that is, that produceth this Royal _Menstruum_, (coming from the _Menstruum_ it self) and that easily without any labour, you may easily conjecture, that it is not any common one, by the help whereof other things are distilled, that can yield such a _Menstruum_ that is not corrosive: which certainly is not any common _Menstruum_, because there is but this one _Menstruum_ that I know, which doth not partake of any corrosive quality, that doth more than any, or all other corrosive waters whatsoever. For all corrosives whatsoever they are, as _aqua fortis_, _aqua regia_, spirit of salt, vitrioll, allome, and nitre cannot together, and at once dissolve the close union of gold, and silver, and other most hard subjects, that cannot be dissolved in waters, though never so caustick.