Copper may be substituted for Iron, and procures the same advantages.
It is very necessary to remove the crucible from the fire as soon as the matter is melted, and indeed to cool it as expeditiously as possible, to prevent the Arsenic from flying off in vapours: for, when once the Regulus is formed, the proportion of Arsenic, with respect to that of the metal mixed with it, is continually lessening while it stays in the fire; so that, after some time, there will be left in the crucible, not a Regulus of Arsenic, but only Iron or Copper, alloyed with a little Arsenic. On this occasion the Copper turns white, and assumes the colour of Silver; but it soon tarnishes in the air.
It is easy to perceive, by what hath been said, that the Regulus of Arsenic made according to this process is never pure, but contains always a considerable quantity of Iron or Copper, whatever precautions be used: but it is difficult to avoid this inconvenience, for the reasons above assigned; and if we attempt to fuse Arsenic alone, with reducing fluxes, the greatest part thereof is dissipated in vapours, long before the very flux begins to melt: and that part of it, which is found metallized, is not collected in one mass at the bottom of the crucible, as in other metallic reductions; but in small particles, dispersed and mixed among the scoriæ. There are nevertheless several expedients for obtaining a Regulus of Arsenic absolutely pure, and unalloyed with any metallic substance.
First: into a little low cucurbit, covered with a blind-head, put Regulus of Arsenic made with Iron or Copper; set this cucurbit in a sand-bath; heat it till the sand begins to grow red, and you will see part of the Regulus sublime into the head, still retaining its metalline splendour. The portion of Regulus thus sublimed is pure Arsenic, or at least contains but a very small portion of the adventitious metal, which may have been carried up with it. What is left in the bottom of the cucurbit is the metal that was added, still containing a little Arsenic, which continues obstinately fixed with it, and which the violence of fire is unable to force away from it in close vessels.
Secondly: mix your Arsenic in equal parts with the black flux; put the mixture into such a cucurbit as that last mentioned; and apply to it the strongest degree of heat that can be procured by a sand-bath; arsenical flowers, of a blackish grey colour, will first sublime into the head, and after them a Regulus of Arsenic of a white metalline colour, which is pretty glossy, but tarnishes very soon in the air. This Regulus hath no solidity: it is exceedingly friable; but it is pure.
Thirdly: I have also made a Regulus of pure Arsenic by another method, which produces a much greater quantity thereof, with a much smaller degree of heat. For this purpose I powder the Arsenic, and mix with it any Fat Oil; so that the mixture may be like a liquid paste: this paste I put into a little phial of thin glass, like one of those used by apothecaries; I set this phial in a sand-bath, and gradually heat it, till the bottom of the pot containing the sand begin to be red. Part of the Oil first rises out of the phial in vapours, which must be suffered to pass off. After this the upper part of the phial is gradually lined, on the inside, with a glittering metallic crust, which makes it look like a quick-silvered glass. This crust is the Regulus of Arsenic. When it begins to sublime, the mouth of the phial must be slightly stopped with a bit of paper, and the heat increased a little, till you see that nothing more rises.
If you break the bottle after the operation, you will find its upper part crusted over with a coat of Regulus, thicker or thinner in proportion to the quantity of Arsenic employed. The Regulus is in a mass, of a beautiful brilliant colour, which to me seems to stand the air better than that of any Regulus made by other methods; probably because of the great quantity of fat matter with which it is united, and by which it is defended.
This Regulus of Arsenic is absolutely pure, and a much greater quantity thereof is obtained, by this method, than by treating it with the black flux; because the Arsenic is much sooner and more easily combined with the inflammable matter: and hence it comes to pass that part of the Arsenic doth not rise at first in grey flowers, as in operating with the black flux. Moreover, by our process, all the Arsenic is sublimed in Regulus: whereas, when the black flux is employed, a pretty considerable part of the Arsenic unites with the alkaline part of the flux, and remains fixed therewith. In our operation there is nothing left at the bottom of the phial, except an oily, light, but very fixed coal.
Regulus of Arsenic, in whatever manner made, may be easily reduced into white, crystalline Arsenic, by the means of a Fixed Alkali, or of Mercury, applied in the same manner as for separating Arsenic from Sulphur.