56 



be supposed to deprive the arsen. of its phlogist. : as for example, in making 

 arsen. acid by solution in aq. fort., the nitrous acid is known to have a great 

 disposition to lay hold of phlogiston, & there are strong reasons for 

 thinking that the dissolving of metallic substances in that acid is a very 

 powerful method of depriving them of it, as I shall take notice of by and by. 

 It seems likely too, that the nitrous acid may have the same effect in calcining 

 arsen. & nitre together as in the common way of making neut. arsen. salt ; 

 but the above-mentioned way of making neut. arsen. salt by calcining the 

 simple combination of arsen. and f. alk. is a more especial example hereof, 

 since the natural effect of exposing metallic substances at the same time to 

 heat & the open air is to deprive them of their phlogiston : the last exper. 

 too shews that the presence of the open air is almost, if not quite, necessary 

 to produce this change, since the mixture seems in that exper. to have suffered 

 but a small part of the change necessary to turn it into neut. arsen. salt, 

 though exposed both to a greater heat, & for a longer time, than in the 

 former exper.: perhaps too, if the vessel had been more perfectly closed, it 

 wd have suffered still less change. 



P. 17. The nature of the difference between the arsen. acid & plain 

 arsen. in some measure favours this opinion, since arsen. acid differs from 

 plain arsen. much in the same manner as that does from the regulus of 

 arsenic ; for the regulus of arsen. is indissoluble in water, & has no affinity 

 to f. alk. ; white arsenic is in some measure dissoluble in water, & has a 

 very evident affinity to f. alk., thereby manifesting something of an acid 

 property : the arsen. acid is much more dissoluble in water than white 

 arsen., has a strong affinity to f. alk., & seems in all respects a real acid. 



If these arguments should seem too hypothetical, the following will most 

 likely be allowed to be satisfactory, namely, that the arsen. acid is easily 

 reduced into regulus by subliming it with inflammable substances. A small 

 quantity of arsen. acid was put into an apothecary's vial with about ^ its 

 weight of linseed oil ; it grew soft, mixed uniformly with the oil, & sublimed 

 in the form of regulus, with a less heat than sufficient to make the glass red 

 hot. 



The red fumes which issue in the distillation of the neut. arsen. salt & in 

 the dissolution of arsen. in aq. fort. (& consequently the blue aqua fortis, 

 which is only these fumes condensed), can be nothing else, I imagine, than 

 the nitrous acid combined with & volatilized by the phlogiston of the 

 arsen., though I am quite ignorant why they should differ so much both in 

 colour & their greater degree of volatility from the same acid impregnated 

 with phlogiston by dissolving other metallic substances in it. As it appears 

 from a former experiment that little or no arsen. is elevated in drawing off 

 the nitrous acid from a solution of arsenic in aq. fort., & as the arsen. acid 

 made by that means so much exceeds in weight the arsen. it was made from, 

 it does not seem likely that these fumes contain any arsen. 



P. 19. Though what I am going to say has not much relation to the 

 present subject, I will beg leave to offer a few conjectures concerning the 

 solution of metals in acids. It is remarkable, that though in general the 

 nitrous acid has the least affinity to metallic substances of any acid, yet it 

 dissolves them with the greatest ease of any : this has been with great reason 



