55 



tarn all the nitrous acid (if there was any in the cap. mort.) under the form 

 of calcarious nitre, but not much of the calc. arsen. salt ; but in order to 

 free the calc. nitre more effectually from a little of the calc. arsen. salt, 

 which still remained suspended in the water, the liquor was evap. to dry- 

 ness, & the solid contents washed with water in order to dissolve all that 

 was soluble : these washings were found by the addition of a little f. alk. 

 and evaporation to contain very little, if any, nitrous salt. 



It appears from this experiment that this cap. mort. is the pure arsen. 

 acid, (or the substance, which when united to f. alk. forms the neut. arsen. 

 salt), without any sensible mixture of the nitrous acid." 



It also seems to possess all the properties of an acid (unless perhaps it 

 should fail in respect to taste, which I have not thought proper to try), since 

 it effervesces with & neutralizes the fixed & volatile alcalies, & calcarious 

 earths & magnesia, turns syrup of violets red, & also unites to the earth 

 of alum, which last the sedative salt & sulphur (substances which possess 

 some of the properties of acids, but not all) are not able to do. 



The excess of the weight of the cap. mort. above that of the arsen. it was 

 made from must be owing, I suppose, to its retaining some of the water of 

 the aq. fort, used in making it ; for the following exper. shews that there is 

 none of the nitrous acid enters into the composition of the arsen. acid, as it 

 shews a way of making the neut. arsen^ salt without any thing which con- 

 tains the nitrous acid. 



3*0-0 of arsen. was mixed with 3"1'13 of pearl ashes dissolved in water, so 

 that there was, as well as I can guess, about -rV part more of alk. in pro- 

 portion to the arsen. than in the neut. arsen. salt : this was boiled till the 

 arsen. dissolved, & then evap. to dryness : some of this mixture was 

 pounded fine & calcined over charcoal in a broad shallow earthen pan, care 

 being taken to keep it frequently stirred : the heat was as great as the matter 

 could bear without caking together. Some of it was taken out now & 

 then, & dissolved in water with solut. silver : the colour of the precipitate 

 formed thereby changed gradually, the more the matter was calcined, from a 

 pale yellow, which it was of at 1st, to a purplish-red, the same as that 

 made by neut. arsen. salt : it was then taken off the fire & dissolved in 

 water : as the f. alk. bore too great a proportion to the arsen. some spt of 

 salt was dropt into it, till it began to efferv. with f. alk. : soon after the spt 

 of salt had been added, it grew muddy, & a small quantity of white 

 sediment fell to the bottom, which seemed to be arsen. ; it was then evap. : 

 there 1st shot some crystals resembling neut. arsen. salt, & afterwards 

 some crystals of sal. Sylvii : some of the crystals resembling neut. arsen. 

 salt were dissolved in water ; the solut. efFerv'd with whiting & f. alk., 

 reddend the colour of blue paper, made the same coloured precip. with solut. 

 silver, & blue vitr., as the neut. arsen. salt ; in a word, I could perceive 

 no difference between that & the neut. arsen. salt made in the common 

 manner. 



P. 16. I think these experiments shew pretty plainly that the only dif- 

 ference between plain arsenic & the arsen. acid is that the latter is more 

 thoroughly deprived of its phlogiston than the former : for all the ways I 

 know of making arsen. acid or neut. arsen. salt are such as may reasonably 



