54 



from the solut. on saturating it with f. alk., & the saturated solution" yielded 

 on evaporation crystals of nitre, mixed with other crystals of a different shape, 

 which proved to be neut. arsen. salt. The success of this exp. induced me 

 to try whether by dissolving arsen. in aq. fort. & driving off the acid by heat I 

 could not procure the arsen. which had suffered the above-mentioned change 

 (or the arsenical acid, if you will allow me to call it by that name) by itself; 

 the success of which was as follows : 



I'O-O of arsen. was put into a flor. flask with 4'15'15 of purified aq. fort., 

 another flor. fl. with a hole in the bottom being inverted into the neck of the 

 1st, & luted to it : with the assistance of heat it efferved & discharged 

 fumes, which filled both flasks & in part escaped at the hole at top ; part of 

 the fumes seemed to condense in the upper flask, & run down in the form 

 of a greenish liquor ; after having been kept over the lamp some time it was 

 taken off : the arsen. was not near dissolved ; the liquor whilst hot was of a 

 reddish-yellow, like the fumes which were discharged from it, but when cold 

 changed to a deep bluish-green, but which went entirely off in 2 or 3 days : 

 in all probability it proceeded at 1st only from some of the red fumes which 

 were condensed in the flasks & fell down into the liquor : the remainder of 

 this ounce of arsen., & also 3 oz. more were afterwards dissolved in it : the 

 aq. fort, seemed as if it wd have dissolved still more : the solut. was quite 

 clear and transparent, & did not let fall the least sediment as is usual in 

 solutions of metallic substances in this acid : the mixture was found to have 

 lost 16'0 by evap. during this process ; this solut. was put into a retort & 

 distilled to dryness in a sand heat ; it did not require much heat to do it : at 

 the end of the operation almost as great a heat was given it as the furnace 

 would admit of: no arsen. sublimed ; no red fumes nor volatile vapour rose 

 during the distillation ; some of the distilled liquor was saturated with f. 

 alk., it seemed to contain little or no arsen., as it made no sensible precip. 

 with blue vitr. or solution of silver or mercury. 



P. 12. The caput mortuum remaining in the retort after the distillation 

 weighed 4* 13*6, id est, about i part more than arsen., from which it was 

 made : it attracted the moisture of the air, though but slowly ; it requires very 

 little water to dissolve it, I beheve scarcely more than -^ its weight ; but it 

 does not dissolve fast without the assistance of heat. 



Into a solut. of this cap. mort. was dropt some f. alk. ; it made a strong 

 efferv. ; more f. alk. was dropt in till the efferv. was almost, but not quite, 

 ceased : on evap. it furnished crystals which differed in no respect from the 

 neut. arsen. salt made in the common manner, 



The following experiment was made to see whether this caput mort. con- 

 tained any of the nitrous acid used in the making of it. 



The arsen. acid, as I found by an exper. which will be mentioned after- 

 wards, when thoroughly saturated with a calcarious earth, forms a substance 

 which is insoluble in water, & which I beg leave to call calcarious arsenical 

 salt : some of this caput mortuum therefore was dissolved in a good deal of 

 water & saturated with whiting : it made a great efferv., & the calc. arsen. 

 salt fell in flakes to the bottom : after having stood some time, the clear 

 liquor was strained from the insoluble part : this liquor it is plain must con- 



