of Arsenic from other Substances. 235 



one grain of arsenic in twenty-eight thousand grains of water 

 (or four imperial pints), and have obtained therefrom, upwards 

 of one hundred distinct metallic arsenical crusts. 



Similar results have been obtained with perfect success from 

 three pints of very thick soup, the same quantity of port wine, 

 porter, gruel, tea, coffee, &c. &c. 



It must, however, be understood, that the process was allowed 

 to proceed but slowly, and that it required several days before 

 the mixture used ceased to give indication of the presence of ar- 

 senic, and, also, a much larger portion of zinc and sulphuric 

 acid was employed from time to time, than when working Avith 

 the small bent tube apparatus, in consequence of the large quan- 

 tity of matter operated on under this arrangement. 



With the small apparatus, I have obtained distinct metallic 

 crusts, when operating on so small a quantity as one drop of 

 Fowler's solution of arsenic, which only contains one-120th part 

 of a grain. 



The presence of arsenic in artificial orpiment and realgar, in 

 Scheele's green, and in the sulphuret of antimony, may be rea- 

 dily shewn by this process, when not more than half a grain of 

 any of those compounds is employed. 



In conclusion, I beg to remark, that although the instruments 

 I have now finished describing, are the form I prefer to all that 

 I have employed, yet it must be perfectly evident to any one, 

 that many very simple arrangements might be contrived. In- 

 deed, I may say unequivocally, that there is no town or village 

 in which sulphuric acid and zinc can be obtained, but every 

 house would furnish to the ingenious experimentalist ample 

 means for his purpose ; for, a two-ounce phial, with a cork and 

 piece of tobacco-pipe, or a bladder, with the same arrangement 

 fixed to its mouth, might, in cases of extreme necessity, be em- 

 ployed with success, as I have repeatedly done for this purpose. 



The only ambiguity that can possibly arise in the mode of 

 operating above described, arises from the circumstance, that 

 some samples of the zinc of commerce themselves contain arse- 

 nic ; and such, when acted on by dilute sulphuric acid, gave out 

 arsenuretted hydrogen. It is, therefore, necessary for the ope- 

 rator to be certain of the purity of the zinc which he employs, 

 and this is easily done by putting a bit of it into the apparatus. 



