On Tests for Arsenic. ^33 



instantly discover in what state the arsenical salt exists^ 

 Whether it be an arsenile or arsenate : with copper the hr.t 

 i. of a yellowish gr.en, and the arsenate is a bluish green j 

 but with silver, the arsenile is of a bright yellow, and the 

 arsenate of a hkk red. In describing these appearaiices of 

 Ihe 5/7z;£'r-test, Dr. R. has copied me nearly verbaiinr . 



So little indeed do these tests differ in the mode ot apply- 

 in^ ihem, and in their habitudes with alkalies and arsenic, 

 thlt I might have contented myself when 1 first announced 

 the 5iW-test, bv merely recommending it to the notice ot 

 your numerous readers, to be substituted for that ot copper, 

 as beins more elieible. . 



It is also evident, that the ^i«^ of alkali was with mease- 

 condary object in the experiments with which I commenced 

 the enquiry; for, in the letter already quoted I have said 

 -thou'rh my process answer very well with potass, or 

 even with lime-water, I^am inclined to pretet.lhe common 

 Eub-carbohate of soda." . ; » t ^ j^ 



I must insist, that the name due to^the test IS to be de- 

 rived from the metallic salt I employ, that is, ^ny soluble 

 salt of which silver is the base ; and not from the akali or 

 alkaline earth, more than from the acul or fluid, all ot whicli 

 the operator may have his choice. In this particular pomt. 

 Dr. Roaet has not done me justice ; he might have quoted 

 niv test", in the way several rcspeciable authors have done, as 

 the " Nitrate of Silver ;" he might have copied Dr. Henry at 

 least as uell as Mr. Thomson and some others, and not 

 mock his friend by ascribing to him a new tof when, ake 

 it in the niosl favourable light, it can only ht A mod, feat ion. 

 It is sin.rulaf that Dr. Roget did not act by me as he has 

 done in minting Dr. Bostock.-Hcl-e he mentions the sul- 

 phate of copper, but leaves the reader to guess at the alkali 

 which this able philosopher employed. 



Dr Rogel has told you that '• it was onlv at the moment 

 of sending his paper to the press that Dr Marcet met with 

 the quotanon in Dr. Henry's Elements ot Chemistry, which 

 led Dr. H. to read my letters; and, you are likewise in- 

 formed, that Dr. Roget did no more than quote the proper 



authorities. , ,. i t-^ n » ^ ,,» 



Now, in all these documents, where did Dr. Roget find 

 that mV process "appears less convenient in its practical 

 application" than any ether? That my test might prove so 

 in some hands is not to be doubted ; but it was incumbent 



• Medic, and Phys. Journ. 1810, 



Vol. 40. No. 176. Dec. 1812. Ec Of. 



