of Copper and of Iron. 145 



To the ufe of alcohol, in order to get rid of the exccfs of 

 acid, as mentioned above, there is not the fame objeftion 

 that there might be to evaporation, or to an alkali : it can 

 combine with that acid only which is free; and an excefs of 

 it can, in no way, affeft the metallic falls. 



I have given the preference to lead, above every other me- 

 thod of combining arfenic, to determine its quantity in any 

 other bodv, having found arfeniate of lime, which has 

 been hitherto recommended, as well as all other earthy arfe- 

 niates, to be nearly as foluble in water as fulphate of lime. 

 Lead prefented alfo much facility as to the proportions of its 

 arfeniates ; and a few experiment?, inllituted to arrive at 

 them, atTordcil fufficient accuracy. But, firft, it was necef- 

 fary to ai'certain how much acid a given quantity of metallic 

 arfenic could aftord ; and, finding that it was in vain to afpire 

 at a greater degree of prccifion than that which Mr. Prouft 

 had obtained, I have adopted his refults. By them it ap- 

 pears, that 133 of white oxide and i^j^ of acid, contain each 

 100 of real arfenic, the relt being oxvgen. But, 100 of 

 metallic aifcnic, acidified by nitric acid, neutralized by aa 

 alkali, antl precipitated by nitrate of lead, gave 463 of arfe- 

 niate of lead; that is, 100 of arieniate of lead contain 7^'^ and 

 a fraction of arfenic acid; and, on the other hand, my own 

 experiments informed me, that lead, diffolved in nitric acid, 

 and precipitated bv arfeniate of ammonia, gave a proportion 

 of 63 ; and 4 were expelled by heat from this ialt. The 

 compofilion of arfeniate of lead, therefore, is, 



Arfenic acid ~ ~ S3 



Oxide of lead ' " 63 



^\'ate^ - - - 4 



100 

 This experiment, repeated feveral times, never gave i per 

 cent. ditlVrence in the refults. Another method, which may 

 be deemed (Iiortcr, and perhaps even more accurate, to ana- 

 lyfe arfeniate of copper, is as follows: — After the quantity 

 <jf water has been eltimated, tiie remainder may be treated 

 by either of the fixed alkalis, which will eombiiie with the 

 acid, and leave i.lie bn)wn, the onlv real oxide of copper, in 

 the fame Itatc as that iti which it exilled in the ore; the al- 

 kaline lifjuor may be neutralized as above, and the propor- 

 tions determined in the fame manner. 



believe it contains a little cir- on, prodiu t.i hy the (liconipofition of the 

 ulc(i>ol, and is tlien-forf a lubcai buret of potafh; but ttibun can be of no 

 corltijucnce, in the generality of ex|Kriu)enis, in huniiil docimrilia. The 

 l.imc mciiiod, employed witli carbonate of foda, ij the only one to procure 

 UjA ^ lii a ftare of crmal ]ii.irity. 



Vol. XII , No. 46. K No. I. 



