300 Mr. Fhillips's Reply to Dr.Christisoiu [Oct. 



^planation of what happened is rendered probable by an expe- 

 jriment of Dr. WoUastou's, as related by Dr. Thomson, in which 

 he found that arseuious acid when heated with hme was converted 

 into arsenic and arseniate of hme. This method, therefore, can- 

 not be adopted with propriety, for as extremely minute portions 

 of arsenious acid are usually operated upon, a very considerable 

 proportion of it must reuiam in combination with the poiash in 

 the state of arsenic acid, and which would render the experi- 

 ment of sublimation more decisive by increasing the quantity of 

 sublimed metal, if charcoal were present to decompose the 

 arsenic acid, or to prevent its formation ; added to this, unless the 

 heat be greater than required when charcoal is used, there also 

 remains a large quantity of arsenious acid in combination with 

 the potash ; but after the long application of a strong red heat, 

 and when the quantity of carbonate of potash used was five times 

 greater than that of the white arsenic, arseniate of potash only 

 appeared to remain in the crucible. 



Having now shown that Dr. Christison's opinions on the methods 

 .of reducing arsenious acid are inaccurate, and that his state- 

 ments of the advice of authors on that subject are incorrect, I 

 shall proceed to notice his animadversions upon my proposal for 

 using animal charcoal. The author twice asserts that I propose 

 to boil the suspected fluid with animal charcoal. If Dr. Chris- 

 tison had not actually quoted the passage in which I describe 

 the process in question, I should conclude that he had never read 

 it, but had acquired an imperfect knowledge of it from hear?$ay. 

 He does, how ever, quote it, and no mention whatever is made 

 oi boiling the suspected fluid; my words are, " I mixed some 

 of it with animal charcoal." The fact is, that I merely agitate 

 the mixture, as I presently again more particulaj'ly mention, and 

 without heating it at all. Having shown what Dr. Christison 

 has added to the process, I shall now notice what he has 

 omitted. It is well known that animal charcoal contains 

 muriatic salts ; .in order to a})ply the silver test, I direct that 

 it should be washed. This necessary part of the operation must 

 have been totally neglected by Dr. Christison: alluding to a 

 solution which he had decolorized, I presume by boiling with 

 animal charcoal, he says, *' Lime water has no effect, the cop- 

 per test produces an exceedingly scanty azure blue, and the 

 silver test an abundant cream white precipitate.'* Now this 

 abundant precipitate was evidently chloride of silver formed 

 froti;^ the salt contained in the animal charcoal. 



Thus tl^en has Dr. Christison committed two errors, either of 

 which would have been fatal to the experiment, and then assures 

 his readers that "it was hardly necessary to search for the cause 

 of (lie failure of Mr. Phillips's process ; " and this is what the 

 >dtithorico«8ider8 " a fair trial." It is, perhaps, proper to inform 

 IDtdCSlif litis on that I was acquainted with Mr. Thomson's expe- 

 riments on the Beparation of arsenious acid from solution by 



