1)04 Dr Turner's Analysis of Euchroite. 



tion of that salt by sulphuric acid, and neutralizing the clear 

 solution with potash, nitrate of silver was added. The brick- 

 red arseniate of silver subsided without any admixture of the 

 yellow phosphate. Another portion of the arseniate of lead 

 was heated before the blow-pipe, on charcoal. Decomposition 

 readily ensued, with evolution of copious arsenical vapours. 

 Numerous globules of metallic lead were procured, but not 

 the slightest trace of the characteristic phosphuret of lead 

 could be detected. Phosphoric acid cannot, therefore, enter 

 into the composition of Euchroite. 



I shall only remark, with respect to the atomic constitution 

 of Euchroite, that the proportion established by analysis is 

 not satisfactory in theory. Supposing an atom of the per- 

 oxide of copper to be eighty, and an atom of arsenic acid 

 sixty-two, (the estimate of Dr Thomson,) we shall require 

 almost four per cent, more acid than is given by analysis, to 

 establish a due proportion ; and, even then, the water of cry- 

 stallization would not agree. The proportions of Berzelius 

 are still more discordant. But we are not warranted, I con- 

 ceive, in assuming, on speculative grounds, so great an error 

 in an analysis, unless it bear internal evidence of inaccuracy. 

 The quantity operated on was, indeed, of necessity, small, and, 

 therefore, the unavoidable errors of analysis would have con- 

 siderable influence on the result ; but as they were rendered 

 trifling by careful manipulation, and the employment of an 

 exceedingly delicate balance, the whole error could hardly 

 amount to one per cent. 



It is pleasing to see an analytical result square neatly with 

 the doctrine of proportions ; and when it does so happen, it is 

 no small confirmation of the accuracy with which the analyst 

 has operated. So general, indeed, are the laws of combina- 

 tion, that an analysis may sometimes be regarded as incorrect 

 which does not correspond with theoretical considerations. Such 

 an inference, however, is by no means admissible wherever ar- 

 senic acid is concerned ; for our knowledge of its atomic con- 

 stitution is far less precise than that of most other substances. 

 To justify this observation, I need only mention, that the two 

 celebrated analysts, Professors Thomson and Berzelius, who 

 are deservedly held as our first authorities on this as on many 



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