256 Prof. Giseke's Observations on the Tests/or Arseiiic. 



blue colour is restored by the addition of concentrated sul- 

 phuric acid when the experiment has been made with arsenious 

 acid, but not when subUmate has been used. Thus iodine- 

 starch will serve as a true criterion to distinguish between 

 those two kinds of poison. 



Being one day present with my friend Moritz Hecker, at a 

 judicial investigation of an alleged poisoning by arsenic, and 

 the absence of both arsenic and sublimate having been ascer- 

 tained by the acknowledged re-agents, we also tried iodine- 

 starch. The liquid under examination having completely de- 

 prived it of colour, we were led to make a series of experiments 

 on the decolorization of iodine-starch and the restoration of its 

 colour. They completel}' confirmed what had already been 

 aftiimed by Stromeyer in Gilbert's Aimalen, vol. xlix. p. 150, 

 and by Colin and Gaultier de Claubry [Jouni. vol. xiii. old 

 series, p. 453 — 457) ; and nothing can be added, according 

 to our experiments, to the substances mentioned by these che- 

 mists as decolorizing the iodine-starch, except prussic acid 

 and prussiate of mercury. That protonitrate of mercury as 

 well as sublimate would deprive it of colour might have been 

 expected. But it deserves to be noticed, that whilst arsenious 

 acid quickly decolorizes the iodine-starch, pure arsenic acid 

 does not, although arseniate of potash and arseniate of am- 

 mohia render it colourless ; after which, an addition of sul- 

 phuric acid restores the colour. The fuming sulphuric acid 

 of Nordhausen, concentrated or somewhat diluted, completely 

 decolorizes iodine-starch; wherefore the blue coloui*, if de- 

 stroyed by arsenious acid, cannot be restored by the addition 

 of fuming sulphuric acid, but only by the rectified acid. 



On developing the fuming substance of the Nordhausen sul- 

 })huric acid by warmth, and introducing it into iodine-starch, 

 it decolorizes it immediateh'. Even by warming in a phial 

 one part of black oxide of manganese and two of Nordhausen 

 sulphuric acid, with the view of converting the sulphurous 

 acid that might perhaps be contained in the oil of vitriol into 

 sulphuric acid, and introducing from time to time the vapours 

 into iodine-starch, the latter was deprived of colour even 

 after the mixture had been boiling for a quarter of an hour. 

 Still it would be a mistake to ascribe the quality of decoloi'i- 

 zing the iodine-starch to the fuming substance, which, ac- 

 cording to F. C. Vogel's and Doebei'einer's and Bussy's ex- 

 periments, is nothing but sulphuric acid free from water; misled 

 by the peculiar, and as yet impexfectly explained, affinity of 

 the fuming substance to another colouring matter — indigo. On 

 the contrai'y, the iodine-starch was found in tliese experiments 

 the most powerful re-agent on the sulj)hur()us acid. For by a 



repeated 



