Prof. Giseke's Observatiofts on the Tests for Arsenic. 251 



repeated treatment of Nordhausen sulphuric acid with black 

 oxide of manganese, I obtained at last tlie fuming substance 

 in such a state that, being perfectly free from sulphurous acid, 

 it no longer decolorized the iodine-starch. Another experi- 

 ment gave a further proof that it is only the sulphurous acid 

 contained in the Nordhausen sulphuric acid which discolours 

 the iodine-starch. For by introducing only as many drops of 

 this fuming sulphuric acid into the starch as will just deco- 

 lorize it, a little shaking of the liquid in the glass will repro- 

 duce the lost colour sometimes in a surprisingly short time. 

 The restoration also takes place by exposing the iodine-starch, 

 decolorized as stated, for some time to the influence of the at- 

 mosphere. But the colour is reproduced still more quickly by 

 adding a few drops of nitric acid, just as it is in iodine-starch 

 decolorized by sulphurous acid. 



If the colour is removed by sulphuretted hydrogen, it is only 

 restored by nitric or sulphuric acid, if no more sulphuretted 

 hydrogen has been employed than was just necessary to re- 

 move the colour. For if the sulphuretted hydrogen be in 

 excess, the colour can only be restored by the liquid's being 

 boiled, and thus the excess of sulphuretted hydrogen removed. 

 I made the attempt to remove this excess by freezing, but 

 did not succeed : whence it may perhaps be asked, whether the 

 separation of the hydrosulphuret of arsenic by freezing de- 

 pends only on the expulsion of the excess of sulphuretted 

 hydrogen, or on the power of crystallization, favoured by the 

 cold. 



It is also deserving of notice, that a decoction of onions de- 

 colorizes iodine-starch, and that the colour is afterwards re- 

 stored by sulphuric acid, just as in the case of arsenic. — Of 

 this, however more hereafter. 



In conclusion, I would make the following remark on 

 iodine-starch in its relations to arsenic. Neither Brugnatelli 

 nor any one else has noticed that it is only fresh iodine-starch 

 which is applicable as a re-agent on arsenic. Yet in the lec- 

 tures in our university it was found that, unless perfectly fresh, 

 it was useless for such experiments : — even if it was but one 

 day old it was of no use, as will be seen in the sequel. 



Iodine-starch (made by dropping a solution of iodine in 

 alcohol into a solution of boiled starch in cold water, which 

 had become clear by standing) was distributed in equal quan- 

 tities and strength of colouring into four glasses. The glass 

 a contained iodine-starch just prepared ; b, such as was one day 

 old ; c, such as had been a fortnight in a closed glass ; and d. 



Vol. 66. No. 330, Oct. 1825. K k such 



