258 Prof. Giseke's Observatio7is Ofi the Tests for Arseiiic. 



such as had been for a fortnight in an open glass. The starch 

 in glass a was decolorized by four drops of a solution of 

 arsenic, and that in glass b by twelve drops ; whilst twenty- 

 four drops of the same solution did not perfectly decolorize 

 the iodine-starch in glasses c and d. After two hours the li- 

 quid in glass c was completely colourless, but that in d was 

 still violet. The restoration of the colour was perfectly ef- 

 fected in glass a by eight drops of sulphuric acid, and in b 

 less pei'fectly by twelve drops ; the liquid in c and d received 

 its blueish colour again after the addition of sixteen drops, 

 but did not assume a full blue even with a greater addition of 

 sulphuric acid. It would perhaps not be easy to determine 

 what change may have taken place in the iodine-starch by a 

 short exposure to the atmosphere, in which the colour suffers 

 no alteration whatever. — Perhaps I shall find an op}>ortunity 

 to make further experiments on the subject. 



3. Affinity of the Decoction of Onions for some Rc-agciifs. 



As it has been frequently said that the decoction of onions 

 bears the same affinity for the re-agents as arsenious acid (See 

 l&exze\msl„ehrbuche derChemie,\o\. ii. p. 154), the subject was 

 considered in our lectures, in order to show the similarity of 

 the precipitates as they appear at first, and also the difference 

 which is perceptible on a closer inspection. 



Repeating this examination, I made the following experi- 

 ments. A small onion was cut and boiled for a quarter of an 

 hour in distilled water, and the decoction being strained off", 

 it was treated with the following re-agents : 



1. The red solution of camelion mineral was changed 

 yellow. 



2. The blue solution of ammoniated copper turned to a 

 greenish hue. 



3. Iodine-starch was decolorized, and the blue colour re- 

 stored by the addition of sulphuric acid. 



4. Sulphuretted hydrogen produced no change in the de- 

 coction. 



5. Lime-water coloured it yellow, and produced after a 

 little standing a yellowish deposit, but which was not dis- 

 solved, like the arsenite of lime, in salts of ammonia, nor was 

 it altei'ed by weak acids. 



6. Nitrate of silver produced a white precipitate, which in- 

 creased and became yellow by the addition of ammonia. On 

 more ammonia being added it disappeared, but was repro- 

 duced on being neutralized with nitric acid ; and again disap- 



peaied 



