8 Thomson, Notes on Arsenic. 



should have the effect of doing away with the influence 

 of the iron ; in other words that the deposition of 

 cadmium in a spongy condition on the zinc (which is the 

 metal dissolved by the acid) should have the effect of 

 raising the supertension at which the hydrogen is evolved, 

 whilst the iron still remains and must presumably act as 

 it did before the addition to it of a spongy covering. I 

 have repeated his experiment with a sample of electro- 

 lytic zinc prepared by Brunner, Mond & Co., which 

 contains a small proportion of metallic iron. It retains 

 minute quantities of arsenic added to the sulphuric acid 

 used in the Marsh-Berzelius apparatus. We added 

 2 grammes of cadmium sulphate to this zinc, as recom- 

 mended by Chapman and Law, and tried experiments by 

 introducing minute quantities of arsenic into the apparatus, 

 with and without cadmium sulphate, but we failed to find 

 that the cadmium sulphate made any difference. The 

 cadmium was thrown down on the zinc, and the evolution 

 of hydrogen much reduced thereby, but in our hands it 

 remained as insensitive as it was before, although the 

 strength of the sulphuric acid was increased so as to give 

 about the same flow of hydrogen. 



Chapman and Law mention, as I previously found, 

 that magnesium is insensitive. I endeavoured to obtain 

 magnesium free from arsenic and antimony, but failed. 



1 took, however, the sample which contained the smallest 

 quantity of arsenic, and made the test by dissolving com- 

 pletely I "4 grammes of magnesium. The experiment was 

 repeated with the addition to it of 0'2 gramme cadmium 

 sulphate. In the latter case a slightly larger mirror was 

 obtained than in the former. 4 c.c. of the standard 

 arsenic trioxide solution was then added with 1-4 grammes 

 of magnesium, and a mirror obtained equivalent to from 



2 to 2^ c.c. On repeating the experiment with the addi- 



