Manchester Memoirs, Vol. I. (1906), No. 1*^. 7 



C.C. of standard 

 solution. 



Graphite ... ... ... ... ... i 



Lead ... ... ... ... ... 9*0 



Cadmium... ... ... .. ... 2*5 



*Zinc ... ... ... ... ... 9*o 



Iron ... ... ... ... ... 5"o 



The iron electrode was made by wrapping closely 

 round a glass tube pure fine iron wire, such as is used 

 for making standard solutions. 



Part of the arsenic seems to combine with the iron 

 and to form such a combination that it is not eliminated 

 as AsHg when the current is passed. When iron which 

 has previously been used in an arsenic test is used 

 immediately afterwards with acid free from arsenic, it 

 gives a perfect blank after working for 45 minutes, but 

 on being left till the surface oxidises, and then repeating 

 the test with pure acid, a mirror of arsenic is obtained. 



Insensitive zinc in the Marsh- Berzelius apparatus. 



When zinc contains small quantities of iron, etc.» 

 it becomes insensitive when used in the Marsh-Berzelius 

 apparatus, i.e., it gives off hydrogen free from arseniu- 

 retted hydrogen, and even when minute quantities 

 of arsenic are added it still fails to produce a mirror. 

 Chapman and Law {^Analyst, January, 1906, vol. 31, 

 p. 358), say that it is due to the hydrogen being given off 

 at a lower electrical supertension, owing to the metal, 

 existing as an impurity, being capable of liberating 

 hydrogen at such lower supertension. They say, however, 

 if 2 grammes of cadmium sulphate be added to the 

 solution, that the insensitive zinc becomes sensitive. It 

 seems remarkable that a metal such as cadmium which 

 liberates hydrogen at a lower supertension than zinc 



* In working with As.iO^ the 9 parts would be represented by the 

 standard mirrors as 10 for that time of the experiment. 



