6o6 
POISONS : THEIR EFFECTS AND DETECTION. [§ 776. 
time no brown ring is seen, the testing liquids are presumed to be 
pure. Iwo c.c. of amyl alcohol are run into the inner cell by means 
of the tap funnel, and followed by the solution to be tested. The 
solution, if quantitative results are to be obtained, should be con¬ 
centrated down so as not to be more than from 30 to 50 c.c. 
Obviously, no air must be admitted, and the stem must remain full 
of liquid, the last portions of the liquid being rinsed into the flask by 
distilled water. 
The final operation is to preserve any ring formed in an atmosphere 
of hydrogen, which is accomplished very simply as follows:—The 
stopper of the funnel is opened, and a small pointed flame directed 
against the narrow tube at a point 3 cm. from the deposit, between 
the deposit and turned-up end of the tube, and drawn off; the electric 
current is now interrupted, and the tube, still full of hydrogen, heated 
and drawn off near the shoulder. 
Hy. Julius Salomon Sand and John Edward Hackford 1 have 
modified the apparatus by replacing the platinum electrodes by 
those of lead, and maintain that lead cathodes give better results 
than platinum. A. C. Chapman and H. D. Law have also experimented 
with various cathodes, and have obtained good results with lead, tin, 
and cadmium. 2 
§ 776. Gautier 3 has also devised a process by which the most 
minute quantity of arsenic can be separated ; the process is based on 
the fact that iron oxide in precipitating from a solution carries down 
with it any arsenic. Gautier uses a solution of ferrous sulphate, freed 
from every trace of arsenic, as follows :— 
100 grins, of ferrous sulphate are dissolved in 1J litres of water, and 
after the addition of 25 grms. of pure sulphuric acid heated with SH 2 , 
any precipitate is filtered off, and the solution oxidised with 28 grms. 
of arsenic-free nitric acid. The iron is now precipitated by ammonia, 
the precipitate filtered, washed, and dissolved in the cold by means of 
dilute sulphuric acid. Granulated zinc is added, and the solution heated 
to boiling under diminished pressure for two days. The solution is 
again oxidised with nitric acid, the iron precipitated, washed, etc., and 
finally dissolved in dilute sulphuric acid. 
After destruction of organic matters in the way before indicated, to 
the final solution in water so small a quantity of the iron solution is 
added that, after such addition, there is no reaction with ferrocyanide ; 
the precipitate which forms contains no arsenic, and is filtered off. The 
filtrate is now precipitated with 5 c.c. of the iron solution, and boiled. 
Ammonia is added to neutral reaction. The resulting precipitate is 
dissolved in a mixture of nitric and sulphuric acids, the nitric acid 
1 Joum. Ghent. Soc., T., 1904, p. 1018. 2 Ana i ysty 190 6, p . 12 . 
3 Compt. Rend., cxxxvii. 158. 
