Manchester Memoirs, Vol. xlvii. (1903), No. 15. 5 



been given by a piece of tissue paper 3 or 4 inches long 

 by I inch wide, folded in the centre and hung over the 

 tube for receiving the mirror, over which water is allowed 

 to drop rapidly, the two hanging folds being cut to a point 

 to allow the water to run off in a single stream into a 

 glass placed underneath. A roll of several layers of filter 

 paper is not so effective for cooling, as the cold water 

 takes some time to penetrate the several layers of paper. 



Brozvn - metallic looking mirrors and black arsenic 

 deposits. 



It has been suggested by the Joint Committee above 

 mentioned, and by other chemists, that the presence of 

 oxygen or air, if mixed with the hydrogen /rom the 

 generating apparatus, tends to produce black deposits 

 rather than metallic arsenic mirrors ; this seems to be 

 so, but the explanation appears to be that the heat 

 produced by the burning of the oxygen and hydrogen 

 at the red-hot portion of the tube, evaporates the 

 minor after it has been deposited in the brown-metallic 

 condition, and it then deposits a little further on in the 

 black form. On gently warming the brown-metallic 

 mirror with a small Bunsen flame, whilst the hydrogen is 

 still flowing, the metallic mirror is evaporated and de- 

 posited a little further on as a black deposit. I have 

 tried thus converting the metallic mirrors into black 

 deposits, with the view to ascertain whether such deposit 

 would form a better measure of the quantity of arsenic 

 present than the metallic mirror, but have found that the 

 quantities are better indicated by the brown-metallic 

 mirrors than by the black deposit. 



I have studied somewhat more minutely these two 

 forms of arsenic. The first, or brown form, with metallic 



