224 Royal Society :— 
mixture, substituting the same quantity of cyanide; on heating 
both these mixtures in small reduction tubes, I got the charac- 
teristic metallic rigs in both, but better defined in the case of 
the ferrocyanide. | 
I finally took 2°5 grains of the mixture of sulphuret and glass, 
containing about z;%oth parts ofa grain of sulphuret of arsenic, 
and treated them in the same manner, using in one case 2°5 grains 
of ferrocyanide, and in the other 2°5 grains of cyanide, and 
obtained in each case a minute metallic ring, which, however, 
was much more distinct and satisfactory where the ferrocyanide 
had been used as the reducing agent. 
The same comparative experiments were made with arsenious 
acid, when results similar to those in the case of the sulphuret of 
arsenic were obtained. 
The ferrocyanide, therefore, is a most delicate reducing agent 
in the case of arsenical compounds, and where very minute quan- 
tities have to be detected, appears from my experiments to give 
more satisfactory results than the cyanide. 
Whether the addition of dried carbonate of soda would improve 
the ferrocyanide for some cases of reduction, I am not at present 
able to say; but in one experiment which I made with the sul- 
phuret of arsenic, I obtained as good results, using the fer- 
rocyanide alone, as where it was mixed previously with its own 
weight of dried carbonate of soda. In many cases the ferro- 
cyanide may be used as a reducing agent in a state of powder 
without separating its water of crystallization ; but, in most cases, 
it will be rendered a far better reducing agent by being previously 
dried at 212° in a water-bath or oven; and in this dried condition 
it may be kept for any length of time in a good-stoppered or 
well-corked bottle. 
Though as yet my experiments have been chiefly confined to 
the reduction of different compounds of arsenic and mercury, I 
entertain no doubt that the ferrocyanide of potassium will be 
found an equally effective reducing agent im the case of the 
combinations of other metals, and that it may with great advan- 
tage be substituted for the cyanide of potassium in many cases 
where the latter salt is used as a reducing agent. 
XXXV. Proceedings of Learned Societies. 
ROYAL SOCIETY. 
(Continued from p. 153.] 
April 26, 1860.—Sir Benjamin C. Brodie, Bart., Pres., in the Chair. 
NHE following communication was read :— 
“On the Effect of the Presence of Metals and Metalloids upon 
the Electric Conductivity of Pure Copper.’ By A. Matthiessen, Esq., 
and M. Holzmann, Esq. 
After studying the effect of suboxide of copper, phosphorus, 
