the Ferrocyanide of Potasstum in Chemical Analysis. 223 
intimately mixed with the substance without any appreciable 
absorption of moisture. I made the following comparative ex- 
periment to ascertain the relative absorptive properties for mois- 
ture of the two salts under the same circumstances. Having 
thoroughly dried in a water oven, till it ceased to vary in weight, 
some finely powdered ferrocyanide, I placed 50 grains of it in a 
counterpoised watch-glass, and powdering in a warm mortar 
some fresh cyanide of potassium, I placed the same quantity of 
itn a similar counterpoised watch-glass, and left them both 
exposed to the air. On examining them after four hours’ expo- 
sure, I found that the former had only gained ;8>th parts of a 
grain of moisture, whereas the latter had taken up 3°6 grains, 
or sixty times as much moisture under the same circumstances. 
After two days’ exposure I found that nearly all the cyanide had 
passed into the liquid condition, having taken up 46 grains of 
water ; whereas the ferrocyanide appeared perfectly dry, and 
had only absorbed 1°4 grain. 
The great fusibility of the cyanide is sometimes rather a dis- 
advantage, which has to be lessened by mixing it with a certain 
proportion of dried carbonate of soda; but the ferrocyanide not 
fusing at so low a temperature, does not require in most cases 
this admixture to lessen its fusibility. Again, the ferrocyanide is 
not a poisonous salt, whereas the cyanide is highly so, and must 
be used with great caution; and lastly, the former salt is little 
more than half the price of the latter. Combined with the above 
advantages, I find that the ferrocyanide is equally effective in re- 
ducing metallic oxides and sulphurets, and is especially con- 
venient for the reduction of different combinations of arsenic and 
mercury, which are reduced by it with the greatest ease. 
I made several comparative experiments with the dried ferro- 
cyanide and with the cyanide as reducing agents for the sulphuret 
of arsenic and arsenious acid, employing the same quantity of 
arsenical compound with each salt under similar circumstances ; 
and in almost every case, particularly where the quantities 
operated on were minute, I obtained more satisfactory results 
with the dried ferrocyanide than with the cyanide. 
The followmg were amongst my experinents :—I mixed the 
zath of a grain of sulphuret of arsenic with 3 grains of the dried 
ferrocyanide, and made a similar experiment, substituting the 
same quantity of cyanide; and on heating the mixtures in similar 
glass tubes, obtained almost identically fine and characteristic 
rings of metallic arsenic. 
I then intimately mixed the same quantity of sulphuret of 
arsenic with 49-9 grains of very finely powdered glass, and taking 
5 grains of this mixture, containmg the ygoth part of a grain 
of the sulphuret, mixed it with 5 grains of the dried ferrocyanide, 
and made a comparative experiment with another 5 grains of the 
