THE ACTION OF CHEMICAL AFFINITY. 
185 
In order to test this, pare sesquisulphocyanide of iron was required ; but I found 
the greatest difficulty in preparing such a salt ; and even when obtained in tolerable 
purity, it was always liable to spontaneous decomposition, giving rise to protoxide of 
iron and a yellow powder (hydropersulphocyanic acid or pseudo-sulphocyanogen). 
The nearest approach to a pure salt was made by decomposing sulphocyanide of 
barium by an equivalent quantity of ferric sulphate, but this was not absolutely free 
from either protoxide of iron or sulphuric acid. Six equal portions of this were 
taken : one was kept as a standard ; to the other five were added respectively equal 
portions of the nitrate, hydrochlorate, sulphate, acetate, and citrate of potash. In 
each case the colour was reduced. Column I. in the annexed Table shows the amount 
to which the standard had to be diluted before it was brought down to an equality in 
colour with the different mixtures. Column II. represents a similar experiment, in 
which a red mixture of one equivalent of sesquichloride iron, and twelve equivalents 
of sulphocyanide of potassium, was employed instead of the actual ferric sulpho- 
cyanide. The two following columns give the results of two experiments selected 
from a number of very early ones, the testimony of all of which was similar. In III. 
a mixture of ferric sulphate and sulphocyanide of potassium was employed ; in IV. 
the same, with a large excess of sulphuric acid. 
I. 
II. 
III. 
IV. 
Volume of original solution + salt added ... 
70 m. 
64 m. 
34 111. 
60 m. 
Mixture containing the nitrate 
80 
77 
41 
62 
Mixture containing the chloride 
90 
90 
56 
68 
Mixture containing the sulphate 
150 
160 
124 
85 
Mixture containing the acetate 
270 
220 
164 
120 
Mixture containing the citrate 
trace of red 
green 
These experiments might be varied ad infinitum, all proving the influence on the 
resulting colour of the nature of a substance mixed with the ferric sulphocyanide. 
Other organic acids, such as the oxalic and the tartaric, were found to reduce the red 
very rapidly*. As might be expected also, the diversity of effect is not confined to 
differences in the acid present. The addition of a protosalt of iron has a very great 
effect in reducing the colour of the ferric sulphocyanide: baryta salts act powerfully; 
lime salts have so much influence that the experiments here detailed would have been 
rendered abortive if spring water had been used for dilution, and even the quality of 
the distilled water employed could be detected by its effect on some of the succeeding 
experiments. A solution of chloride of mercury (as was observed long ago) very 
speedily removes the colour. 
The colour of a mixture not dependent on the manner in which the constituents were 
originally arranged. — On more closely examining the above results, it will be seen 
that whether ferric citrate be mixed with sulphocyanide of potassium, or ferric sul- 
phocyanide with citrate of potash, the resulting liquids contain but the merest trace of 
* Pelouze has observed the different effects of different acids, Ann. Chim. et Phys. xliv. 216. 
