THE ACTION OF CHEMICAL AFFINITY. 
191 
phocyanide of potassium added to a ferric salt will absolutely convert the whole of it 
into the sesquisulphocyanide of iron. Yet we can easily judge where this result will 
be very nearly attained. Thus 400 equivalents of the sulphocyanide, added to one of 
the ferric nitrate, must give a close approximation. A somewhat larger quantity will 
do the same with the sulphate. Indeed, it was found by experiment that 500 equiva- 
lents of the sulphocyanide added to each of the three principal ferric compounds, 
caused as nearly as possible the same intensity of colour. Such a mixture was made, 
assumed to be the proper tint for an equivalent of the ferric sulphocyanide, and em- 
ployed as a standard. Mixtures were then made of three equivalents of sulphocyanide 
of potassium with one equivalent of the various ferric salts, each occupying 330 gr. 
meas. The standard red was then diluted till it was equal in colour to these several 
mixtures. The annexed Table gives the different amounts of dilution required. 
1 eq. ferric nitrate-|-3 eq. sulphocyanide of potassium . . 1700 gr.m. 
1 eq. ferric chloride-1-3 eq. sulphocyanide of potassium . . 1900 gr.m. 
1 eq. ferric sulphate-f 3 eq. sulphocyanide of potassium . . 2650 gr. m. 
1 eq. ferric acetate-1-3 eq. sulphocyanide of potassium . . 7000 gr. m. (about) 
This affords us the elements requisite for a calculation of the actual amount of 
ferric sulphocyanide present in each of these mixtures ; and had it been desirable, 
almost every observation given ab,ove might have been thus reckoned. The ratio be- 
tween the volumes of the diluted standard ferric sulphocyanide, and that of any one 
of the other red mixtures (330 gr. meas.), gives the ratio between one equivalent and 
the fractional part existing in the said mixture. The four observations calculated on 
this principle give the following results : — 
1 eq. ferric nitrate-1-3 eq. sulphocyan. of potas. give 0*1941 eq. ferric sulphocy. 
1 eq. ferric chloride-f 3 eq. sulphocyan. of potas. give 0*1737 eq. ferric sulphocy. 
1 eq. ferric sulphate-1-3 eq. sulphocyan. of potas. give 0*1245 eq. ferric sulphocy. 
1 eq. ferric acetate-1-3 eq. sulphocyan. of potas. give 0*0471 eq. ferric sulphocy. (about) 
If this mode of reckoning involve no fallacy, the proportion between these four 
numbers should be the same as that given near the commencement of this inquiry, 
where the colours produced on a different occasion by adding three equivalents of 
sulphocyanide of potassium to one equivalent of the ferric salts, were directly com- 
pared. That they do agree almost exactly will be seen from the following Table, 
where column I. gives the numbers of the former experiment, and column II. those 
of the last calculation reduced to the same unit of comparison. 
I. 
II. 
1 equiv. ferric nitrate + 3 equivs. sulphocyanide of potassium 
1 equiv. ferric chloride + 3 equivs. sulphocyanide of potassium 
1 equiv. ferric sulphate + 3 equivs. sulphocyanide of potassium 
1 equiv. ferric acetate + 3 equivs. sulphocyanide of potassium 
100 
89*4 
65-2 
20 
100 
89-5 
64-2 
24-2? 
