THE ACTION OF CHEMICAL AFFINITY. 
183 
tions, it was necessary to have vessels of colourless glass of a uniform character. 
Ordinary precipitating glasses, holding about five ounces, were found peculiarly fitted 
for the purpose : being blown, and not moulded, they are very translucent ; they are 
easily obtained devoid of colour ; and, although not strictly uniform in size, it was 
easy to pick out a sufficient number which would furnish every requisite for the ex- 
periment. This was tested by dividing a coloured solution into two equal parts, and 
pouring one-half into one glass, and the other half into another ; if the two solutions 
appeared then of a perfect equality of tint, nothing more could be desired. When 
two or more coloured solutions were to be compared, the best method was found to 
be to place the glasses containing them on a stand before the window, across the pane 
of which was stretched a piece of tissue-paper about 3 inches in depth. The experi- 
ments were almost always performed when the sun was shining, but not on the win- 
dow itself, as that was found to be disadvantageous. By observing the coloured 
solutions against the evenly illuminated tissue-paper, most exact results could be 
obtained, especially after a little practice. If the object was to observe the amount 
of dilution necessary to reduce one coloured solution to the same tint as another, 
distilled water was added, and thoroughly mixed with it, until equal bulks of the two 
solutions appeared alike. The amount of water added was of course easily measured 
in a graduated vessel. My own observation was always checked by that of my 
assistant, and if we differed I generally adopted his view, since having no idea of 
what result was to be expected, his judgment was the more impartial. I may also 
state in this place, that it was found unnecessary to let a freshly mixed solution con- 
taining a sulphocyanide stand any length of time, for it assumed instantaneously its 
proper amount of colour: two mixtures, similarly prepared, were always found to be 
of precisely the same shade; and everything conspired to give me great and increa- 
sing confidence in the validity of testimony drawn from the colour of a solution. 
The first object to be determined evidently was, whether on mixing three equiva- 
lents of sulphocyanide of potassium with one equivalent of the ferric salt, say the 
chloride, the full depth of colour possible from the combination of all the sulpho- 
cyanogen with all the iron was actually obtained. That this was not the case was 
seen at once, for on the addition to such a mixture of either more sulphocyanide of 
potassium, or more chloride of iron, the colour was increased. This showed also the 
influence of mass, which will be exhibited quantitatively in due course ; but before 
doing so it is necessary to advert to another part of the inquiry, viz. — 
The dependence of the amount of the coloured salt on the nature of the other sub- 
stances present in the solution, hut which are not immediately concerned in its forma- 
tion . — In order to investigate this point, 25 gr. rneas. of each of the ferric salts, which 
it will be remembered contained exactly the same amount ot iron, were mixed with 
6'25 gr. meas. of the sulphocyanide of potassium solution, that is one equiv. of the 
former to three of the latter. The five mixtures were equally diluted. At a glance it 
was evident that a widely different amount of red sulphocyanide of iron had been 
