188 
DE. A. VEENON HAECOUET ON THE VAEIATION WITH 
to have attracted less attention than it deserves, for, as will be seen in the Appendix, 
most of the Chemists who have made observations on the variation with temperature 
of the rate of chemical change have calculated from other formulae which give 
numbers agreeing less well with their experimental results. 
Tlie following account of work done relates to an attemjet to measure the influence 
of temperature in another case of gradual chemical change. 
When solutions of ferric chloride and stannous chloride are mixed, ferrous chloride 
and stannic chloride are formed more or less gradually according to the concentration 
and temperature of the solution. Stannic chloride is decomposed by water, hydrogen 
chloride and a gelatinous precipitate of stannic hydroxide being formed; but if 
hydrogen chloride has been added in sufficient quantity the liquid remains clear. If 
the liquid is hot, a larger proportion of hydrogen chloride is needed to resist the action 
of water than when it is cold. For the observation of a colour change, it is necessary 
that the liquid should remain clear. A ferric salt loses its colour when it is reduced, 
but the colour is too pale, and therefore the change of colour too little, to serve for 
observation ; hut when a sulphocyanide has been added the fading of the blood-red 
colour can he followed by the eye from minute to minute, or at less intervals of time 
when the rate of cliange is greater. 
With stannous chloride in excess the whole of the ferric salt is reduced and the 
liquid becomes colourless ; but it is not possible to fix with any approach to accuracy 
the time when this final stage is reached, for the rate of change continually decreases 
as the residue of ferric chloride grows less and less, becoming at last indefinitely slow. 
Iffie plan followed was, therefore, that of the police-trap, namely, to have two fixed 
stations or standards and to take as accurately as possible the time at which the 
change passed each of these. Glass cylinders were used of about 100 c.c. capacity, as 
similar as possible ; one was the reagent glass, two served as standards, filled with a 
darker and with a paler red liquid. Into the reagent glass, for each observation, water 
with a small proportion of hydrogen chloride, solutions of ferric chloride and of 
potassium sulphocyanide, were delivered from pipettes, making up a total volume 
of 75 C.C., and lastly, when the liquid had been brought to the desired temperature, 
5 c.c. of an acid solution of stannous chloride. Exactly the same routine was followed 
for each observation, the only difference being of the temperature of the liquid. 
The darker standard was made by filling one of the glasses to the level at which 
80 c.c. stood in the reagent glass, with a mixture of a few drops of ferric chloride with 
a large proportion of hydrogen chloride and potassium sulphocyanide, such as to 
produce a colour sufficiently translucent to be well distinguished, and considerably less 
deep than that of the mixture in the reagent glass. In the absence of an excess of 
hydrogen chloride and potassium sulphocyanide, ferric sulphocyanide is gradually 
bleached when exposed to sunlight, but the presence of an excess of one or other or 
both of these substances seems to increase its stability. If the conditions of this 
action of light have not been investigated that would be worth doing. Many attractive 
