192 
DK. A. VERNON HARCOURT ON THE VARIATION WITH 
The mixture made for each observation was an aqueous solution, 80 c.c. in volume, 
of four substances, in the following masses and molecular proportions :— 
Gram. 
Mol. props, 
Ferric chloride, FeCP. 
. . -0145 
ro 
Potassium sidphocyanide, KCNS . 
. . -312 
36-1 
Stannous chloride, SnCP. 
, . -106 
6-3 
Hydrogen cldoride, HCl. 
. . -289 
88-8 
No simple numerical relations were aimed at. A large excess of sulphocyanide was 
taken in order to increase the depth of colour. Dr. Gladstoxe having shown that 
this increase extends to a proportion thirty or more times that of the ferric salt. The 
proportions of hydrogen chloride and of stannous chloride were chosen, after many 
trials, such as to avoid separation of a stannic compound, and to give rates convenient 
over the whole range of temperatures. The stannous chloride was taken in large 
proportion relatively to the ferric chloride, so that the rate of decrease of depth of 
colour when the change was passing the pale standard might be mainly that due to 
the diminution of one only of the acting substances. 
The conditions were now varied by increasing the amount of hydrogen chloride ; the 
rate was nearly trebled. The short-time observations were made by removing the 
cork and thistle-funnel and stirring with the thermometer. One set only was made, 
and gave the following times of change at the same series of temjieratures as before :— 
Temperatures 
. 9° 
12° 
15° 
18° 
21° 
24° 
27° 
o 
O 
CO 
Minutes . 
. 15-8 
12-2 8-8 
G-8 
5-0 
4-1 
2-8 
2-2 
(Calculated) . 
. lG-35 
12-26 
8-99 
6-82 
5-02 
3-83 
2-91 
2-19 
The numbers in the lower line are calculated from the equation 
a-j = 40’14 X 
i 273 
\273 + t) ’ 
40'14 being the estimated number of minutes in which the portion of change would 
be completed at 0° C. 
A few further observations were made upon the influence on the rate of change of 
other acids. 
Phosphoric acid changes the colour of the liquid to a pale yellow. Sulphuric and 
nitric acids can be used, and have curiously different effects upon tlie rate of change. 
The former, instead of increasing the rate, as an addition of hydrogen chloride does, 
diminishes it greatly. The times of reduction of the same amount of ferric chloride by 
the same amount of stannous chloride (l) in the mixture, whose composition is given 
above; (2) in the same mixture with an additional quantity of hydrogen chloride ; 
