70 
Proceedings of the Royal Society of Edinburgh. [Sess. 
by an ordinary eyepiece with cross-wires and the instrument used as a 
spectroscope. 
First of all the effect was tried of mixing uranyl nitrate in succession 
with cobalt chloride, copper sulphate, potassium dichromate, and nickel 
sulphate, and of mixing cobalt chloride in succession with copper sulphate, 
potassium dichromate, and nickel sulphate. In order to obtain the greatest 
accuracy, a differential method was used. Double glass cells of the form 
shown in the diagram were made for us by Leybold’s Nachfolger, Cologne, 
each compartment measuring 2x2x1 cm. inside. Solutions of the two salts 
to be mixed were prepared of a suitable strength. One compartment of each 
cell was filled with the one solution and the other compartment with the other 
solution. The cells were then placed above one another, one before each 
slit, so that the light passed through both solutions in each cell, and the 
position of the nicol read carefully for different wave-lengths. It was, of 
course, practically the same for all colours. Then the top cell was emptied. 
// // 77 
y 
a mixture prepared of equal parts of the two solutions, and both compart- 
ments of the cell filled with this mixture. The cell was then replaced and 
the nicol again read. Any difference in the nicol readings could thus be 
due only to the mixing. 
The effect of mixing was also examined spectroscopically. This method 
is most suitable when dealing with sharp bands, the centres of which can 
be measured with the cross-wires. In this case light which passed through 
the mixed solutions was compared straight away with light which had 
passed through the solutions in succession. 
If c denote the concentration of the solution in gram-molecules 
per litre and d the thickness of solution passed through, then A the 
“ molecular extinction coefficient ” is defined by 
i = i 0 io- Acd , 
where I and I 0 are respectively the intensities of the light before and after 
passing through. 
