PEOFESSOR 0. MASSON ON IONIC VELOCITIES. 
337 
The nature of these cell solutions is all-important to the theory and practice of the 
method. They may be distinguished as the anode solution and the cathode solution 
respectively. They must fulfil four conditions, as follows :—In the first place, each 
must possess a strong and characteristic colour; but the anode solution must owe its 
colour to its cation, and the cathode solution must owe its colour to its anion. In 
the second place, the coloured ions must not be such as to act chemically on the salt- 
jelly, so as to form a precipitate in the tube through which they are to travel. In 
the third place, the cell solutions themselves must not, during an experiment, undergo 
such chemical change as to lead to the production of a new sort of cathion (e.p., H ions) 
in the anode cell, or of a new sort of anion ((?.(/., OH ions) in the cathode cell. The 
fourth condition, which will be explained more fully later, is that the coloured ions 
must be specifically slower than the corresponding ions of the salt-jelly. A suitable 
anode solution in most cases is made with copper sulphate, provided that the anode 
be made of copper, to prevent, or at least minimise, the production of free acid, i.e., of 
H ions. A generally suitable cathode solution is made with potassium chromate and 
sufficient bichromate to prevent the formation of free alkali, i.e. of OH ions.* The 
cathode should be of jjlatinum. The strength of these solutions should be known, but 
need not be proportioned to that of the salt in the jelly tube. All that is necessary 
is that there shall be plenty of coloured ions in the neighbourhood of the electrode 
and tube for the carriage of the current into the latter. 
During the experiment, the procession of the original cations (say K) of the jelly 
is followed through the tube by a corresponding procession of blue Cu ions, while the 
opposite procession of original anions (say Cl) is followed by a corresponding procession 
of yellow Cr 04 , or of mixed Cr 04 and Cr. 207 . Thus the tube is soon seen to contain 
blue (CuCb) jelly at one end, colourless (KCl) jelly in the middle, and yellow (K 2 Cr 04 ) 
jelly at the other end, of which the first and third continually grow in length at the 
expense of the second, intermediate, part. The ratio of the lengths of blue and yellow 
is constant, and these eventually meet, to the extinction of the colourless portion. 
There is no mixing of K with Cu, nor of Cl with Cr 04 . The blue and yellow 
boundaries remain quite clear cut, and may be sharply located at any points in their 
course, the former marching steadily through the solid jelly towards the cathode, the 
latter towards the anode, till they meet. It may be mentioned that the blue boundary 
is always slightly convex in the direction of its migration, while the yellow boundary 
is always slightly concave, so that each presents a meniscus with its convexity towards 
the cathode. 
At intervals throughout the experiment observations are taken of the positions of 
the blue and yellow boundaries, of the time, of the current, and of the voltage ; also of 
* The passage of Cr 04 and Cr 207 ions into the jelly-tube, where they meet with K or other cations, 
does not chemically affect the gelatine. In fact, a transparent orange half-normal bichromate-jelly, 
containing 12 per cent, of gelatine, may be prepared, solidified, and remelted, without any precipitation of 
the gelatine \ but precipitation occurs on the addition of free mineral acid. 
VOL. CXCII.—A. 2 X 
