PROCEEDINGS OF SECTION B. ZEt 
TII. SimuLTANEOUS CoMPARISON OF Four Ions. 
When at the end of such an experiment as those just 
described, Cu cations meet Cr O, anions, they act on each 
other to form a Cu Cr O, precipitate which shows as a dark- 
coloured film across the jelly. The current now rapidly falls 
off and soon almost ceases, the film being impermeable by the 
ions; and the experiment necessarily stops. If, however, 
Ni SO, and a nickel electrode be substituted for Cu SO, and a 
copper electrode in the anode cell, this is not the case; for 
Ni Cr O, is soluble. The experiment may then be continued with- 
out interruption after the coloured boundaries have crossed 
each other, note being taken of the progress of the Ni through 
chromate towards the cathode and of that of the Cr O, through 
Ni salt towards the anode. After the first stage of the experi- 
ment, during which the jelly is divided into three parts as al- 
ready described, there comes the moment of crossing at which 
it consists of only two parts, each having a distinctive colour. 
Then follows the second stage, when the jelly is again divided 
into three parts with different colours; and now the central 
one (Ni Cr O,) continually grows at the expense of each of the 
others, but faster towards the anode than towards the cathode. 
Ii the jelly contained originally KCl, the three portions 
(reckoning from the cathode end) are in the first stage, 
ior O.. KCl NivCh;) mm the sseeond stage, “K.. Cr’ Og 
Ni Cr O, Ni Cl, The green colour of the nickel is not 
so readily distinguished in a fine tube as the blue of copper, 
but correct observations are still possible in a tube of 2.2 mm. 
diameter; and the boundaries are easily seen after they have 
crossed. It is now possible to compare the velocities of Ni and 
Cr O, ions under equal conditions as to ionisation and poten- 
tial slope, and it at once becomes evident that the Cr O, is 
about double as fast as the Ni. But it is also possible to obtain 
comparable measures of the velocities of all four ions at the 
moment of the crossing of the boundaries, and further to assign 
values to the concentrations of all the four salts. For this purpose 
the results of an experiment are plotted in the manner shown in 
the accompanying figure. (Plate IV.) Here the abscissze repre- 
sent the time in minutes from the first closing of the circuit, 
and the ordinates represent the positions of the various bound- 
aries measured in cm. from the cathode end of the tube. The 
figure is seen to consist of four curves which cut one another at 
a single point. Curve « shows the advance of the Cl—Cr O, 
boundary through a K region; curve # shows the advance of the 
K—Ni boundary through a Cl region; curve vy shows the 
advance of the Cl—Cr O, boundary through a Ni region; and 
curve © shows the advance of the K—-Ni boundary through a 
Cr O, region. Between the curves are regions of definite salts ; 
K Cl between « and 8, Ni Cl, between 8 and vy, Ni Cr O, be- 
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