336 
PROFESSOR O, MASSON ON IONIC VELOCITIES. 
an etched scale from end to end. The two ends of the tube can be fitted, water-tight, 
into the short side necks of two cells of relatively large capacity, so that the tube 
forms a horizontal connection between them. For an experiment the tube is filled 
(by means of tubular elbow-pieces and rubber connections) with a molten jelly, which 
is then allowed to set at the experimental temperature. This jelly contains a known 
quantity per cub. centim. of the salt, whose ionic velocities it is desired to observe, 
c.g. KCl. The ends of the jelly, after removal of the elbow pieces, are shaped true 
with a knife ; and the tube is then connected with the empty cells, and the whole 
apparatus is placed in a large bath of water, so that only the mouths of the cells 
Fig. 2. 
IV IV = constant temperature bath. 
t = thermometer. 
T = jelly-tube. 
yl = anode cell. 
K — cathode cell. 
C = low resistance galvanometer for current measurement. 
V = high resistance galvanometer for voltage measurement. 
J! = added resistance in voltmeter circuit. 
S — mercury connections. 
Ji = flattery. 
remain above the surface. The temperature of tlie bath is kept constant at any 
desired point below the melting-point of the jelly, e.g. 18° C. The graduations of the 
tube are easily read through the water in a good light, and parallax is avoided by 
having the tube marked both l)ack and front. The electrodes, which have surfaces 
very large in comparison with the bore of the tube, are placed in the cells and fixed 
so as to be close to the eiids of the tube without touching it. They are connected 
with a battery of sufficient voltage, as constant as possible. A low-resistance 
galvanometer serves to register the curi’ent, and one of high resistance is arranged in 
parallel circuit to indicate the voltage. Fig. 2 shows the whole arrangement diagram- 
matically. The main circuit is completed and the experiment started by filling the 
cells with aqueous solutions, and a stop-watch is started simultaneously. 
