CHEMICAL EFFECTS OF THE CURRENT. 675 



into two test-tubes, and adding to one a drop or two of dilute 

 sulphuric acid, and to the other a few drops of solution of caustic 

 soda. The colour of the liquid in the first tube will turn to a beau- 

 tiful red, in the other it will change into a bright green. 



The electrodes must be formed of platinum foil, because all metals 

 with the exception of platinum and gold are attacked by some of 

 the products of the decomposition, and the results would thus be 

 vitiated. 



The experiment may be made on a very small scale by bending a 

 short piece of glass tubing, 2 or 3 mm wide, so that both branches are 

 parallel and nearly close together. About 6 or 10* from the bend 

 a cut is made with the triangular file, and the portion above the cut 

 broken off. A very small U-tube, like that in fig. 341, is thus ob- 



FIG. 341 (real size). FIG. 342 (real size). 



tained, which may be supported by heating the bent part and sticking 

 it into a square piece of sealing-wax, which serves as its foot. Two 

 small pieces of platinum wire are connected by clamps with the 

 terminals of the small battery in fig. 336, and dipped as far as shown 

 in fig. 341 into the U-tube. which is filled with the blue liquid. 

 After a few minutes the liquid at the anode becomes red, at the 

 cathode green. If platinum wire cannot be had copper wire may 

 be substituted ; the copper is attacked by the sulphuric acid which 

 appears at the anode, but not by the soda which is disengaged at 

 the cathode. 



A U-tube of somewhat larger dimensions is, however, preferable 

 for the experiment. The branches should be several centimetres 

 long, and the diameter of the tube at least l cm . It is filled with 

 the liquid, clamped in the retort-stand, and two platinum strips, 4 

 or 6 mm wide and from 3 to 5 cm long, are used as electrodes. Pla- 

 tinum wires are soldered to the strips, and bent so that they may 

 hang over the ends of the tube, as shown in fig. 342 ; the wires are 

 connected outside with the terminals of the battery by clamps. 

 When the liquid, after one portion has become red and another 



x x 2 



