186 Mr. W. C. Dampier Whetham. 



whereas in other solutions, such as chlorides, it is about 0'0030, that 

 is, about 46 times as great. Now acetic acid at the concentration 

 mentioned above has an abnormally low conductivity, only the 1/62 

 part of that of an equivalent solution of hydrochloric acid, so that, in 

 such cases, the immediate cause of the low conductivity appears to be 

 a reduction in the ionic velocities. 



An attempt was made to complete the investigation of acetic acid 

 by measuring the velocity of the acetate group C 2 H 3 2 . I thought 

 the red colour, which acetates give with ferric salts, might be used as 

 indicator, and for this purpose set up solutions of ferric chloride and 

 ferric chloride coloured red by ferric acetate. These ferric salts are 

 said to be decomposed in solution into ferric hydroxide and the acid. 

 Besides the chemical reasons in favour of this hypothesis, it is sup- 

 ported by the conductivities. Ferric chloride, which gives hydro- 

 chloric acid, is known to have abnormally great conductivity, and 

 measurements I have carried to great dilution show that the ^mole- 

 cular conductivity reaches a maximum at a certain concentration, 

 and, as the dilution is pushed further, sinks again. This behaviour 

 is characteristic of the solutions of acids. In the case of ferric 

 acetate, acetic acid is produced, and the molecular conductivity is 

 abnormally low. It seemed likely, then, that the red colour pro- 

 duced by acetates, when added to solutions of ferric salts, might be 

 used as a means of measuring the velocity of the acetate group in 

 acid solutions. When the experiment was made, however, it was 

 found that the colour boundary travelled in the wrong direction for 

 an anion, viz., with the current, the specific velocity being 0'00028 cm. 

 per second. Now it is unlikely that an ion should behave thus, and 

 an experiment on the migration phenomena of a solution of acetic 

 acid showed that there was no accumulation of acid round the 

 kathode. The result of further investigation was to show that the 

 red colour of such solutions is due to the presence of soluble ferric 

 hydroxide, and that, under the influence of a current, this is trans- 

 ported through the solution without decomposition in the direction of 

 the current. Among other experiments, a direct measurement of the 

 velocity of the transport was made. If a solution of ferric chloride 

 be dialysed through parchment paper, t hydrochloric acid escapes, 

 while a red solution of soluble ferric hydroxide, known as " dialysed 

 iron," remains. This was used to colour a solution of ferric chloride, 

 which was set up in contact with an ordinary aqueous solution of 

 ferric chloride of the same concentration. The specific velocity of the 

 hydroxide could thus be determined by observing the motion of the 

 colour boundary, and came out O00033 cm. per second in the direc- 

 tion of the current. It is evident that this is what we were measuring 

 in the case of the acetate described above. 



The conductivity of the dialysed iron solution is very low, and an 



