PROFESSOR O. MASSON ON IONIC VELOCITIES. 
335 
Neiv Method of Observing Ionic Velocities. 
This method resembles Lodge’s, in so far as it makes use of electrolytic jellies and 
of visible moving boundaries, but differs from his in its essential principle, as will be 
explained, and also in the fact that it seeks to avoid such sources of error as change 
of temperature, the use of mixtures of unknown composition, and the introduction of 
indicators that react with the ions under observation. The use of gelatine necessarily 
mtroduces a small amount of electrolytic impurity which must have some disturbing 
effect; but the solid gelatine used in making the jellies contains less than '5 per cent, 
of its weight of mineral matter, and a plain 12 per cent, jelly was found to have a 
conductivity so small as to be practically negligible in comparison with those of the 
salt jellies used for experiment. That this is so is shown by the results obtained; 
but the fact that the best available gelatine has some conductivity of its own would 
introduce a real difficulty in any attempt to apply the method to solutions of small 
concentration.* 
Fig. 1. 
A sketch of the apparatus is shown in fig. 1. A straight tube of convenient 
length and uniform narrow bore, the dimensions of which are known, is graduated by 
a complex cuprammonium ion. He says {loc. cit., p. .344): “ The first solutions used were those of copper 
and ammonium chlorides with just enough ammonia added to each to bring out the deep blue colour of 
the copper.” This is certainly not the colour of the copper ion. In support of this statement it may be 
mentioned that the deep blue ion of Fehling’s solution, which has as much right to be called copper as 
has the deep blue ion of Whetham’s experiment, can be proved by direct observation to be a negative 
ion, which travels towards the anode while its associated K ions carry the current towards the cathode. 
This observation led the author, in conjimction with B. D. Steele, to an investigation of the cupro- 
tartrates, which they propose to commimicate to the Chemical Society. Attention is there directed to 
earlier evidence of the same fact adduced by others. 
* Some triais were made with agar-agar in place of gelatine. It proved inferior, however, in respect to 
freedom from electrolytic impurity; and, though it affords jeliies of high meiting-point, and otherwise 
admirable, they have the fatal habit, after setting firm in the tube, of contracting away from its walls and 
exuding an aqueous solution. It is then easy to blow the whole cylinder of jelly out of the tube by the 
application of slight pressure at one end. The gelatine jellies used by the author showed no such 
tendency; nor did any extension from the tube occur in the course of the experiments in the manner 
described by Lodge. The cbfference may be due to the use of stiffer jellies, and particularly to the use of 
a constant temperature bath to prevent heating of the jelly by the current. 
