Page:Philosophical Transactions of the Royal Society A - Volume 184.djvu/341

Rh The agreement between Dr. ’s observed value for the velocity of hydrogen and that deduced by is too close to be accidental, but some further experiments in which the velocities of various ions were determined by watching the formation of precipitates as they travelled along did not give such good results. Thus, Dr. found Ba$$= .00012$$, Sr$$= .00015$$, and Cl$$=$$Br$$=$$I$$= .00024$$ centims. per second, while gives Ba$$= .00033$$, Cl$$= .00053$$, I$$= .00060$$ (‘Wied. Ann.,’ vol. 6, p. 206).

There are several disadvantages in this method. The greatest appears to be the impossibility of keeping the solutions of a definite strength—the mere fact of producing a precipitate must withdraw a certain amount of salt from the sphere of action, and so reduce the strength of the solution. This will increase the resistance of that portion of the tube in which the precipitate has been formed, and the potential

gradient will cease to be uniform and calculable. When jelly is used, the experiments are easy to make, but it has a considerable conductivity of its own, and its great viscosity must offer a high resistance to the passage of the ions through it. Moreover, it gradually exudes from the tube in a direction opposite to that of the current—perhaps owing to electric endosmose. If ordinary liquid solutions are employed, the precipitates cause serious mechanical disturbances which prevent good results, while the use of indicators restricts the experiments to the cases of acids and alkalis and introduces a foreign substance which may itself serve to convey some of the current.

In order to get over these difficulties, and to submit ’s important and beautiful theory to adequate experimental tests, I undertook a series of experiments in which the substances used acted as their own indicators. The solutions can then be kept at a definite strength, no third substance need be introduced, no precipitate