124 SCIENCE PROGRESS. 



It is well known that some metals have the property of 

 allowing certain gases to pass through them under favourable 

 conditions, the most thoroughly investigated instance of 

 this kind being the permeability of the metal palladium to 

 gaseous hydrogen at moderately high temperatures. At 

 about 300° C. hydrogen can pass quite freely through a 

 palladium septum, and it is difficult to conceive the nature 

 of this phenomenon without admitting the existence of 

 diffusion in the solid. Whether the hydrogen is dissolved 

 in the palladium or forms a compound with it, as has been 

 asserted, is of little consequence, for in the latter case the 

 compound superficially produced must have possessed the 

 power to penetrate the remaining metal, or to allow of the 

 passage of hydrogen through itself. 



Connected with the process of diffusion in solution we 

 have the phenomena of the conduction of electricity in 

 solutions, or electrolysis. Here the electric current is 

 carried by material particles, and the resistance that these 

 experience in their passage through the solution is of the 

 same nature as the resistance offered to diffusion. Helm- 

 holtz, in his Faraday lecture, drew attention to the fact that 

 glass behaves as an electrolyte towards an electric current, 

 i.e., that the current in passing through the glass is as- 

 sociated with two currents of particles moving in opposite 

 directions. The particles travelling towards the negative 

 pole of the battery have since been proved to move faster 

 than those moving towards the positive pole. Lehmann 

 also has shown that when two silver electrodes are immersed 

 in fused iodide of silver, which is afterwards allowed to 

 solidify, and a current of electricity is passed through the 

 solid iodide, one of the electrodes increases in weight at the 

 expense of the other, and that the phenomenon can be 

 reversed by reversing the current. 



These examples will suffice to indicate that we are not 

 without data to establish an analogy between the behaviour 

 of certain solids and the behaviour of ordinary liquid solu- 

 tions. Since the appearance of van't Hoff's original paper 

 on the subject a considerable number of researches have 

 been published more or less directly bearing on the question, 



