Wave-trains through Layers of Electrolyte. 537 



The complete results for distilled water are given in 

 curve 1, fig. 8. It is seen that, at least for such a poor con- 

 ductor as distilled water, any slight absorption is completely 

 masked by the interference. As I mentioned in the intro- 

 duction, the intensity of the transmitted ray does not uniformly 

 decrease, but the transmission follows the same general law 

 as for light with a thin plate : we are in fact dealing with a 

 " thin plate " — a plate whose thickness is comparable with 

 the wave-length of the radiation used. A minimum is trans- 

 mitted by a plate a quarter wave-length thick, a maximum 

 by a plate a half wave-length thick, and so on. The fact that 

 the maxima in our curve (1. fig. 3) get successively lower is 

 only in part due to the absorption : it is chiefly owing to the 

 fact that we are here dealing, not with a steady wave-train 

 like light, but with a rapidly damped wave-train. If the 

 head of such a wave-train be only interfered with by the tail, 

 it is not much affected : complete interference is quite im- 

 possible. 



It was desirable to know how far the phenomenon would 

 still be noticeable with electrolytes of higher conductivity. 

 To test this question, a few drops of a strong solution of zinc 

 sulphate were added to the jarful (some 14 litres) of distilled 

 water, and the relative intensities of the transmitted radiation 

 determined as before. The results are given in curve 2, 

 fig. 3. Curve 3 of the same figure was determined with a 

 slightly stronger solution. In both curves the first maximum 

 is very well marked. Both solutions were, however, very 

 dilute : their specific gravities with reference to water at the 

 same temperature were approximately 1*00028 and 1*00038, 

 and their specific resistances 5100 and 4030 ohms x cm. 

 respectively, as measured with the Kohlrausch bridge and 

 telephone. To give an idea of the corresponding opacities I 

 may add that radiation traversing the first solution would be 

 reduced to 0*41 of its initial intensity in a metre, or to 0*32 

 of its initial intensity in traversing a metre of the second 

 solution. 



With a view to making certain of the nature of the periodic 

 effect, experiments were also carried out with liquids of a 

 different specific inductive capacity. Curve 1, fig. 4, was 

 determined with 95 per cent, alcohol, and curve 2 with a 

 mixture of three volumes of the same alcohol with one volume 

 of water. In both cases the maxima are very well marked. 



