Changes produced m Matter hy Electric Wares. 453 



ditions as the diminution of resistance noticed in the case of iron 

 filings. These two specifically diff'erent efi*ects have to be recognised, 

 and it would be advisable, to avoid misunderstanding, to use a simple 

 term to indicate both these effects, and distinguish them from one 

 another, by calling the one positive and the other negative. The term 



coherence " applied to the normal cUminution of resistance exhibited 

 by certain metals by the action of electric waves cannot be applied in 

 all cases, as there is, as has been said before, another class of sub- 

 stances which exhibits under normal conditions an increase of resistance. 

 The term " decoherence " has been used to indicate the effect of 

 mechanical tapping on fatigued substances of the former class; this 

 produces an increase of resistance, and at the same time restores the 

 sensitiveness. The action of tapping on fatigued specimens of the 

 latter class is, however, a diminution of resistance. 



I have shown in a former paper that the seat of sensitiveness is con- 

 fined mainly to the surface layer of the sensitive substance, and that 

 the nature of the substratum has little or no effect on the sensitiveness. 

 Thus, a substance which exhibits a strong diminution of resistance, if 

 coated with an extremely thin layer of a substance of the other class 

 which shows an increase of I'esistance, will now exhil^it an increase of 

 resistance. It is seen that the action is one of the bounding layer 

 or skin of the sensitive substance. There is a Sanskrit word, ttoach, 

 which means the skin ; and as the phenomenon dealt with in the pre- 

 sent paper is one of sensitiveness of twach, I shall use the expression 

 " electric touch " in the restricted sense of contact sensitiveness to elec- 

 tric stimulus, the touch being regarded as positive when electric oscilla- 

 tion produces an increase of conductivity or diminution of resistance, 

 and negative when the contrary effect is produced. Substances which 

 exhibit a decrease of resistance will be called positive, and those which 

 show an increase will be regarded as negative. The above terms are 

 to be regarded as convenient substitutes for long descriptive phrases. 



The phenomenon ©f contact-sensitiveness seems at first to be 

 extremely anomalous, and there appears to be little relation between 

 substances which exhibit similar electric sensitiveness. Taking iron 

 as an example of a very sensitive substance, it is seen to be easily 

 oxidised, and from this it may be inferred that a slight oxidation on the 

 surface is favourable for sensitiveness. This view obtains some support 

 from the consideration that the so-called noble metals are not as sensitive 

 as iron. But the metals nickel and cobalt, which are bright and not 

 easily oxidised, are also very sensitive. The very sensitive metals 

 iron, nickel, and cobalt are all magnetic, and it might be thought that 

 magnetic property is favourable for electric sensitiveness, but a dial 

 magnetic substance like bismuth is also found to exhibit a fairly strong 

 sensitiveness. Again, from the strong diminution of resistance ex- 

 hibited by magnesium, it may be inferred that the sensitiveness depends 



