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pereha has a rhangcalilc. amount of conductivity, that is to say, 

 its conductivity increases with increase of temperature ; the tem- 

 perature at which it is 40 times more conductive than india- 

 rubber is 75 Fain 1 . The diagram, Fig. 1, Plate 1, shows this 

 remarkable change ; the abscissae represent temperature, and the 

 onli nates relative resistance to the galvanic current. 



With regard to the inductive capacity, india-rubber has also 

 the advantage of being superior to gutta-percha in the ratio of 

 in to 7. 



With regard to fusibility by heat, india-rubber has also a 

 decided advantage over gutta-percha, being capable of resisting 

 the heat of boiling water perfectly, whereas gutta-percha softens 

 at a temperature of 120 Fahr., and melts at 130 or 140. Great 

 care had, therefore, to be used not to expose gutta-percha covered 

 conductors to the direct radiation of the sun or other sources of 

 heat. With regard to hydrostatic pressure, it has been proved 

 that neither india-rubber nor gutta-percha are in the least altered 

 by great pressure ; it may, therefore, be safely assumed that they 

 will remain at the bottom of the deepest ocean perfectly unchanged. 

 No doubt there is compression, and this compression has a remark- 

 able effect upon gutta-percha, as I shall presently show. On the 

 occasion of making a line of telegraph for the French Government, 

 which was put into very deep water, I tested gutta-percha and 

 india-rubber also, in a tank, under a pressure of 300 atmospheres ; 

 a remarkable result was produced, shown by diagram Fig. 2, Plate 1. 

 The insulation at freezing-point and at atmospheric pressure is 

 measured by the ordinate at the starting-point of the curve (on 

 the left), and the succeeding ordinates represent the varying 

 electrical resistances due to increase of hydrostatic pressure. It 

 will be observed that under 300 atmospheric pressures the resist- 

 ance of gutta-percha was nearly three times as great as under 

 atmospheric pressure. I had expected that india-rubber would 

 follow very nearly the same law, but to my surprise, in the case of 

 india-rubber coated wire, the insulation decreased visibly with 

 increase of pressure, returning, however, always to the original 

 high electrical resistance when the pressure was relieved. I then 

 thought that this decrease might possibly be due to the infiltration 

 of water through the pores of the india-rubber ; accordingly, I 

 submitted to the test a wire that had been first coated with india- 



