12 THE ADDRESSES, LECTURES, ETC., OF 



rubber and then with gutta-percha, the gutta-percha making a 

 complete tube round the india-rubber. This wire followed a law 

 of increase represented by a line between the two, thus clearly 

 showing that we have to deal with a specific quality of the material 

 itself, the precise nature of which has not yet been assigned to any 

 general physical law. For further particulars see Report of the 

 British Association for 1863, page 688. 



An important question to be asked in reference to our subject 

 is the following : Are these substances which we employ for 

 insulating submarine conductors subject to decay ? If exposed to 

 air and light they are both very subject to gradual decay, but 

 there is this difference, that gutta-percha becomes brittle by ex- 

 posure to light and air, whereas india-rubber, at least when put 

 upon copper wire, turns into a viscid liquid and thereby becomes 

 unserviceable ; but if submerged, neither of these results takes 

 place ; and we may safely affirm that both materials are imperish- 

 able when submerged in sea water to any considerable depth. 

 They, nevertheless, undergo a gradual change by absorption of 

 water, unless they are protected by an outer sheathing. Gutta- 

 percha absorbs sea water to a very moderate extent, and in doing 

 so, its conductivity does not sensibly increase. India-rubber, on 

 the other hand, absorbs water at a somewhat greater ratio, and 

 after a full exposure for 100 days to sea water it absolutely begins 

 to dissolve superficially. An experiment, which was continued 

 over 300 days, clearly establishes this result, and goes to prove, 

 moreover, that the rate of absorption is independent of external 

 pressure. For particulars see Proceedings of Institution of Civil 

 Engineers, vol. xxi., page 523. This action may, however, be 

 prevented by the application of an impervious coating, say of tape, 

 saturated with paraffin. 



An important advantage in favour of gutta-percha is that joints 

 can be made very perfectly, in fact, joints are now made which 

 are quite equal in insulation to the uniform covering of the wire, 

 whereas, with regard to india-rubber, there is still some degree of 

 difficulty, though I do not mean to say that that difficulty cannot 

 be overcome. There are other substances suitable for insulation 

 which are mostly compounds of india-rubber. A compound of 

 india-rubber and paraffin has been proposed, and its insulation is 

 certainly very remarkable. The compound of india-rubber and 



