SCIENCE AXD PRACTICE. 385 



could a line be insulated safely with pure india-rubber. 

 The want of plasticity which would be an advantage to it 

 in one sense, is, however, fatally opposed to its employment 

 in another; for while gutta-percha can, at a moderate 

 temperature, be put upon the wire by means of a die in an 

 unbroken tube; pure or masticated, india-rubber must be 

 joined, and the joint must, however the material is applied, 

 reach from end to end. This is a great objection to its 

 use. By the method most commonly employed, the india- 

 rabber is put upon the wire in a spiral, and the joint between 

 the overlaps secured by subjecting it, for a while, to the 

 temperature of boiling water. This joint is very neat but 

 extensive in its dimensions. Mr. Siemens has invented a 

 process by which pure india-rubber is put on cold, under 

 pressure ; the joints running longitudinally on opposite sides 

 of the conductor, are formed by the adhesion of freshly- cut 

 surfaces. This is also a pretty joint, and the plan which is 

 followed by the inventor of making the joints in concentric 

 coverings at right angles to each other, adds to its security. 

 But this is no exception to the general rule : joints, however 

 made, are the weak points of a cable. Another drawback 

 against the fortunes of india-rubber as an insulator for sub- 

 marine lines, is the property it has of turning into a viscid 

 mass when in contact with other bodies. 



It was the opinion of Dr. Miller that this phenomenon might 

 be the result of a process of oxidation. Were this the case, 

 however, the decomposition would take place more slowly when 

 oxygen is excluded than when in contact with the air. 

 Experience shows, however, that under exclusion of air, 

 india-rubber turns viscid more rapidly than otherwise. For 

 example, in those wires covered spirally and cemented by 

 means of heat, we have observed that the decomposition 

 commences invariably next to the wire, and seldom or never 

 extends to the outside of the coating ; further, when an 

 india-rubber covered wire is coated with some other substance 

 impervious to the air (such as gutta-percha), the process goes 

 on more rapidly than when uncoated. Under water also the 

 process seems to thrive. 



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