SCIENCE AND PRACTICE. 387 



metres broad. One set of such, samples was immersed in a 

 bath of distilled water, and another set in a bath of water 

 containing 5 per cent, of sea- salt, both being kept at the 

 mean temperature of the laboratory. 



It appears from this, that the absorption goes on more 

 rapidly in fresh than in salt water. On removing the pieces 

 of india-rubber from the baths, previously to weighing them, 

 their surfaces were invariably found to be slimy, an obser- 

 vation which first lead to the belief that india-rubber was 

 soluble, to a certain extent, in water. This fact is corroborated 

 by the curves of this material being depressed toward the 

 ends, as well as by the weights of the specimens, taken after 

 the series of tests was concluded, showing a decrease of from 

 0.4 per cent, to 1'2 per cent. It became also apparent that 

 un vulcanised india-rubber is likewise subject to solution in 

 water, but in a less degree. The results of experiments 

 instituted with the view of ascertaining the dependence of 

 the absorption from the thickness of the material, only served 

 to express a law, which might have been foreseen, that thicker 

 plates absorb comparatively less than thinner ones. The 

 difficulty of procuring materials physically equal leaves only 

 a remote chance of arriving at satisfactory results in this 

 respect. 



92. Mr. Siemens attempted to overcome all these bad 

 qualities of india-rubber and to take advantage of all the 

 good ones by covering it up in a coating of gutta-percha. 

 By this it was anticipated that the absorption of water and 

 the danger of bad joints would be prevented, while a cable so 

 insulated would possess the small inductive capacity and high 

 insulation due to an india-rubber covering. The conductors 

 were usually coated first with Chatter ton's compound, then 

 with one or two coats of india-rubber, and lastly with a tube 

 of gutta-percha. 



Wires insulated in this way gave splendid results ; but it 

 is to be feared that age does not spare them. It would seem, 

 indeed, that covered up in the tube of gutta-percha, india- 

 rubber shows more disposition to decompose than otherwise, 

 and in some instances, bursts the outer tube in expanding. 



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