1864. | Farreairn on Submarine Telegraph Cables. 629 
We have made numerous experiments upon the effect of tempera- 
ture and hydrostatic pressure on both gutta-percha and caoutchouc. 
They necessarily occupy a very considerable time, and are otherwise 
difficult to perform. ‘The general results appear to be that tempera- 
ture has a very marked effect upon gutta-percha, but that pressure 
appears to consolidate the material and improve the insulation, of both 
gutta-percha and india-rubber. 
The results may be briefly stated, as follows:—With the gutta- 
percha in ordinary use for submarine cables, the insulation at 72° Fahr. 
was not one half as good, and at 92° not one fourth as good, as it was 
at 52°, and at 52° it was not one third as good as at 52°. Perfectly 
pure gutta-percha was a far superior insulator, and suffered little loss of 
insulation, until it attained a temperature of between 72° and 92°. 
India-rubber and Wray’s compound, which are very far superior as in- 
sulators to the gutta-percha which has been ordinarily in use, exhibit 
very little loss of insulating power until they attain temperatures 
far above 92°. 
The experiments at a very high temperature showed that, whilst 
india-rubber withstood a heat of 200° Fahr., and Wray’s compound one 
of 152°, gutta-percha-covered wire was entirely spoiled at a tempera- 
ture a little over 122°. At 90°to 100° gutta-percha does not change 
its shape, but at a higher temperature a wire, when covered with this 
gum, easily becomes eccentric by the mere process of coiling. Gutta- 
percha-covered wire should in no case be exposed to heat the exact 
amount of which cannot be defined and regulated. The material is 
therefore not a desirable one for cables which have to be conveyed 
through, or laid in, the tropics, unless means be found for ensuring 
that the cable be maintained at a low temperature. 
When immersed in water gutta-percha, india-rubber, Wray’s 
compound, and Chatterton’s compound, absorb a portion. Professor 
Miller’s experiments, in which gutta-percha and india-rubber were 
subjected to pressure of three tons per square inch for a period of six 
weeks, show that the absorption of water by gutta-percha is almost nil 
in sea-water, and only trifling, though appreciable, in fresh-water. 
The absorption of water by caoutchouc is always sensible, the surface 
being rendered white and opaque. The absorption, however, only 
reaches to a small depth, and does not destroy, nor in any way impair, 
the insulating power of the subjacent portion. The white aspect dis- 
appears as the substance dries. The amount of absorption is dependent 
upon the extent of surface exposed to the action of the water. The insu- 
lation of specimens of gutta-percha and masticated india-rubber, experi- 
mented on by Professor Miller, was in no way impaired by immersion 
under pressure, but the results with virgin india-rubber were not 
equally satisfactory. ; 
The experiments conducted by the writer, at Manchester, on the 
permeability or absorption of water under pressure, and of different 
degrees of temperature, give variable results, as shown in the following 
pages. They were instituted to determine the value of the different 
kinds of insulators under severe pressure, and to ascertain not only the 
amount of absorption under a force equivalent to the known depths of 
