388 THE ELECTRIC TELEGRAPH. 



The properties of tliis gum have still to be studied. 



India-rubber has been applied to wire in a variety of other 

 forms, amongst which that of Mr. Hooper is perhaps the 

 most likely to obtain a place as a mode of insulating long 

 lines. Mr. Hooper's method consists in covering the copper 

 conductor first with a coating of pure india-rubber, then with 

 a coating of india-rubber worked up with oxide of zinc, and 

 lastly with a coating of india-rubber worked up with flowers 

 of sulphur. This triply coated core is then baked for four 

 hours in a temperature of about 250 Fahr., by which the 

 india-rubber jacket becomes vulcanised, and sufficient sulphur 

 penetrates the interior mass to make the whole combine into a 

 compact and possibly a durable insulator. The joints are secured 

 by baking them in a steam-jacket constructed for the purpose. 



There is another substance which, a few years ago, was 

 much talked of, and whose merits, perhaps, have been some- 

 what overlooked : we mean Wray's mixture. Had gutta- 

 percha not obtained itself so secure a position as a cable 

 insulator, it is probable that Wray's compound would have 

 come into extensive use. 



Gutta-percha contains a volatile oil which is expelled from 

 it in time by exposure to the air, and more quickly by over- 

 heating. In either case, as soon as the oil has left it, the 

 material becomes brittle and cracks. When submerged, 

 however, or enclosed in an air-tight space, this volatile oil, 

 which seems to be essential to its plasticity, cannot escape, and 

 the gum lasts unimpaired for an indefinite time, which is equi- 

 valent to saying that cables will never fail through spon- 

 taneous deterioration of the gutta-percha after they are in 

 the water. 



Gutta-percha still holds its ground, and is likely to do so, 

 because electrically it is all we can wish ; and the advent 

 of a successful rival would, in all probability, cause the price 

 of gutta-percha to be so reduced as to recover its position as 

 an insulator. 



93. The Cable in the Ship. At present all cables of any 

 importance are sent to sea in water-tanks on board the trans- 

 port ships. The t inks are circular, wit'i as large a diameter 



