70 SUBMAKINE CABLE LAYING AND REPAIBING. 



work in classification, and reported to the Government in 1883. 

 But the supplies had been seriously depleted by that time. 

 The other species of gutta are very varied in composition, 

 those containing most resin being of a less tough and durable 

 nature. It is a curious fact that the less the percentage of 

 pure material in any natural gutta or composition the higher 

 ia the insulation of that particular kind. 



Commercial gutta is composed of pure gutta, resin and 

 water, and where the proportion of pure gutta is low there is 

 more reain present. To a certain extent the various gums 

 may be and are mixed to improve the electrical properties, but 

 this must not be carried too far or it may seriously affect the 

 fibrous texture. The purest guttas are fibroup, tough and 

 mechanically strong, and although showing a somewhat low insu- 

 lation have the essential quality of durability. Gutta containing 

 a large proportion of resin or resinous gums shows a higher rate 

 of electrification and a lower specific capacity than the pure 

 material. But resin has an action upon gutta which, in course 

 of time, weakens it mechanically and in its electrical properties. 

 A very high insulation resistance is neither necessary nor desir- 

 able, and it is usual, for the above reason, to specify a maximum 

 limit in insulation of the dielectric as well as a minimum. It 

 should not be lower than about 250 megohms per naut, and 

 the superior limit is about 7,000 megohms. 



The best means of telling that a given core has a good gutta- 

 percha dielectric is in the gradual rise in resistance of the insu- 

 lator with time as it passes through the stages in the factory. 

 This is the maturing or ageing effect, and more reliance is, as 

 a rule, placed upon this than upon any specially good test 

 figures of the specific values. 



In collection a good deal of admixture of different yields 

 takes place, and in most of the gutta regions no care is taken 

 in keeping the gum clean and free from woody litter, con- 

 sequently the raw material, as marketed, is of very varying 

 quality and contains impurities. Before purchase in this 

 country for cable purposes an analysis is made, or a few hun- 

 dred yards of core made up at the factory to test the sample of 

 gutta in the ordinary way under water, so that the best qualities 

 only are accepted. Mr. Charles Bright, F.R.S.E., in his ex- 

 cellent book " Submarine Telegraphs," enters very fully into 



