100 THE SCIENTIFIC PAPERS OF 



ordinary atmospheric temperatures, and therefore is less liable to 

 receive accidental injuries. It is not liable to become sticky, or 

 semi-fluid, when exposed to the atmosphere, and lastly, it resists 

 the action of water more perfectly than india rubber. 



The absorption of water by insulating materials, including 

 gutta percha, india rubber, and compounds of india rubber, 

 such as vulcanized india rubber, Wray's mixture, and a com- 

 pound with mica, under various pressures at different tempera- 

 tures, and from water containing different degrees of salt in solu- 

 tion, is a subject which has been very fully investigated by Dr. 

 Werner Siemens and the author. The results of this inquiry, 

 extending over three hundred days, are partly contained in Ap- 

 pendix No. 7 of the Keport of the Joint Committee on Submarine 

 Telegraph Cables, but have been much extended since the publica- 

 tion of that document. The results are graphically shown in a 

 diagram in Plate 7. The ordinates in that diagram show the 

 percentage of increase in weight of the materials examined at 

 different periods after submersion : the abscissas represent the time 

 of immersion in days. The experiments were made on plates, as 

 nearly equal in size as were procurable 1 millimetre thick, 100 

 millimetres long, and 50 millimetres broad, of the following 

 materials, namely : Raw india rubber, unvulcanized block india 

 rubber, india rubber and mica, vulcanized india rubber, and gutta 

 percha. The data given in relation to the latter material are taken 

 from the results already published in the above report. The 

 specimens were immersed in a bath of distilled water, and in anothe 1 * 

 bath containing 5 per cent, of sea salt, both vessels being kept at 

 a temperature of from fiO Fahr. to 70 Fahr. 



The following are the principal deductions to be derived from 

 the experiments : 



1st. Increase of pressure up to the limit of 50 Ibs. per square inch 

 does not increase the rate of absorption in any of the materials 

 operated upon. Absorption is favoured, however, to some extent, 

 by a vacuum, owing probably to the absence of condensed air upon 

 the surface of the material. 



2nd. The absorption is more rapid from pure water, than 

 from sea water ; and more rapid from sea water, than from brine. 



Notwithstanding the faculty of absorption of the gutta percha in 

 both sea and pure water, and that of the remaining four materials in 



