386 



THE ELECTRIC TELEGRAPH. 



91. Another reason against the employment of india- 

 rubber, is its greater absorption of water, first pointed out by 

 Mr. W. Fairbairn. Dr. Siemens had experiments made in 

 his laboratory at Berlin, by which, at the end of three hun- 

 dred days' immersion, he concluded the relative absorptions 

 of different sorts of india-rubber and gutta-percha to be as 

 follows : 



In fresh water. In salt water. 



Raw india-rubber 25 per cent. 3 percent. 



TJnvulcanised block ditto 23 3 '8 ,, 



India-rubber and mica ... 19 ,, 3'9 ,, 



Yulcanised india-rubber .10-14,, 2'9 ,, 



Gutta-percha 1'5 ,, I'O ,, 



The details of these measurements are graphically shown in 



er cent, of increase 

 in weight. 



Eaw india-rubber. 



Unvulcanised block indit 

 rubber. 



India-rubber and njica. 



Vulcanised India-rubber. 



india-rubber and mica. 

 Unvxilcanised block India- 



rubber. 



Vulcanised india-rubber. 

 Eaw india-rubber. 



joodtys 



zoo (lays 



Fig. 177. 



the curves, Fig. 177. The ordinates represent the per-centage 

 increase in weight, and the abscissse the periods after immer- 

 sion. 



The experiments were made with pieces of equal dimen- 

 sions, 1 millimetre thick, 100 millimetres long, and 50 milli- 



