1889.] 



Properties of Vulcanised India-rubber. 



245 



The breaking strains for the different specimens were found to be 



The section is that at the moment of rupture. 



These numbers, therefore, are not the forces required to break a 

 length of india-rubber of one square inch section when unstrained. 

 To obtain the force requisite for this [purpose, the numbers given 

 above must be divided by the extensions of the unit length at the 

 moment of rupture. They are given directly by the termination of 

 the curves A, B, 0, and are about 820 lbs. for all three kinds. 



The tensile strength, however, is dependent in some measure on 

 the time for which the force is applied, a long-continued application 

 of force causing rupture when the force itself is not sufficient to pro- 

 duce the maximum extension. This is particularly noticeable in the 

 case of the hard grey india-rubber. 



There was some difficulty in obtaining a direct measure of this co- 

 efficient, owing to the very large pressures which have to be employed 

 to produce any measurable compression. 



The plan which succeeded best was to enclose the india-rubber in a 



Soft grey. . 



Red 



Hard grey, 



8100 



(6.) Volume Elasticity. 



Fig. 6. 



