1889.] Properties of Vulcanised India-rubber. 



237 



the particles separate from one another in one direction and approach 

 one. another in a direction at right angles to this. As long as this 

 approach merely involves the flow of the intervening fluid, the dis- 

 tortion takes place with comparative ease ; but when the approach of 

 the particles brings them into actual contact with one another, the 

 conditions change. There is no longer a store of fluid between, say, 

 the vertical layers of particles which can be drawn on to supply the 

 increased distance between the horizontal layers, and if the strain is 

 augmented it must imply either a distortion of the hard particles 

 themselves or an increase of volume of the whole mass. 



The latter is what happens in the cases just mentioned, the dull 

 surface being the result of the fluid being sucked or rather pushed 

 inwards by atmospheric pressure to supply the extra volume required 

 in the interior, thus leaving the surface comparatively dry. 



The dry patch which is seen for a short time to surround fresh 

 footsteps on some kinds of wet sand, is an example of the same kind 

 of action. 



I will now describe the various experiments by which the results 

 given in the table were obtained. 



(1.) Statical Measure of Young's Modulus. 



The apparatus used is shown in fig. 1. The specimen of india- 

 rubber is attached at one end to the balance beam and at the other to 



Fig. 1. 



