1889.] 



Properties of Vulcanised India-rubber. 



233 



At moments, we were conscious of dark lines crossing the spectrum, 

 but the unfavourable conditions under which the observations were 

 made prevented us from ascertaining by measurement or otherwise, 

 whether any of these lines were Fraunhofer lines. — July 5.] 



VIII. " The Physical Properties of Vulcanised India-rubber." 

 By A. Mallook. Communicated by Lord Rayleigh, Sec. 

 R.S. Received May 9, 1889. 



Considering the wide use now made of india-rubber, it seems curious 

 that the elastic constants which define its properties should not be as 

 well "known as the corresponding quantities for iron or brass. 



The only published quantitative measure, however, with which 

 I am acquainted, relating to the s abject, is contained in a paragraph 

 of Thomson and Tait's « Natural Philosophy ' (p. 230, Part II, New 

 Edition), where the resilience of vulcanised india-rubber, i.e., the 

 amount of work restored by the substance when allowed to return to 

 its equilibrium form, after having been stretched to a maximum short 

 of rupture, is stated to be equivalent to its own weight raised through 

 1200 metres. 



In 1885 I made some measures of the value of Young's modulus 

 for india-rubber, and also examined the effect of continued strain on 

 the material, but at that time I was not aware how much different 

 kinds of india-rubber differed from one another in these respects, and 

 the experiments were made on one kind of vulcanised rubber only, 

 namely, a soft grey sort, which when cut, shows small spots of a 

 yellowish- grey scattered throughout its substance. This year I 

 resumed the experiments, using specimens of three different kinds of 

 vulcanised india-rubber made at Silvertown. The specimens were 

 cut from a sheet half an inch thick, and were square in section, and 

 one foot long. 



One was a soft grey kind, apparently identical in properties with 

 that experimented on in 1885. The next was the well-known red sort, 

 and the third a dark grey, much harder and stiff er than the two- 

 former. 



On these specimens experiments were made to determine the three 

 elastic constants, viz., Young's modulus, the simple rigidity, and 

 volume elasticity. The apparent viscosity was also measured, and 

 the behaviour of the materials under great strains, and strains con- 

 tinued for long periods, observed. 



Young's modulus and the simple rigidity were each measured in 

 two ways, statically and dynamically. The statical measurements 

 being made by observing the extension and angle of torsion produced 

 by a known force and moment ; while for the dynamical measures the 



