674 



SCIENCE. 



[N. S. Vol. VIII. No. 202. 



matrix of soft viscous matter with hard fibres 

 imbedded and interlaced in all directions. 



If this physical constitution of rubber is 

 granted, it is clear that its elasticity must be due 

 to the hard fibrous component, and that, there- 

 fore, these hard fibres are elastic. 



If now the rubber be subjected to stress, the 

 resistance at the beginning of the strain will be 

 due to the forced change of shape of the indi- 

 vidual elastic fibres, the bent ones being 

 straightened and vice versa. As the strain be- 

 comes greater, however, the fibres are drawn 

 out longitudinally and the applied force is less 

 and less opposed by the resistance to change of 

 shape of the fibres, and more and more opposed 

 by their direct tension. In the extreme case, 

 where the fibres are supposed straightened out 

 lengthwise of the specimen, the resistance will 

 be almost wholly due to the direct tensile 

 strength of the fibres and the cohesion between 

 them. 



The action may be likened to that which 

 occurs with a spiral spring. Thus, when the 

 spring is first subjected to stress it opposes the 

 applied force wholly by the resistance to change 

 of shape of the wire, but later, when the stress 

 becomes so great as to draw out the spiral, it 

 acts less by its resistance to change of shape 

 and more by virtue of its direct tensile strength. 

 Finally, when the wire has become straight, its 

 resistance is entirely a function of the tensile 

 strength. Of course, in the rubber there are 

 no fibres extending the length of the specimen, 

 but consisting, as it does, of a network of im- 

 bedded and interlaced elastic fibres, it acts, in 

 opposing the stress, like the spiral spring, 

 first, by the resistance to change of shape of 

 the fibres, and later by their cohesion and re- 

 sistance to direct stress. That the action in the 

 two cases is similar may be inferred from a com- 

 parison of the stress-strain diagram of the rub- 

 ber given by Professor Thurston, with the data 

 of the accompanying table of tests upon a spiral 

 spring of one-fourth-inch diameter and one-inch 

 length : 



Load (lbs.). Elongation (inches). 



1 1 



2 2.7 



3 9.2 



4 14.7 



5 16.7 



6 17.7 



7 18.2 



8 18.65 



9 18 95 



10 19.19 



11 19.38 



12 19.51 



These data will plot a curve very much re- 

 sembling the corresponding diagram for the 

 rubber, and showing the same peculiar increase 

 of the ratio of the stress to the strain. 



Perhaps a better illustration of the supposed 

 action would be given by the stress-strain rela- 

 tions of a chain with circular links. Here, 

 after the links are flattened out by the stress, 

 the applied force is opposed entirely by the 

 direct tensile strength of the material, while in 

 the early stages of the strain it is mainly opposed 

 by the resistance to change of shape of the links. 



It seems to the writer that some such simple 

 explanation of the phenomena of the stress- 

 strain relations of rubber naturally suggests 

 itself from what is known of its physical consti- 

 tution. 



C. M. Beoomall. 



Media, Pa., October 17, 1898. 



[On reference to our columns, in issues of 

 February 19th and April 15th, our correspond- 

 ent will find that Professor Fessenden some 

 years ago (See Jour. Frank. Inst., September, 

 1896) oflfered this same interpretation of the 

 singular behavior of this peculiar substance, 

 and has, furthermore, furnished experimental 

 illustration of artificial reproduction of the phe- 

 nomena described by Dr. Thurston. The com- 

 parison between the data for the rubber and for 

 the helical steel spring, here given, is interest- 

 ing also ; but it is to be observed that the rub- 

 ber finds an elastic limit only at its point of 

 rupture, while the steel exhibits a change of 

 law on passing an elastic limit far within that 

 limit, usually if not always, and it certainly is 

 not composed of such a mixture of ' hard and 

 horn-like substance with jelly-like matter in its 

 pores,' as is described by Fessenden. Experi- 

 mental investigation must still be continued in 

 order to reveal the secret of this curious ex- 

 ception to the ordinary behavior of materials of 

 construction and of commerce. — Ed. Science.] 



