22 Schwartz & Kemp, Physical Properties of Rubber, 



(i) The load, expressed in pounds per square inch of 

 the original cross-sectional area of the test piece, 

 is constant for a given extension within certain 

 limits. 



(2) The work done in extension, in retraction, and in 



the rubber itself, is proportional to the cross- 

 sectional area of the test piece within certain 

 limits. 



(3) The areas of the hysteresis loops become constant 



after about the 6th cycle. 



(4) The increments of extension, obtained on subject- 



ing the specimen to a series of cycles of extension 

 and retraction with a given maximum extension, 

 follow a logarithmic law with respect to the 

 number of the cycles. 



In conclusion, the authors wish to express their thanks 

 to the Principal and Committee of the School of Technol- 

 ogy, Manchester, to Messrs. F. Shaw, B.Sc, and J. Davies, 

 for assistance in carrying out the experiments. 



