[coKER A m'kergow] THERMAL CHANGE TO TENSION 15 



yield point is reached no visible change takes place in the structure 

 of the crystalline aggregate, but as soon as the stress is increased beyond' 

 this, slip bands appear and continue to grow in number as the stress is 

 intensified. There seems no reason to doubt that the large heating 

 effect produced in plastic strain is due to the production of these slip 

 bands, and that in their absence we may expect the Thomson law to hold. 



From the experiments of Joule it appears that the thermal stress 

 relation may be expected to hold generally for elastic bodies, and this 

 is well shown in some experiments on the compression of a short column 

 of cement, which had been kept in the McGill University testing 

 laboratory for more than ten years. Specimens recently made show 

 little or no sign of elastic properties, but in specimens of great age, the 

 cement appears to exhibit approximately perfect elasticity to near the 

 crushing load. 



The laboratory record book showed that the specimen examined 

 was made from " Peters' " brand of cement on November 10th, 1893, 

 and the average tensile strength of the neat cement at the end of 28 

 days was 220 pounds, and the compression strength was 1,600 pounds; 

 the tests for fineness, soundness and blowing tests were stated to be 

 satisfactory. The specimen originally formed part of a bar, one inch 

 square and nine inches long, which broke with a central load of 54 

 pounds at the end of a week; a similar specimen, at the end of four 

 weeks, broke with a load of 84 pounds. This specimen was cut down 

 to 2 '5 inches, and faced in a lathe, and was then subjected to stress in 

 a small press provided with an accurately calibrated hydraulic 

 diaphragm for measuring the stress. Several experiments were made 

 at different loads, and finally the specimen was stressed until fracture 

 took place. In this way the following readings were obtained: — 



