726 Dr. G. W. C. Kaye on the Expansion and 



III. Thermal Hysteresis. 



Having regard to the expansion curve o£ fused silica, we 

 should expect that any thermal hysteresis it may exhibit 

 would depend very considerably on the temperature treat- 

 ment. Callendar * remarks that if the temperature be kept 

 constant at any point above 1000°, silica continues slowly to 

 expand ; and furthermore, after such an expansion it does 

 not return to its original length on cooling, but remains 

 slightly longer. Randall f has noticed that this gradual 

 lengthening at constant temperature in the region of 1100° 

 is accompanied byanisotropic expansion, the worked surfaces 

 of the silica ceasing to be plane. The distortion is moreover 

 permanent and remains even after cooling. These results 

 are of course not surprising in view of the existence of a 

 change-point at 1000° C. 



At low temperatures, DorseyJ noticed that for a range of 

 about 60° on either side of — 80°, fused silica shows this 

 peculiarity; above —80°, when warmed it first contracts 

 slightly and then expands; similarly when cooled, it first 

 expands a trifle and afterwards contracts. For temperatures 

 below —80° the converse of this is true. Dorsey could not 

 trace the effect below about —140° or above about —20°, 

 nor did he notice it in any other substance. 



"With temperature treatment which is not extreme, one 

 may infer from a review of the observations of Holborn and 

 Henning, Minchin, and Randall (see above), that the residual 

 length alteration after a temperature cycle would be very 

 slight if the silica has been annealed and if the temperature 

 has not exceeded say 400° or 500°. The existence of such 

 hysteresis for moderate temperature ranges has been defi- 

 nitely established and measured at the National Physical 

 Laboratory by Mr. L. F. Richardson. 



To fix one's ideas quantitatively, thermal hysteresis may 



be defined as follows: — Let I be the original length of a 



specimen, w T hich is subjected to a rise of temperature of t°, 



and is maintained at that temperature for say a day or two. 



If, when it is cooled to the original temperature, its length 



£/ 1 

 (after half an hour or so) is found to be (I + &1), then -j . - is 



adopted as a measure of the linear thermal hysteresis H. 



Since the mean coefficient of expansion = -y- .-, where Al 



* Callendar, Chem. News, lxxxiii. p. 151 (1901). 

 t Randall, Phys. Rev. xxx. p. 216 (1910). 

 X Dorsey, Phys. Rev. xxv. p. 88, July 1907. 



