326 



J. P. Iddings — Columnar structure in the 



moment of rupture will be equal to twice the tangent of a, then 

 2 tan a represents the limit of tension and will be constant for 

 any given substance. 



As the conductivity of a cooling body is not directly pro- 

 portional to the degree of radiation from its surface the differ- 

 ence between the contraction of successive layers of a rapidly 

 cooling mass will be greater than between those of one cooling 

 less rapidly, and what may be styled the angle of contraction 

 will be greater in the former case than in the latter. If a cer- 

 tain rate of cooling causes a single rupture in a given extent of 



mass, represented in fig. 1, then a greater rate of cooling which 

 would produce in the same extent of mass a contraction repre- 

 sented by a greater angle, /9, will cause as many ruptures as 



. . tan 6 



the ratio 



tan a 



If the forces producing contraction are unequally distributed 



