518 



Dr. C. Chree. On the Determination of [Dec. 21, 



showed no phosphorescence of any kind even on the metal plate 

 (compare paragraphs 4 and 5, Part I). 



48. These observations at 100 confirm the view (paragraph 38) 

 that the effect of low temperature is to increase the insulating power 

 of the molecules, and thereby to enable some substances to store 

 chemical energy which are unable to do so at all at higher tem- 

 peratures, e.g., barium platino cyanide ; and to increase the storage 

 capacity of other substances in which the secondary phosphorescence 

 is very short-lived at higher temperatures, e.g., potassium chloride and 

 bromide. In the case of calcspar, the insulating capacity at 

 ordinary temperatures is already so excellent, as shown by the 

 persistence of secondary phosphorescence, that there is comparatively 

 little gain at the lower temperature. 



49. If, as seems most probable, the coloration of solids by the 

 /3 rays is due to the presence of ions, it is interesting to note that the 

 different salts of potassium give perfectly distinct colours, the chloride 

 being a red violet and the bromide and iodide a greenish blue. This 

 difference is most likely due to the modification of the colour of the 

 potassium ions by the presence of the haloid ions, chlorine, bromine, 

 or iodine.] 



' Note on the Determination of the Volume Elasticity of Elastic 

 Solids."* By C. CHREE, Sc.D., LL.D., F.K.S. Received 

 December 21, 1904, Eead February 2, 1905. 



In a recent paperf Mr. A. Mallock gives an ingenious and simple 

 method of determining the coefficient of volume elasticity (bulk 

 modulus) of metals by direct observation of the extension of a hollow 

 right circular cylinder under uniform internal pressure. 



The method depends on a result of the mathematical theory which 

 seems capable of being proved in a more direct and complete way, but 

 which at the same time requires to be restricted by conditions to which 

 Mr. Mallock does not seem to refer. Further, the method is only one 

 of several whicji seem equally worthy of consideration. 



When dealing with isotropic material I shall employ the following 

 notation : 



E = Young's modulus, rj = Poisson's ratio, 



k = E/(1 - 2rj) = bulk modulus. 



* [The main results of the first part of this paper (Case i) were worked out as a 

 verification before Mr. Mallock's paper was printed ; they were considered to have 

 been sufficiently indicated in a footnote appended to the paper. J. L.] 



t ' Eoy. Soc. Proc.,' vol. 74, p. 50. 



