Dielectric Constants of Esters at Low Temperatures. 481 



It is important to notice that for the test of a power law 

 the first nine elements are as important as the other seventy- 

 three taken together. In the upper part of the table the 

 rate of variation of stopping power and of atomic number 

 is not nearly so rapid, and many rules could be devised 

 which would tit the results over a limited range of the 

 periodic table. The logarithmic plotting which is used in 

 the figure brings out this fact, whereas the method of direct 

 plotting which von Traubenberg uses disguises it. 



It seems evident, therefore, that while von Traubenberg's 

 experimental results are a valuable addition to our know- 

 ledge, his rules fail either in simplicity or in generality 

 or in both. 



5. In view of the complexity of the phenomena asso- 

 ciated with the passage of a rays through matter, it is 

 rather surprising that a simple relation can be found 

 between stopping power and atomic number. The index 

 number suggests that the stopping power of an atom is 

 related to the number of electrons in the atom in much 

 the same way as the cross section of a sphere is related to 

 its volume. A theory of the passage of a. rays through 

 matter has been worked out by Bohr (Phil. Mag. vol. xxx. 

 p. 581. 1915), and it has been shown by him to express the 

 results for light atoms. The data necessary for applying it 

 to heavy atoms are unfortunately not available. The two- 

 thirds power rule is therefore propounded as a simple 

 working rule until such time as a general theory of the 

 phenomena is available. 



Cavendish Laboratory, 



Cambridge, 



October 11, 1921. 



LIV. The Dielectric Constants of some of the Esters at Low 

 Temperatures. By L. 0. Jackson, M.$e* 



THE dielectric constants of the members of homologous 

 series of organic compounds, when measured at 

 ordinary temperatures, are generally found to change from 

 member to member in a manner that can be correlated with 

 the structure of the compounds. This is exemplified in the 

 case of the two series of esters (formates and acetates) for 

 which the values of the dielectric constants as determined 

 by Drudet by his well-known method, using high-frequency 

 oscillations, are given below. It will be seen that the values 



* Communicated by the Author. 



t Zeit. Phys. Chem. xxiii. p. 267 (1897). 



Phil. Mag. Ser. 6. Vol. 43. No. 255. March 1922. 2 I 



