of some of the Esters at how Temperatures. 485 



(Deutsch. Phys. Ges. Ver. xxi. pp. 572-577, Sept. 30, 1919), 

 and more complicated methods involving the use o£ more 

 than two valve circuits have been described by Pungs and 

 Premier (Phys. Zeit. xx. pp. 543-545, Dec. 1, 1919) and 

 Falckenberg (Ann. cler Phys. Ixi. 2, pp. 167-172, Jan. 15, 

 1920). The latter worker has applied his method to the 

 determination of the dependence on pressure of the dielectric 

 constants o£ water, ethyl alcohol, methyl alcohol, and 

 acetone. 



Purification of the Materials. 



The substances used in the investigation were methyl, 

 ethyl, n-pr^opyl, n-butyl formates and the corresponding- 

 acetates. The esters were obtained from the firm of Poulenc 

 Freres of Paris, and before use were fractionated, washed, 

 very carefully dried, and re-distilled. It was found, however, 

 that after the most elaborate precautions had been taken to 

 eliminate water, acids, or other possible impurities, the 

 methyl, ethyl, and, to a less extent, n-propyl esters still 

 possessed a conductivity, while liquid at ordinary temper- 

 atures, which, though verv small, was sufficient to so reduce 

 the intensity of the sound in the telephones as to make the 

 method inapplicable, quite apart from any change in the 

 value of the effective dielectric constant due to the finite 

 conductivity of the dielectric. (Had it been possible to 

 obtain readings of C 1? the true dielectric constant could have 

 been calculated in terms of the known resistance of the 

 dielectric and the constants of the circuit : cf. P. T. Lattey, 

 Phil. Mag., June 1921. Possibly some arrangement by 

 which the sound in the telephones could be considerably 

 amplified would enable this to be done.) 



In the case of the butyl esters it was found that a much 

 less rigorous method of purification than had failed with the 

 other esters would suffice to reduce the conductivity, even 

 when the substances were liquid to a value small enough to 

 make the method applicable. 



The facts that the conductivity of the esters remained after 

 very rigorous purification, and that this residual small con- 

 ductivity decreased from member to member as one proceeded 

 up the series, would seem to show that this conductivity was 

 a property of the esters themselves. 



The conductivities of the esters when solid at liquid-air 

 temperatures were so small as to make the present method 

 quite applicable. In general, the method provides a very 

 simple and direct means of measuring the dielectric constants 

 of substances of sufficiently small natural conductivity. 



