Cooper, — The Physical Features of Kauri-gum. 491 



quite moderate temperatures the pressure rises rapidly. Mea- 

 sured by an open mercury manometer this was on one occa- 

 sion found to be over three atmospheres. If the vessel is now 

 opened the whole mass froths up violently. A sample of the 

 gum melted in a sealed tube showed marked surface dichro- 

 ism, as do many of the fractions obtained by Mr. Prideaux 

 by fractional distillation. It is intended to follow this part 

 of the subject up more thoroughly. 



The gum is insoluble in water, ether, carbon-bisulphide, 

 or mineral oil. It is converted by chloroform into a jelly-like, 

 sticky substance, but not wholly dissolved. On evaporation 

 it resolidifies in a slightly modified form. 



2. Optical Properties. 

 The refractive index was measured by grinding several 

 prisms and using the method of minimum deviation. The 

 results found give 1-54 as a mean value. No trace of double 

 refraction could be found, even when the gum was compressed 

 between crossed nicols up to the breaking-point. It will be 

 interesting to examine the fluorescent dichroic oils for anoma- 

 lous dispersion. 



3. Thermal Properties. 



A rough determination of the specific heat by the method 

 of mixtures gave as a mean result about 0-46. 



4. Electric Properties. 



The gum is easily electrified by friction, and retains its 

 charge well. This argues a high insulating-power. An at- 

 tempt was therefore made to determine the conductivity by 

 cutting a parallel slab and providing it with tinfoil discs on 

 either side. With a water battery of 70 cells, and a Despretz 

 and D'Arsonval galvanometer, no deflection could be observed. 

 A deflection of a quarter of a scale division could easily have 

 been detected. This would correspond to a specific resistance 

 of about 10" ohms per centimetre cube. The specific resist- 

 ance is thus higher than that of either ebonite or paraffin. 

 No attempt has yet been made to see how this changes at 

 higher temperatures. 



Surface conduction appears also to be very slight. The 

 resistance between two plates 7 cm. by 5 cm. separated by a 

 layer of powdered gum 1 mm. thick was over 9 x 10^ ohms. 

 This is due most probably to the fact that the gum is 

 scarcely, if at all, hygroscopic. 



It was found impossible at the time to obtain a satis- 

 factory measure of the specific inductive capacity. The 

 method used was one due to Borgman and Petrowski,* in 



* " Comptes Eendus," vol. cxxviii., p. 420. 



