Art. XLVI.—The Physical Properties of Kauri-gum.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 36, 1903, Page 490

Art. XLVI.—The Physical Properties of Kauri-gum. By J. S. S. Cooper, M.A., B.Sc. [Read before the Otago Institute, 13th October, 1903.] [Preliminary Communication.] The fossil gum from the kauri has many well-known properties peculiar to itself. It was thought worth while to make it the subject of a systematic physical investigation. Although this work is far from finished, it seems best to publish what few results have been obtained, and to supplement them at a later date. The samples used were of good quality, clear-yellow or milky in appearance, and in some cases of considerable size—up to a pound, or so in weight. They were obtained from Messrs. Heather, Roberton, and Co., of Auckland. 1. General Properties. The specific gravity varies with the milkiness—i.e., with the vesicularity—so that the specific gravity of the pure substance would appear to be very constant. Specific gravity of clear sample 1.076 " milky sample 1.049 " very milky sample 1.037 On being heated in the open the gum rapidly decomposes, and may be destructively distilled into a very large number of fractions. On heating under pressure in a confined space the gum may be melted as a whole, and then resolidifies nearly unchanged, but slightly more dense. During the heating to

quite moderate temperatures the pressure rises rapidly. Measured by an open mercury manometer this was on one occasion 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 dichroism, 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 doable 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 anomalous 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 attempt 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 Co a specific resistance of about 1011 ohms per centimetre cube. The specific resistance 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 × 109 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 satisfactory measure of the specific inductive capacity. The method used was one due to Borgman and Petrowski,* “Comptes Rendus,” vol. cxxviii., p. 420. in

which a spectrum-analysis vacuum-tube is used. A condenser made of a slab of the kauri-gum 1.38 cm. thick was compared with one made of a slab of vulcanite 0.73 cm. thick. With the utmost care it was found impossible to obtain concordant results. The value of the specific inductive capacity appears to lie between 3.9 and 4.3, if the S.I.C. of the vulcanite in question be taken as 3.15. The experiments were begun in 1899, but were mainly carried out at Canterbury College in 1900.