Introduction.Thiele in his comprehensive study of the unsaturated compounds1 concludes that, as a rule, with a conjugate system of double bonds, more especially C = C -C = C and C = C -C = 0 addition takes place on the 1,4 position rather than on the 1,2 or 3,4 position. Subsequently it developed that Thiele's conclusions were not justified, for, as shown by Straus,2 *e. g., the addition of bromine on the system C = C -C = C takes place on both the 1,2 and 1,4 positions. Staudinger* has shown conjugate system, C = C -C = O. W. J. Hale, of the University of Michigan, has by letter taken issue with Bailey and Moore as to the constitution of their CeHsCH = N -N = 1 "Die Ketene," p. 2 (1912).2 Ann., 336, 95 (1907).

The dielectric constant, power factor, and conductivity of purified rubber and of a series of its compounds with sulphur were determined at temperatures from -75°to 235°C. The dielectric constant and power factor were measured at five frequencies from 60 to 300,000~and the apparent conductivity at about 0.002 second and at one minute after the application of potential. The results of the measurements are expressed in both tabular and graphic form. At 25°C . and 1,000^the dielectric constant of purified rubber containing no sulphur was 2.37.With increasing sulphur content the dielectric constant increased to a maximum of about 3.75 at 11.5 percent sulphur, then decreased to a minimum of 2.70 at 22 percent sulphur and again increased to 2.82 at 32 percent sulphur. Under similar circumstances, the power factor increased from 1.6X10 -3 for the rubber alone to a maximum of 93.8X10 -3 for the compound containing 13.5 percent sulphur; it then decreased to about 4.0 X 10~3 at 20 percent sulphur, and again slowly increased to 5.1 X 10 -3 at 32 percent sulphur. The 1-minute conductivity was 2.3X10 -17 mho/cm for the rubber alone. With 12 percent sulphur, it was only 0.5X10 -17 , while at 18 percent sulphur it passed through a sharp maximum of 38X10 -17 , and then decreased to values between 1 and 1.5X10 -17 for compositions between 22 and 32 percent sulphur. Changes of the temperature or the frequency at which the measurements were made shifted the maxima and minima in these curves and modified their heights. For example, at -25°C , the maximum dielectric constant at 1,000-^was 2.8, and was obtained with a compound containing 4 percent sulphur, while at 145°C. the maximum was 4.5 and was obtained for a compound containing about 28 percent sulphur. Comparison of the results of this investigation on purified rubber with previous work done with crude rubber indicates that purification alters the values obtained for the electrical properties, but it does not modify the general manner in which these properties vary with changes in composition, temperature, or frequency.The results may have practical bearing on the selection of rubber compounds for specific uses and in pointing out the manner in which the properties of rubber are related to temperature or frequency. CONTENTS Page 179 3. Change in dimensions with temperature 179 IV.3. ELECTRODES The electrodes were circular disks cut from aluminum sheet and were approximately 0.025 cm in thickness. They were used in pairs 24 and 26 cm in diameter, respectively, as indicated in figure 1. 205-33

Purified rubber was prepared by the digestion of crude rubber or latex with water at about 190°C, followed by extraction with water and with alcohol, and drying in an atmosphere of inert gas. The digestion hydrolyzed the proteins, and the extraction removed the hydrolysis products, resins, and other impurities. The purified rubber contained about 99.5 per cent of rubber hydrocarbon. Properties of the rubber hydrocarbon at 25°C. were: Density, 0.9060; refractive index,

1. Two dilatometers have been described which are suitable for measuring the change of volume of rubber on stretching. 2. The volume of pure gum rubber remains constant up to a critical elongation, above which it decreases. Coarse fillers and other inhomogeneities tend to increase the volume because of the formation of vacuoles on stretching. Carbon-black compounds, however, decrease in volume above a critical elongation, which is lower than for pure gum. 3. The decrease in volume of rubber on stretching is greater the higher the elongation, the lower the temperature, or the longer the time the rubber is kept stretched. 4. After the first few minutes the volume of stretched rubber decreases at an approximately uniform rate with the logarithm of the time. No final or equilibrium state was attained in periods up to 3 or 4 weeks. 5. Stretched rubber shows a greater coefficient of expansion than unstretched rubber. 6. Poisson's ratio for rubber in the absence of coarse fillers is 0.5 or greater, the numerical value being a function of the composition, the elongation, the temperature, and the time after stretching.

The change of volume on stretching was studied for four rubber compoundsj two were pure gum, and one each contained 10 percent by volume of carbon black and whiting, respectively. Samples were made in the form of narrow molded rings. Measurements of the change with time up to about 20 minutes were made with a water-filled dilatometerj for longer periods a mercury-filled dilatometer was used. The whiting compound increased in volume on stretching, presumably on account of the formation of vacuoles around the coarse particles of filler. With the other compounds the volume remained constant up to an elongation of 200 or 300 percent, above which it decreased by an amount which was greater the higher the elongation, the lower the temperature, and the longer the time the rubber was kept stretched. The decrease was an approximately linear function of the logarithm of the time from a few minutes after stretching, to about 4 weeks, the duration of the experiment. The volume thermal expansivity of the stretched rubber was greater than that of the unstr etched. Increased vulcanization of a given compound, in general, decreased the volume change on stretching. The results indicate that, in the absence of coarse fillers, Poisson's ratio for rubber is 0.5 or greater, the numerical value depending upon the composition, the temperature, the time after stretching, and other factors.