at all appeared to have taken place in experiments (1) and (2), but in (3) the samples of No. 2 rubber showed a slight discoloration on the surface, similar to that observed before with this rubber.

17. From the above experiments it appears that Para rubber, which is the kind generally used in the manufacture of cables, is more easily attacked by ozone or other gas generated by the action of alternating stress on the air, than is the rubber called No. 1. This decomposition has, however, only been detected when there has been comparatively free access of air to the rubber. In the case of the rubber called No. 1, although in the condensers F and G it was very severely stressed, being kept for 3000 hours under an alternating stress having a maximum value of 50,000 volts per cm., no deterioration in insulating properties could be detected; but the fact that this rubber becomes harder and rather brittle on keeping makes it unsuitable for cable-work. 18. The principal conclusions which appear to be justified by the foregoing experiments are:

(1) That in the case of the okonite cable the current during the first hour's electrification increases as the charging pressure increases.

(2) That this does not necessarily prove the dielectric not to follow Ohm's Law, since the true conduction-current must be considerably less than the total current unless electrification is continued for very lengthened periods.

(3) That in the case of the okonite cable and the paraffinpaper condenser, the charging current is a power function of the time reckoned since electrification commenced, and on this may be superimposed a current of true conduction.

(4) That for the okonite cable, the mica, and the paraffinpaper condensers, the discharging current is a power function of the time which has elapsed since discharge began; the currents at any particular charging pressure but varying time of charge being given, in the case of the okonite cable by equations of the form

where -X=K,T; t=time in seconds since discharge commenced; T-time of charge in seconds; K, K1, K2, K3, and are constants.

(5) That the only deterioration in insulating properties of indiarubber due to alternating pressure was in the case of pure Para, and appeared to be due, not to any effect of alternating stress on the rubber itself, but to chemical action taking place between the rubber and gases produced by the action of alternating stress on the air.

L. Note on Accidental Double Refraction in Liquids. By BRUCE V. HILL, A.M., Former Fellow in Physics, University of Nebraska *.

IN

N a former paper + the writer gave the results of a series of experiments upon solutions of gelatine and gumarabic in water. These solutions were subjected to a strain by being placed between two parallel rotating cylinders. When the cylinders were set in motion the liquids became double-refracting. In the case of gelatine this doublerefraction increased to a maximum, then decreased, changed sign, and finally increased in the opposite direction. The solutions of gelatine were then placed under a static strain, and were still found to become double-refracting. This strain was maintained by a solution having only 5 gm. of gelatine in 100 c.c. of water. Although these dilute solutions when examined in a test-tube seemed as perfectly mobile as water, it appeared from the above that they were not true solutions, but quasi-solid substances capable of sustaining only very small strains without rupturing, and further experiments upon them seemed necessary.

To examine jellies too dilute to bear their own weight they were placed in thin-walled brass tubes. These tubes were 42.55 cm. in length and 2.77 cm. in diameter. To the ends of these tubes glass caps were fastened by slipping short pieces of rubber-hose over them. The tubes were strained, so that their cross-section was elliptical, by means of clamps. The effect of double-refraction decreases so rapidly with rise of temperature that at room temperature-about 23° C.-no effect was visible in the dilute jellies. The tubes were then surrounded by ice, and the formation of dew on the glass ends was obviated by slipping over the tube a second one, also having a glass cap and containing a little phosphoric anhydride.

With tubes of this length it was very difficult to make the compression uniform throughout, but with shorter tubes the distortion had to be carried beyond the limits of perfect elasticity of the jellies in order to obtain a measurable effect.

The amount of double-refraction was measured as before by means of a half-shade polariscope. With jellies containing 3 gm. (or less) of gelatine in 100 c.c. of water, the optical sensibility was 0.005 of rotation of the plane of polarization or 000028 x relative retardation of the two *Communicated by Prof. D. B. Brace. + Phil. Mag. Dec. 1899.

component rays. A number of solutions of gelatine were examined. In all cases the tubes were surrounded by ice. The following tables give typical results, the light being red, 781 to 639 μμ.

μμ

I. 5 gm. in 100 c.c. having been on ice for 16 hours.

The depolarization prevented further observations.

II. 5 gm. in 100 c.c., after being 18 hours on ice.
Compression in mm.

III. 5 gm. in 100 c.c., after being 48 hours on ice.

IV. 3 gm. in 100 c.c., after being 40 hours on ice.

V. 3 gm. in 100 c.c., after being 48 hours on ice.

A solution of 2 gm. in 100 c.c. showed (only) a trace of double-refraction after 48 hours on ice.

A solution of ·1 gm.

in 100 c.c. gave no effect after 48 hours.

Two tubes con

taining 1 gin. in 100 c.c. were then allowed to stand for 120 hours on ice with the following results:

VII. ·1 gm. in 100 c.c., after being 120 hours on ice.

As stated previously these solutions behave as if they were solids having a low elastic limit. When a stress is applied the strain increases to a certain point beyond which no further strain is produced, as shown in Tables V., VI., and VII. If the distortion be carried further rupture takes place and the strain diminishes, as shown in Tables II., IV., and V.

The solid structure grows in time though not in proportion to it, and not always with the same rapidity for solutions of the same concentration. The growth is more rapid in the more concentrated solutions. Frass found that solutions containing more than 7 per cent. of gelatine attained a maximum rigidity in from 24 to 32 hours, while the solution containing 1 per cent. used in this experiment requires a much longer time to attain a maximum rigidity. Along with double refraction, depolarization also appeared. This was very small at first, but in the case of a solution containing 5 per cent. of gelatine it became so great in 24 hours as to interfere seriously with the accuracy of observation. After 24 hours more it was again greatly decreased. This seems to indicate that the structure does not grow uniformly throughout the liquid, and so, at the end of the first 24 hours, it was not homogeneous. After the second 24 hours there was greater homogeneity, and consequently_a_more uniform strain and less scattering of the light. This may

* Wied. Ann. vol. liii. p. 1074.

also suggest a possible explanation of the fact that the plane of polarization is not always rotated in the same direction. If the growth in the liquids is not uniform it is possible that internal strains are set up so that one is really relieving rather than producing a strain by compressing the tube. As the rate of diffusion of gelatine is very slow, it is also possible that the stirring which would produce uniformity in a crystalloid solution fails to do so here.

Physical Laboratory,

University of Nebraska, Lincoln.

LI. Notes on Dielectric Strain. By Louis T. MORE, Ph.D., Professor of Physics, University of Cincinnati*.

Na recent number of the Philosophical Magazine, Dr. Paul Sacerdote† has published a critique of a former paper by met, "On the Supposed Elongation of a Dielectric in an Electrostatic Field," in which I stated that I had been unable to notice any change in length in glass and hard rubber-tubes when they were subjected to an intense electrostatic strain after extraneous effects had been eliminated. Believing my experiments to have been carefully performed I concluded that, if the effect exists at all, it must be excessively small, and that the results of former investigators were perhaps due, for the most part, to extraneous causes. Dr. Sacerdote argues that this conclusion is unjustifiable, for, "By the very disposition of his experiments the tube should not experience any appreciable elongation; thus the negative result of his experiments merely proves that they were carefully performed."

At the time I wrote my paper, I unfortunately was not able to consult the recent and important work of Dr. Sacerdote and of Professors Cantone and Sozzani¶. It is on the experiments of these writers that Dr. Sacerdote bases his theory and his criticism given above. He agrees with me in this, that the results of Righi, Quincke, and all former investigators are inaccurate and much too large. He is, however, satisfied that Prof. Cantone's results are conclusive in proving the existence of the effect and the establishment of its law.

* Communicated by the Author.

Phil. Mag. [6] vol. i. pp. 357–359 (1901).

Ibid. pp. 198-210, August 1900.

Annales de Phys. et Chem. sér. 7, t. xx. p. 289; Journ. de Phys.

sér. 3, t. viii. Sept.-Oct. (1899).

| Rend. d. R. Acc. dei Linc. ser. 4, vol. iv. pp. 344, 471.

¶ Rend. d. R. Ist. Lomb. ser. 2, vol. xxxiii. (1900).