Page:Proceedings of the Royal Society of London Vol 60.djvu/466

434 they, too, exhibited unequal conductivities in the two directions of absorption and transmission.

Nemalite, unfortunately, is difficult to obtain, and the specimens I could get here were too small to make the necessary measurements. I have, however, in my possession two specimens which I brought from India; of these, one is a perfect specimen of a fair size, and I obtained with it strong polarisation effects. The second piece is not as good as the first, and rather small in size. I cut from this latter piece a square of uniform thickness, the adjacent sides of the square being parallel to the directions of transmission and absorption respectively. The resistances of equal lengths in the two directions (with the same cross section) were now measured.

The first specimen I gave to Messrs. Elliott Brothers for measurement. They informed me, on the 13th of October last, that the resistance in the direction of transmission was found to be 35,000 megohms, and that in the direction of absorption, only 14,000 megohms.

It will thus be seen that the direction of absorption is also the direction of greatest conductivity, and the direction of transmission is the direction of least conductivity.

My anticipations being thus verified, I proceeded to make further measurements with other specimens. From the perfect specimen of nemalite in my possession, I cut two square pieces, A and B. The size of piece A is 2·56 X 2·56 cm., with a thickness of 1·1 cm. B is 2·76 X 2·76 X 1·2 cm.

For the determination of resistances I used a sensitive Kelvin galvanometer, having a resistance of 7000 ohms. With three Leclanché cells, 1·4 volt each, and an interposed resistance equivalent to 55,524 megohms, a deflection of 1 division in the scale reading was obtained. The following table (p. 435) gives the results of the measurements which I carried out.

The results given clearly show how the difference of absorption in the two directions is related to the corresponding difference in conductivity.

I then proceeded to make measurements with chrysotile. The specimens I could obtain were not very good. I cut two from the same piece, and a third specimen was obtained from a different variety. The ratios of conductivities found in the three specimens were 1 : 10, 1 : 9, and 1 : 4 respectively. In every case the direction of absorption was found to be the direction of maximum conductivity.

[A fibrous variety of gypsum (CaSO4), popularly known as Satin-spar, also exhibits double absorption; and in this case, too, the conductivity in the direction of absorption is found to be very much greater than in that of transmission.