434 Prof. J. C. Bose. On the Selective Conductivity 



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 respec- 

 tively. 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 measure- 

 ment. 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 meg- 

 ohms. 



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 2'56x2'56 cm., with a thickness of 1/1 cm. B is- 

 276 x 2-76x1-2 cm. 



For the determination of resistances I used a sensitive Kelvin gal- 

 vanometer, having a resistance of 7000 ohms. With three Leclanche 

 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 (CaS0 4 ), popularly known as Satin- 

 spar, also exhibits double absorption ; and in this case, too, the con- 

 ductivity in the direction of absorption is found to be very much 

 greater than in that of transmission. 



