On the Selective Conductivity of Polarising Substances. 433 



" On the Selective Conductivity exhibited by certain Polarising 

 Substances." By JAGADIS CHUNDER BOSE, M.A., D.Sc., 

 Professor of Physical Science, Presidency College, Calcutta. 

 Communicated by Lord RAYLEIGH, F.R.S. Received 

 January 14, Read January 28, 1897. 



In my paper " On the Polarisation of Electric Rays by Double- 

 refracting Crystals " (vide ' Journal of the Asiatic Society of Bengal,' 

 May, 1895), and in a subsequent paper " On a New Electro-Polari- 

 scope" (' Electrician,' 27th December, 1895), I have given accounts 

 of the polarising property of various substances. Amongst the most 

 efficient polarisers may be mentioned nemalite and chrysotile. Nemalite 

 is a fibrous variety of brucite. In its chemical composition it is a 

 hydrate of magnesia, with a small quantity of protoxide of iron and 

 carbonic acid. This substance is found to absorb very strongly 

 electric vibrations parallel to its length, and transmit those that are 

 perpendicular to the length. I shall distinguish the two directions 

 as the directions of absorption and transmission. Chrysotile is a 

 fibrous variety of serpentine. In chemical composition it is a hydrous 

 silicate of magnesia. Like nemalite, it also exhibits selective absorp- 

 tion, though not to the same extent. The transmitted vibrations are 

 perpendicular, and those absorbed parallel to the length. Different 

 varieties of these substances exhibit the above property to a greater 

 or less extent. I have recently obtained a specimen of chrysotile with 

 a thickness of only 2'5 cm. ; this piece completely polarises the trans- 

 mitted electric ray by selective absorption. 



The action of these substances on the electric ray is thus similar to 

 that of tourmaline on light. It may be mentioned here that I found 

 tourmaline to be an inefficient polariser of the electric ray ; it does 

 transmit the ordinary and the extraordinary rays with unequal 

 intensities, but even a considerable thickness of it does not completely 

 absorb one of the two rays. 



In Hertz's polarising gratings, electric vibrations are transmitted 

 perpendicular to the wires, the vibrations parallel to the wires being 

 reflected or absorbed. Such gratings would be found to exhibit 

 electric anisotropy, the conductivity in the direction of the wires 

 being very much greater than the conductivity across the wires. 

 The vibrations transmitted through the gratings are thus perpen- 

 dicular to the direction of maximum conductivity or parallel to the 

 direction of greatest resistance. The vibration absorbed is parallel 

 to the direction of maximum conductivity. 



As the nemalite and chrysotile polarised the electric ray by unequal 

 absorption in the two directions, I was led to investigate whether 



