120 FEAGMENTS OF SCIENCE. 



fessor Grovi, of Turin. 1 M. Grovi had been led to examine 

 a beam of light sent through a room in which were suc- 

 cessively diffused the smoke of incense, and tobacco- 

 smoke. His first brief communication stated the fact 

 of polarisation by such smoke ; but in his second com- 

 munication he announced the discovery of a neutral 

 point in the beam, at the opposite sides of which the 

 light was polarised in planes at right angles to each 

 other. 



But unlike my observations on the laboratory air, 

 and unlike the action of the sky, the direction of maxi- 

 mum polarisation in M. Gfovi's experiment enclosed a 

 very small angle with the axis of the illuminating beam. 

 The question was left in this condition, and I am not 

 aware that M. Grovi or any other investigator has pur- 

 sued it further. 



I had noticed, as before stated, that as the clouds 

 formed in the experimental tube became denser, the 

 polarisation of the light discharged at right angles to 

 the beam became weaker, the direction of maximum 

 polarisation becoming oblique to the beam. Experi- 

 ments on the fumes of chloride of ammonium gave me 

 also reason to suspect that the position of the neutral 

 point was not constant, but that it varied with the 

 density of the illuminated fumes. 



The examination of these questions led to the follow- 

 ing new and remarkable results : The laboratory being 

 well filled with the fumes of incense, and sufficient time 

 being allowed for their uniform diffusion, the electric 

 beam was sent through the smoke. From the track of 

 the beam polarised light was discharged ; but the direc- 

 tion of maximum polarisation, instead of being perpen- 

 dicular, now enclosed an angle of only 12 or 13 with 

 the axis of the beam. 



1 ' Comptes Eendus,' tome li. pp. 360 and 66?. 



