90 FRAGMENTS OF SCIENCE. 



By operating upon the fumes of chloride of ammonium, 

 the smoke of brown paper, and tobacco-smoke, I had va- 

 ried and confirmed in many ways those experiments on 

 neutral points, when my attention was drawn by Sir 

 Charles Wheatstone to an important observation communi- 

 cated to the Paris Academy in 1860 by Professor Govi, of 

 Turin.* M. Govi had been led to examine a beam of light 

 sent through a room in which were successively diffused 

 the smoke of incense, and tobacco-smoke. His first brief 

 communication stated the fact of polarization by such 

 smoke; but in his second communication he announced 

 the discovery of a neutral point in the beam, at the op- 

 posite sides of which the light was polarized 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 maximum polar- 

 ization in M. Govi's experiment enclosed a very small 

 angle with the axis of the illuminating beam. The ques- 

 tion was left in this condition, and I am not aware that M. 

 Govi or any other investigator has pursued it further. 



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

 in the experimental tube became denser, the polarization 

 of the light discharged at right angles to the beam became 

 weaker, the direction of maximum polarization becoming 

 oblique to the beam. Experiments on the fumes of chlo- 

 ride 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 following 

 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 beamwas sent 

 through the smoke. From the track of the beam polarized 

 light was discharged; but the direction of maximum polar- 

 ization, instead of being perpendicular, now enclosed an 

 angle of only 12 degrees or 13 degrees with the axis of the 

 beam. 



A neutral point, with complementary effects at opposite 

 sides of it, was also exhibited by the beam. The angle en- 

 closed by the axis of the beam, and a line drawn from the 



" Coraptes Rend us," tome li. pp. 360 and 669. 



