90 FRAGMENTS OF SCIENCE. 
By operating upon the fumes of chloride of ammonium, 
the smoke of bro\vu 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 Wheatstoue 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 tiot 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 beam was 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 
* " Ctomptes Rend us," tome li. pp. 360 and 669, 
