NEW CHEMICAL REACTIONS. S3 
ness, forming particles distinguishable by the naked eye, 
or far beyond the reach of our highest microscopic powers. 
I have no reason to doubt that particles may be thus 
obtained, whose diameters constitute but a small fraction 
of the length of a wave of violet light. 
In all cases when the vapors of the liquids employed are 
sufficiently attenuated, no matter what the liquid may be, 
the visible action commences with the formation of a" blue 
cloud. But here I must guard myself against all miscon- 
ception as to the use of this term. The " cloud " here 
referred to is totally invisible in ordinary daylight. To 
be seen, it requires to be surrounded by darkness, it only 
being illuminated by a powerful beam of light. This blue 
cloud differs in many important particulars from the finest 
ordinary clouds, and might justly have assigned to it an 
intermediate position between such clouds and true vapor. 
With this explanation, the term "cloud," or "incipient 
cloud/' or "actinic cloud," as I propose to employ it, can- 
not, I think, be misunderstood. 
I had been endeavoring to decompose carbonic acid gas 
by light. A faint bluish cloud, due it may be, or it may 
not be, to the residue of some vapor previously employed, 
was formed in the experimental tube. On looking across 
this cloud through a NicoFs prism, the line of vision being 
horizontal, it was found that when the short diagonal of 
the prism was vertical, the quantity of light reaching the 
eye was greater than when the long diagonal was vertical. 
When a plate of tourmaline was held between the eye and 
the bluish cloud, the quantity of light reaching the eye 
when the axis of the prism was perpendicular to the axis 
of the illuminating beam, was greater than when the 
axis of the crystal and of the beam were parallel to each 
other. 
This was the result all round the experimental tube. 
Causing the crystal of .tourmaline to revolve round the tube, 
with its axis perpendicular to the illuminating beam, the 
quantity of light that reached the eye was in all its positions 
a maximum. AVheu the crystallographic axis was parallel 
to the axis of the beam, the quantity of light transmitted 
by the crystal was a minimum. From the illuminated 
bluish cloud, therefore, polarized light was discharged, the 
direction of maximum polarization being at right angles to 
