86 FRAGMENTS OF SCIENCE. 
In all cases, and with all substances, the cloud formed at 
the commencement, when the precipitated particles are 
sufficiently fine, is Hue, and it can be made to display a 
color rivaling that of the purest Italian sky. In all cases, 
moreover, this fine blue cloud polarizes perfectly the beam 
which illuminates it, the direction of polarization enclosing 
an angle of 90 degrees with the axis of the illuminating 
beam. 
It is exceedingly interesting to observe both the perfection 
and the decay of this polarization. For ten or fifteen 
minutes afte/its first appearance the light from a vividly 
illuminated actinic cloud, looked at perpendicularly, is 
absolutely quenched by a Nicolas prism with its longer 
diagonal vertical. But as the sky-blue is gradually 
rendered impure by the growth of the particles in other 
words, as real clouds begin to be formed the polarization 
begins to decay, a portion of the light passing through the 
prism in all its positions. It is worthy of note, that for 
some time after the cessation of perfect polarization, the 
residual light which passes, when the Nicol is in its position 
of minimum transmission, is of a gorgeous blue, the whiter 
light of the cloud being extinguished.* When the cloud 
texture has become sufficiently coarse to approximate to 
that of ordinary clouds, the rotation of the Nicol ceases to 
have any sensible effect on the quantity of light discharged 
normally. 
The perfection of the polarization, in a direction per- 
pendicular to the illuminating beam, is also illustrated by 
the following experiment: A Nicol's prism, large enough 
to embrace the entire beam of the electric lamp, was 
placed between the lamp and the experimental tube. A 
few bubbles of air, carried through the liquid nitrite of 
butyl were introduced into the tube and they were followed 
by about three inches (measured by the mercurial gauge) 
of air which had passed through aqueous hydrochloric 
acid. Sending the polarized beam through the tube, I 
placed myself in front of it, my eye being on a level with 
its axis, my assistant occupying a similar position behind 
the tube. The short diagonal of the large Nicol was in 
the first instance vertical, the plane of vibration of the 
* This shows that particles too large to polarize the blue, polarize 
perfectly light of lower refrangibilit}-. 
