86 FRA OMENTS OF SCIENCE. 



In all cases, and with all substances, the cloud formed at 

 the commencement, when the precipitated particles are 

 sufficiently fine, is blue, 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 after its first appearance the light from a vividly 

 illuminated actinic cloud, looked at perpendicularly, is 

 absolutely quenched by a NicoFs 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 eifect 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, 1 

 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 refrangibility. 



