﻿of the 8/cy, and on the Polarization of Light. 385 



questions glanced at incidentally in the abstract referred to — the blue 

 colour of the sky, and the polarization of skylight. Reserving the 

 historic treatment of the subject for a more fitting occasion, I would 

 merely mention now that these questions constitute, in the opinion 

 of our most eminent authorities, the two great standing enigmas of 

 meteorology. Indeed it was the interest manifested in them by Sir 

 John Herschel, in a letter of singular speculative power, that caused 

 me to enter upon the consideration of these questions so soon. 



The apparatus with which I work consists, as already stated to 

 the Society, of a glass tube about a yard in length, and from 2\ to 3 

 inches in internal diameter. The vapour to be examined is introduced 

 into this tube in the manner described in my last abstract, and 

 upon it the condensed beam of the electric lamp is permitted to act 

 until the neutrality or the activity of the substance has been declared. 



It has hitherto been my aim to render the chemical action of light 

 upon vapours visible. For this purpose substances have been chosen, 

 one at least of whose products of decomposition under light shall have 

 a boiling-point so high that as soon as the substance is formed it 

 shall be precipitated. By graduating the quantity of the vapour, 

 this precipitation may be rendered of any degree of fineness, forming 

 particles distinguishable by the naked eye, or particles which are 

 probably 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 very small fraction of the length of 

 a wave of violet light. 



In all cases when the vapours of the liquids employed are suffi- 

 ciently attenuated, no matter what the liquid may be, the visible 

 action commences with the formation of a blue cloud. I would guard 

 myself at the outset against all misconception as to the use of this 

 term. The blue cloud to which I here refer is totally invisible in 

 ordinary daylight. To be seen, it requires to be surrounded by dark- 

 ness, it only being illuminated by a powerful beam of light. This 

 blue cloud differs in many important particulars from the finest ordi- 

 nary clouds, and might justly have assigned to it an intermediate po- 

 sition between these clouds and true cloudless vapour. 



"With this explanation, the term "cloud," or " incipient cloud," as 

 I propose to employ it, cannot, I think, be misunderstood. 



I had been endeavouring 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 vapour previously employed, was formed in the experi- 

 mental tube. On looking across this cloud through a Nicol's 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 axes 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 perpen- 



Phil. Mag. S. 4. Vol. 37. No. 250. May 1869. 2 G 



