1869.] On the Blue Colour of the Sky, and the Polarization of Light. 223 



IV. "On the Blue Colour of the Sky, the Polarization of Sky- 

 light, and on the Polarization of Light by Cloudy matter gene- 

 rally." By John Tyndall, LL.D., F.R.S. Received December 

 16, 1868. 



Since the communication of my brief abstract " On a new Series of Che- 

 mical Reactions produced by Light," the experiments upon this subject 

 have been continued, and the number of the substances thus acted on 

 considerably augmented. New relations have also been established be- 

 tween mixed vapours when subjected to the action of light. 



I now beg to draw the attention of the Royal Society to two 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 

 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 ac- 

 tivity 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 sufficiently 

 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 darkness, it only being illuminated by a pow- 

 erful beam of light. This blue cloud differs in many important particulars 

 from the finest ordinary clouds, and might justly have assigned to it an in- 

 termediate position between these clouds and true cloudless vapour. 



