82 FRA QMENTS V SCIENCE. 



with cobweb or with gauze would be to liken them to 

 something infinitely grosser than themselves. 



In all cases a distant candle-flame, when looked at 

 through the cloud, was sensibly undimmed. 



2. ON THE BLUE COLOR OF THE SKY, AND THE POLAR- 

 IZATION OF SKYLIGHT.* 



1869. 



After the communication to the Royal Society of the 

 foregoing brief account of a new Series of Chemical 

 Reactions produced by Light, the experiments upon this 

 subject were continued, the number of substances thus 

 acted on being considerably increased. 



I now, however, beg to direct attention to two questions 

 glanced at incidentally in the preceding pages the blue 

 color of the sky, and the polarization of skylight. Reserv- 

 ing the historic treatment of the subject fora more fitting 

 occasion, I would merely mention now that these question's 

 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, addressed to my- 

 self, that caused me to enter upon the consideration of 

 these questions so soon. 



The apparatus with which I work consists, as already 

 stated, of a glass tube about a yard in length, and from 

 2$ to 3 inches internal diameter. The vapor to be ex- 

 amined is introduced into this tube in the manner already 

 described, 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 vapors visible. For this purpose sub- 

 stances 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 vapor, 

 this precipitation may be rendered of any degree of fine- 



* In my " Lectures on Light" (Longmans), the polarization of light 

 will be found briefly, but, 1 trust, clearly explained. 



