a* FRAGMENTS OF SCIENCE. 
C/v 
with cobweb or with gauze would be to liken them ta 
something infinitely grosser than themselves. 
In all cases a distant candle-flame, when looked at 
through the cloud, was sensibly undimmed. 
2. OK 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 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, 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 
%$ 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. 
