HO FEAGMENTS OF SCIENCE. 



this subject were continued, the number of substances 

 thus acted on being considerably increased. 



I now, however, beg to direct attention to two ques- 

 tions glanced at incidentally in the preceding pages 

 the blue colour of the sky, and the polarization of sky 

 light. 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, addressed to myself, 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 vapour to be 

 examined 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 neutral- 

 ity 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 boil- 

 ing-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 far beyond the reach of our highest 

 microscopic powers. I have no reason to doubt that 

 particles may be thus obtained, whose diameters con- 

 stitute but a small fraction ff the length of a wave of 

 violet light. 



In all cases when the vapours of the liquids em- 



