110 FKAGMENTS 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 polarisation 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, 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 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 pro- 

 ducts 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 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 of the length of a wave of 

 violet light. 



In all cases when the vapours of the liquids em- 



