DECOMPOSITION BY LIGHT 113 



passage through which the dry air flowed into the experi- 

 mental tube. 



In this case, the electric beam traversed the tube for 

 several seconds before any action was visible. Decomposi- 

 tion then visibly commenced, and advanced slowly. When 

 the light was very strong, the cloud appeared of a milky 

 blue. When, on the contrary, the intensity was moderate, 

 the blue was pure and deep. In Briicke's important ex- 

 periments on the blue of the sky and the morning and 

 evening red, pure mastic is dissolved in alcohol, and then 

 dropped into water well stirred. When the proportion of 

 mastic to alcohol is correct, the resin is precipitated so 

 finely as to elude the highest microscopic power. By 

 reflected light, such a medium appears bluish, by trans- 

 mitted light yellowish, which latter color, by augmenting 

 the quantity of the precipitate, can be caused to pass into 

 orange or red. 



But the development of color in the attenuated nitrite- 

 of-amyl vapor is doubtless more similar to what takes 

 place in our atmosphere. The blue, moreover, is far 

 purer and more sky-like than that obtained from Briicke's 

 turbid medium. Never, even in the skies of the Alps, 

 have I seen a richer or a purer blue than that attainable 

 by a suitable disposition of the light falling upon the pre- 

 cipitated vapor. 



Iodide of Allyl. Among the liquids hitherto subjected 

 to the concentrated electric light, iodide of allyl, in point 

 of rapidity and intensity of action, comes next to the 

 nitrite of amyl. With the iodide I have employed both 

 oxygen and hydrogen, as well as air, as a vehicle, and 

 found the effect in all cases substantially the same. The 

 cloud-column here was exquisitely beautiful. It revolved 



