106 FRAGMENTS OF SCIENCE. 



a milky blue. When, on the contrary, the intensity 

 was moderate, the blue was pure and deep. In Briicke's 

 important experiments 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 transmitted light yellowish, which 

 latter colour, by augmenting the quantity of the pre- 

 cipitate, can be caused to pass into orange or red. 



But the development of colour in the attenuated 

 nitrite-of-amyl vapour is doubtless more similar to 

 what takes place in our atmosphere. The blue, more- 

 over, 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 precipitated vapour. 



Iodide of Allyl. Among the liquids hitherto sub- 

 jected 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 em- 

 ployed 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 round the axis of the decom- 

 posing beam ; it was nipped at certain places like an 

 hour-glass, and round the two bells of the glass delicate 

 cloud-filaments twisted themselves in spirals. It also 

 folded itself into convolutions resembling those of shells. 

 In certain conditions of the atmosphere in the Alps I 

 have often observed clouds of a special pearly lustre ; 

 when hydrogen was made the vehicle of the iodide-of- 

 allyl vapour a similar lustre was most exquisitely shown, 



