1868.] Chemical Reactions produced by Light. 97 



chlorine itself is molecular and not monatomic. Other cases of this kind 

 I hope, at no distant day, to bring before the Royal Society. 



Production of Sky -blue by the decomposition of Nitrite of Amy I . 



AYhen the quantity of nitrite vapour is considerable, and the light in- 

 tense, the chemical action is exceedingly rapid, the particles precipitated 

 being so large as to whiten the luminous beam. Not so, however, when a 

 well-mixed and highly attenuated vapour fills the experimental tube. The 

 effect now to be described was obtained in the greatest perfection when the 

 vapour of the nitrite was derived from a residue of the moisture of its 

 liquid, which had been accidentally introduced into the passage through 

 which the dry air flowed into the experimental tube. 



In this case the electric beam traversed the tube for several seconds be- 

 fore any action was visible. Decomposition then visibly commenced, and 

 advanced slowly. The particles first precipitated were too small to be dis- 

 tinguished by an eye-glass ; and, when the light was very strong, the cloud 

 appeared of a milky blue. AVhen, on the contrary, the intensity was mode- 

 rate, 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 precipitate, can be caused to pass into orange or 

 red. 



But the development of colour in the attenuated nitrite-of-amyl vapour, 

 though admitting of the same explanation, is doubtless more similar to 

 what takes place in our atmosphere. The blue, moreover, is purer and 

 more sky -like than that obtained from Briicke's turbid medium. There 

 could scarcely be a more impressive illustration of Newton's mode of re- 

 garding the generation of the colour of the firmament than that here ex- 

 hibited ; for 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. May not the aqueous vapour of our 

 atmosphere act in a similar manner ? and may we not fairly refer to liquid 

 particles of infinitesimal size the hues observed by Principal Forbes over 

 the safety-valve of a locomotive, and so skilfully connected by him with the 

 colours of the sky ? 



In exhausting the tube containing the mixed air and nitrite-of-amyl 

 vapour, it was difficult to avoid explosions under the pistons of the air- 

 pump, similar to those which I have already described as occurring with 

 the vapours of bisulphide of carbon and other substances. Though the 

 quantity of vapour present in these cases must have been infinitesimal, its 

 explosion was sufficient to destroy the valves of the pump. 



vol. xvn. H 



