DECOMPOSITION BY LIGHT. 107 



With a suitable disposition of the light, the purple hue 

 of iodine-vapour came out very strongly in the tube. 



The remark already made, as to the bearing of the 

 decomposition of nitrite of amyl by light on the 

 question of molecular absorption, applies here also ; for 

 were the absorption the work of the molecule as a whole, 

 the iodine would not be dislodged from the allyl with 

 which it is combined. The non-synchronism of iodine 

 with the waves of obscure heat is illustrated by its mar- 

 vellous transparency to such heat. May not its syn- 

 chronism with the waves of light in the present instance 

 be the cause of its divorce from the allyl ? 



Iodide of Isopropyl. The action of light upon the 

 vapour of this liquid is, at first, more languid than upon 

 iodide of allyl ; indeed many beautiful reactions may 

 be overlooked, in consequence of this languor at the 

 commencement. After some minutes' exposure, how- 

 ever, clouds begin to form, which grow in density and in 

 beauty as the light continues to act. In every experi- 

 ment hitherto made with this substance the column of 

 cloud filling the experimental tube, was divided into 

 two distinct parts near the middle of the tube. In one 

 experiment a globe of cloud formed at the centre, from 

 which, right and left, issued an axis uniting the globe 

 with two adjacent cylinders. Both globe and cylinders 

 were animated by a common motion of rotation. As 

 the action continued, paroxysms of motion were mani- 

 fested ; the various parts of the cloud would rush through 

 each other with sudden violence. During these motions 

 beautiful and grotesque cloud-forms were developed. 

 At some places the nebulous mass would become ribbed 

 so as to resemble the graining of wood ; a longitudinal 

 motion would at times generate in it a series of curved 

 transverse bands, the retarding influence of the sides 

 of the tube causing an appearance resembling, on a 



