114 FRAGMENTS OF SCIENCE 



round the axis of the decomposing beam; it was nipped 

 at certain places like an hour-glass, and round the two 

 bells of the glass delicate cloud-filaments twisted them- 

 selves 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 vapor a similar lustre was 

 most exquisitely shown. With a suitable disposition of 

 the light, the purple hue of iodine-vapor 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 marvellous transpar- 

 ency to such heat. May not its synchronism 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 va- 

 por 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 com- 

 mencement. After some minutes' exposure, however, 

 clouds begin to form, which grow in density and in 

 beauty as the light continues to act. In every experiment 

 hitherto made with this substance the column of cloud 

 filling the experimental tube was divided into two dis- 

 tinct parts near the middle of the tube. In one experi- 

 ment a globe of cloud formed at the centre, from which, 



