98 



Prof. Tyndall on a New Series of [Recess, 



Iodide of Allyl (boiling-point 101° C). — 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 

 of allyl I have employed both oxygen and hydrogen, as well as air, as a 

 vehicle, and found the eifect in all cases substantially the same. The 

 cloud column here was exquisitely beautiful, but its forms were different 

 from those of the nitrite of amyl. The whole column revolved 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 vapour a similar lustre was most exquisitely 

 shown. 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 illus- 

 trated by its marvellous transparency 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 ? Further experiments on this point are in preparation. 



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, 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 which filled 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 which 

 united the globe with the two adjacent cylinders. Both globe and cy- 

 linders were animated by a common motion of rotation. As the action 

 continued, paroxysms of motion were manifested ; 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 small scale, the 

 dirt-bands of the Mer de Glace. In the anterior portion of the tube those 

 sudden commotions were most intense ; here buds of cloud would sprout 



