108 FRAGMENTS OF SCIENCE. 



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 

 forth, and grow in a few seconds into perfect flower-like 

 forms. The cloud of iodide of isopropyl had a character 

 of its own, and differed materially from all others that I 

 had seen. A gorgeous mauve colour was observed in 

 the last twelve inches of the tube ; the vapour of iodine 

 was present, and it may have been the sky-blue scat- 

 tered by the precipitated particles which, mingling with 

 the purple of the iodine, produced the mauve. As in 

 all other cases here adduced, the effects were proved to 

 be due to the light ; they never occurred in darkness. 



The forms assumed by some of those actinic clouds, 

 as I propose to call them, in consequence of rotations 

 and other motions, due to differences of temperature, 

 are perfectly astounding. I content myself here with a 

 meagre description of one more of them. 



The tube being filled with the sensitive mixture, the 

 beam was sent through it, the lens at the same time 

 being so placed as to produce a cone of very intense 

 light. Two minutes elapsed before anything was 

 visible ; but at the end of this time a faint bluish 

 cloud appeared to hang itself on the most concentrated 

 portion of the beam. 



Soon afterwards a second cloud was formed five 

 inches farther down the experimental tube. Both 

 clouds were united by a slender cord of the same bluish 

 tint as themselves. 



As the action of the light continued, the first cloud 

 gradually resolved itself into a series of parallel disks of 

 exquisite delicacy, which rotated round an axis perpen- 

 dicular to their surfaces, and finally blended to a screw 

 surface with an inclined generatrix. This gradually 

 changed into a filmy funnel, from the narrow end of 

 which the ' cord ' extended to the cloud in advance. 



