toward the Cold. 363 



blunt point, such as that of a glass rod, the camphor vapour is 

 afterwards deposited along the marks. A fact of this kind was 

 pointed out more than a century ago by De Mairan*, who noticed 

 that the hoar-frost was deposited on his window-panes in spiral 

 lines, produced, as he supposed, by the fine sand or ashes used 

 in cleaning the windows ; and although these lines were not 

 visible to the eye, they nevertheless formed ridges sufficiently 

 prominent to catch the condensing vapour. To test this idea, 

 Carenat, in the severe winter of 1814, cleaned four panes of his 

 window with fine sand, rubbing two of them with a circular 

 movement, a third in straight lines from top to bottom, and a 

 fourth in diagonal lines. On the next day and on several suc- 

 ceeding days the hoar-frost was deposited more or less on the 

 lines or furrows produced by the friction. 



In the older memoirs and treatises on dew, the writers are very 

 fond of ornamenting their work with pleasing figures of dew and 

 hoar-frost as it appears on various objects ; and they attribute the 

 differences in the patterns of the watery particles to differences in 

 the radiating powers of the objects in question. It does not seem 

 to have been suspected that the varied patterns also depend on the 

 want of adhesion to the surface. When this is chemically clean we 

 get, not dew, but a sheet of water. All objects exposed to the 

 air contract an organic film ; and when the inner surface of a 

 bottle, for example, as in Dr. Draper's experiment, is rubbed 

 with a blunt point, the film is raised into ridges which catch the 

 camphor vapour more readily than the other parts of the sur- 

 face J. Indeed so sensitive is this vapour in detecting promi- 

 nences, that it will attach itself to, and render visible minute 

 objects on the inner surface of the bottle which before were 

 quite invisible. For example, I have many times washed out a 

 bottle and wiped it dry with a clean cloth. The bottle appeared 

 to be perfectly clean to the eye ; yet after a few minutes' expo- 

 Sure to the sun, with its charge of crude camphor, innumerable 

 filaments, evidently derived from the duster, would start into 

 view from being coated with a very thin layer of camphor. 



In a small volume recently published, entitled " Experimental 

 Essays/' I give some specimens of camphor-figures modified by 

 the presence of vapour of benzole, wood-spirit, naphtha, chloro- 

 form, nitric acid, &c. One of the actions of these liquids is 

 apparently to modify the rate of evaporation of the camphor. 



* Dissertation sur la Glace. Paris, 1/49. 



f Memoires de V Academie Roy ale des Sciences de Turin pour les annexes 

 1813-14. 



X Most of the phenomena of breath-figures (the figures roriques of the 

 French, and the Hauch-Figuren of the Germans) depend on the existence 

 of this film. 



