362 RECENT PROGRESS IN PHYSICS. 



touching the pitch was electrified like the ball : too strong an accumu- 

 lation of electricity was prevented by providing the stamp with a point. 

 After the stamp had been exposed from 20 to 30 minutes to the induc- 

 tive action of the ball, a clear dust image appeared without any dust 

 figure, but irregular spots ajjpeared in the ground, which were not of 

 the color of the image. 



Similar results were obtained when the stamp was placed for several 

 hours in connexion with one pole of a powerful dry pile, while the i 

 electricity of the other pole was conducted off as completely as possible, t 



In these cases, in which generally no dust figures appeared, it was i 

 indifferent whether the stamp was insulated or not, on its removal. 



The color of the irregular spots showed that they originated in the i 

 electricity actually passing from the stamp to the pitch plate at the : 

 places which admitted of a slight current. To avoid these, more ready 

 passage to a conducting medium must be furnished for this electricity, , 

 as in the case when the dust images were produced in rarified air. i 

 Kiess obtained in this manner the most perfect dust images. 



The dust Jigtii^es and images, just considered, are, according to Riess, ; 

 primary electrical delineations ; the figures and images now to be con- i 

 sidered are secondary electrical delineations. 



§ 75. Electrical breath figures. — The surface of glass, mica, &c., . 

 over which an electrical discharge stroke has passed, gives, by breathing 

 upon it, peculiar ramified figures, which stand out from the surface 

 obscured by the breath with a mirror-like lustre. 



The breath figure indicates the path taken by the electrical dis- 

 charge over the surface ; and its form difiers therefore, according to 

 the nature of this surface. On metal, it appears as a round disk ; on 

 resin, as serpentine stripes ; on mica, as fine, many times ramified 

 lines. 



The breath figure is independent upon the hind of electricity em- 

 ployed. 



That these figures do not originate in the electricity which continues 

 to adhere to the surface is established by the fact that they are seen 

 on metallic surfaces, on which they appear after the breathing, as 

 distinct circles, surrounded by more or less obscure rings ; the breath i 

 figures also appear a long time after the discharge stroke has passed i 

 over the surface, or after the surface has been passed over the flame of > 

 a spirit lamp. Hence, the breath figures cannot be owing to adhering ■ 

 electricity ; they are to he ascribed to a change of surface ivhich the sub- 

 stance used has been subjected to, by the electrical discharge. 



On a fresh surface of mica, that is on such as is obtained by a fresh 

 cleavage, breath figures do not appear. This depends upon a peculiar i 

 property of fresh mica surface, which Riess has described in the 67th 

 volume of Poggendorf's Annalen, page 354. 



A clean plate of mica being breathed on, or held over evaporating 

 water, the result is, as with all bodies, that it will be covered with a 

 rapidly disappearing stratum of water, consisting of very small drops, 

 which are not in contact with each other. 



But when the mica has received a fresh surface by cleavage, it re- 

 mains perfectly clear, shining and transparent after being breathed on. 



This phenomenon is by no means owing to the fresh surface not 



