RECENT PROGRESS IN PHYSICS. 363 



condensing vapor of water, for the breath causes it to show the colors 

 'of thin plates ; it is consequently covered with a coherent stratum of 

 'water. 



' A drop of water which stands at rest on an old surface of mica at 

 ■once spreads on a fresh surface, and completely covers it. Hence, a 

 :mica surface made by cleavage possesses, in consequence of its great 

 [purity, so great an attraction for the vapor of water that it condenses 

 'the water into a coherent stratum, while, had the mica been exposed 

 'a long-time to the air, it would have condensed the water in separate 

 'drops. 



While an old surface of mica is an excellent insulator of electricity 

 a, fresh surface discharges an electroscope in a few seconds ; it acts 

 Ihygroscopically by condensing the vapor of water of the atmosphere 

 into a coherent stratum, which conducts electricity. 



This remarkable peculiarity of fresh mica is preserved but a short 

 ftime in the air ; in a few days it may be clouded by breathing upon it. 

 f Very powerful electrical discharges produce not only a change in 

 lithe film of foreign matter covering the body, but they alter the surface 

 lof the body itself. This is the cause of the traces noticed in § 41, occa- 

 isioned by the discharge spark on glass and mica (electrical colored 

 istripes) and of the rings of Priestley , which occur when numerous dis- 

 charges of a battery take place between a point and a polished metallic 

 jSurface, whereby oxidation of the metal forms many colored concentric 

 circles. 



§76. Karsten's Electrical Figures. — The analogy which Eiess de- 

 tecribes in the VI volume of Dove's Repertorium der Pliysih, between 

 lelectrical breath figures and the images of Moser, occasioned Karsten 

 rto examine whether such images could not be obtained in the electrical 

 way. 



I For this purpose he placed (Pog. Ann., LVII, 492) a coin on a 

 imirror, resting on a discharging metal plate, and caused sparks to 

 Istrike from the conductor of the machine upon the coin, thence passing 

 !lto the metal plate, (around the edge of the glass.) After 100 revo- 

 llutions of the machine the coin was removed ; the glass plate seemed 

 jwhoUy unchanged, but when breathed upon the image of the coin 

 ^appeared distinctly. 



' Besides the memoir cited, Karsten has published two others, in 

 ;Poggeudorf 's Annalen, (LVIII, 115, and LX, 1,) on electrical images, 

 •but as he has not succeeded in discovering their true nature, it is un- 

 jnecessary to go further into the details of these memoirs ; and the more, 

 jsince Riess, as we shall see, has correctly ascertained the condition for 

 ^producing electrical images. The report upon Riess' researches will 

 ^therefore suffice to bring the facts at least, to the knowledge of the 

 (reader. 



We must, however, briefly notice, by the way, Karsten's last treatise 

 in one particular. In the beginning he adduces many experiments 

 'which have been made to explain the cause of Moser's images ; besides 

 iMoser's own theory, he presents the opinion of Hunt, Know, Fizeau, 

 'Daguerre, Masson and Moore. Why is the excellent work of Waideles 

 fon this subject ignored ? it appears in the first half of the 59th vol- 

 ■ume of Poggendorf 's Annalen, and after these images had been the 



