RECENT PROGRESS IN PHYSICS. 365 



iiisiilating plates, which are therefore in the condition to produce dust 

 ■figures, oiten even dust images. 



; By simply eltctritying the stamp, the arrangement being the same 

 las for producing dust images, no breath image appears. The alter- 

 nate charge and discharge of the stamp are essentially necessary for 

 ithe formation of these images. 



I By laying a plate of mica on a pitch plate, and placing a metal 

 "stamp on this^ a double discharge of the same kind of electricity takes 

 'place in the same direction in electrifying the stamp, namely, from 

 the stamp to the upper surface of the mica, and from the under sur- 

 'face of the mica to the pitch plate. When a spark is communicated 

 to the stamp from a positively charged jar, the pitch surface, when 

 Sdusted, shows a yellow image of the stamp, surrounded by positive 

 dust figures. If, therefore, in this arrangement of the stamp alternate 

 ■charges and discharges are brought about, the conditions for forming 

 Manifold breath images are fulfilled. 



i A pitch surface being covered with a mica plate and a stamp placed 

 bn it, the latter was charged and discharged by the spark micrometer. 

 After twenty revolutions the upper surface of the mica showed a per- 

 fect breath image, but the under surfaces and that of the pitch pre- 

 isented a most imperfect one. 



I These images are so frequently imperfect because pitch and mica 

 jadhere closely together in consequence of the electricity remaining 

 after each discharge and subsequent discharges is conveyed to places 

 'which lie scattered beyond the image surface ; but a metallic plate 

 being substituted for the pitch plate, a perfect breath image is ob- 

 tained on the upper and lower surfaces of the mica and on the metallic 

 teurtace. 



I The visibility of the breath images is to be explained, according to 

 fRiess, by the fact that the surfaces are freed by electrical discharges 

 Jirom the film of foreign matter with which they are generally covered ; 

 iand he has even proved such a cleansing of the surface by images on 

 [metal. On a perfectly insulating mica surface Iliess produced a breath 

 iimage, and the place where the image appeared conducted as well as 

 !a fresh surface of mica, thus showing that it had been freed from the 

 letratum covering this spot. 



; In most cases breath images are produced by such a cleansing 

 action, but they can be excited also by soiling the plate. 

 i On a fresh mica surface an obscure image of a stamp was obtained 

 jpn a shining ground. On an old surface, which electrified by forty 

 fTevolutions, gave a bright breath image; one hundred revolutions 

 ij)roduced a dull image. 



I The various kinds of dull breath images depend upon the condition 

 lof the plate used and of the stamp, and also upon the strength of the 

 ielectricity ; the clear images appear more frequently only because 

 jfioiled plates and the least possible electricity are generally used, 

 I The origin of the breath images, like that of the breath figures, is 

 ito be ascribed to a change which the electrical discharge produces in 

 'the stratum covering the plate, and consists in an increase or diminu- 

 ition of this stratum, according to circumstances. 

 ' A spark thrown upon a metallic surface injures it when perfectly 



