RECENT PROGRESS IN PHYSICS. 361 



electrified by the discontinuous discharge ; the fact that electricity is 

 already present on the surface, acting attractingly on that issuing 

 from the point, occasions a greater diffusion of the positive electricity ; 

 but the circumstance that the positive electricity spreading forth is 

 i[>artially neutralized by the presence of the negative, causes the radia- 

 ting form of the positive dust figure. 



i To sustain this view^ Riess produced a modification of the phenome- 

 iQon in rarified air. On a plate covered with pitch, placed under a 

 glass receiver, was placed the blunt end of a wire, which received a 

 3park from a jar charged with positive electricity. With the whole 

 pressure of the air the sun appeared on dusting the plate ; but when 

 |the air was exhausted to 27^ lines pressure, only an irregular yellow 

 ^peck appeared ; negative electricity behaved in like manner. The 

 diflerence between the positive and negative figures was no longer 

 pbserved at this degree of rarifaction. 



I When the air was exhausted to 2 or 3 lines the end of the wire left 

 j&nly a point, which, luith positive electricity ivas red, with negative^ 

 yellow ; and consequently caused, not by the transmission of electricity 

 to the plate, but by induction. 



: The penetration of the stratum of air surrounding the plate is, 

 therefore, the origin of dust figures. 



I § 74. Dust images. — If a stamp (as simple as possible, having a few 

 ^•aised letters, and for this reason printing types will answer) be 

 blaced on a single pitch plate, (so Riess calls a copper disk coated on 

 Sne side only with pitch,) and electricity be communicated to the 

 ^tamp, it acts inductively on the pitch surface, the latter becoming 

 electrified at the spot where touched ; and this electricity is opposite 

 Ito that of the stamp, for on removing it and powdering the plate with 

 the mixture mentioned already, a red image of the letter is obtained, 

 [if the stamp is positive ; a yellow one, if negative ; for the flcur of 

 Sulphur attaches itself to the positive,, the minium powder to the nega- 

 jtive spots of the resin plate. 



[ The above described phenomenon underwent numerous modifica- 

 jtions, according to the manner in which the stamp was electrified. 

 \ The stamp being touched by the knob of a charged Ley den jar, 

 and then removed in an insulated condition, leaves au image as above 

 indicated ; it is, however, very little covered with dust ; while the 

 ground, by the formation of dust figures becomes yellow, if the letter 

 is red, or red, if the letter is yellow. 



The stamp being removed uninsulated, the dust figure changes, 

 ^hereby the clearness of the image also suffers. 



By electrifying too strongly, an actual passage of electricity in part 

 occurs at the place where the stamp touches the plate^ so that a dust 

 image appears, partly red and partly yellow. 



I Then we have at the same time a dust image and dust figures. To 

 (obtain the dust image clearly, the formation of the dust figures must 

 be avoided, which Riess accomplished in various ways. 

 ! The knob of a Leyden jar was exchanged for a four-inch ball, and 

 rthe jar fastened horizontally, so that the pitch plate and the stamp 

 jcould be placed under the ball ; the stem of the stamp was half an inch 

 from the ball. By the inductive action of the ball the end of the stamp 



