360 RECENT PROGEESS IN PHYSICS. 



equally strong charges of the jar, are, consequently, in the ratio of 1 

 to 2.77, or the surfaces covered by them are as 1 to 7.67. 



A plate, coated on both sides with pitch, on being brought between 

 the insulated point and the conducting wire, and subjected to the 

 above process, the positive figure appears on one side and the negative 

 on the other. 



When the jar was charged with negative electricity, the disk ap- 

 peared above and the sun below, but the j-ellow sun in this case was; 

 only 2.2 times as large as the red disk. 



The cause ot the negative figures being relatively greater than in 

 the previous experiment was owing to the excess of negative elec- 

 tricity, which was transmitted t© the upper surface ; in fact, a sun ap-j 

 peared on the upper side, which was 3.3 times as great as the redt 

 disk on the under side, when a positively charged jar was used in a 

 similar experiment. 



Riess has shown that the dust figures appear only when the passage- 

 of electricity on the insulating plate is accompanied by a discontinuous! 

 discbarge, which may be recognized generally by a peculiar hissing.; 

 By holding the j3itched plate to the knob of a charged jar a sparki 

 passes with a crashing noise ; a discontinuous discharge thus takeS' 

 place, and a figure appears on dusting; but the plate being placed 

 at such a distance from the knob of the jar that a spark cannot pass, 

 some electricity still gradually goes over, producing a continuous dis- 

 charge. If the plate is dusted after standing from 30 to 70 minutes 

 opposite the knob of the jar, a number of round spots appear irregu- 

 larly distributed — yellow, if the jar had a positive, red, if a negative^ 

 charge. These spots exhibit no trace of rays ; they are perfectly alike 

 in size and form for both electricities. 



Hence, electrical dust figures aiDpear when electricity is transmitted hy 

 a discontinuous discharge to an insulathig plate. 



Upon this fact Riess founds a very ingenious explanation of the 

 diff'erence between positive and negative dust figures. In a discon- 

 tinuous discharge passing over the surface of an insulator the con- 

 densed atmosphere, which covers the surface of all bodies, is forcibly 

 penetrated, and a part of the stratum, containing vapor of water, if 

 projected wiih violence against the surface of the body. 



But Faraday has shown that, if moist air impinges forcibly againsl 

 any body, the latter is negatively electrified; thus, then, in this case'' 

 the surface of the plate becomes negatively electrified in consequencf 

 of the discharge which takes place over the surface; the remaining 

 electricity of this discharge then has only to spread over a negatively 

 electrified insulating surface. 



The surface being charged with negative electricity, it spreads froir 

 the point over an insulating surface already negative ; the circum 

 stances, therefore, not being favorable for the distribution of the nega- 

 tive electricity the figure cannot become enlarged, and a roundec 

 form is assumed. 



The jar being positively charged, the remainder of the positive 

 charge spreads from the point over an insulating surface negatively 



* The experiments of Faraday referred to, scarcely allow of such a conclusion. — (Se' 

 Report for 1856, p. 364.) G. C. S. 



