RECENT PROGRESS IN PHYSICS. 38d 



This phenomenon (of turning the paste of iodide of potassium bine) 

 takes place in the same manner whether the point emit positive or 

 negative electricity ; it is also perfectly immaterial of what substance 

 the point is made, if only an emission of electricity occurs, thus re- 

 futing the view of those earlier physicists who maintained that the 

 od5r was owing to metallic particles carried off by the issuing elec- 

 tricity. 



On holding a platinum or gold plate before the point while the 

 machine is turned, the plate has imparted to it negative galvanic po- 

 larization, which can be demonstrated in the following manner : 



Connect the two mercury cups a and 6, Fig. 83, with the wire ends 

 of a multiplier. Into the cu[) a dip a copper pj^ gg 



wire, to the other end of which a platinum 

 plate p is fastened, the plate having been 

 first soldered with gold to a platinum wire, 

 and this to the copper wire at n. This pla- 

 tinum plate hangs in a glass vessel contain- 

 ing water slightly acidified. 



After having exposed a perfectly similar 

 platinum plate for a time to the electricity 

 issuing from a point, immerse it also in the 

 water of the glass vessel, and as soon as the 

 copper wire of the second plate is immersed 

 into the cup of mercury 6, a considerable 

 divergence of the galvanometer needle takes place, and in a direction 

 which indicates that the platinum plate which had been exposed to 

 ozone behaves negatively toward the other ; that is, the deflection is 

 in the same direction which would have been indicated had a zinc wire 

 been placed in a, a copper wire in 6, and these wires then immersed 

 in the liquid of the glass vessel. This current, however, is only 

 transient. 



The vdiole subject of galvanic polarization we will discuss in an- 

 other place ;* it is only mentioned here as one of the effects which 

 accompany the emission of electricity from points. 



All these reactions disappear when sulphuretted hydrogen, ammo- 

 nia, olefiant gas, &c., are diffused in the air of the room where the 

 experiment is made. 



If the emitting point be raised by the flame of a sph'it lamp to a 

 red heat, and the machine put in operation immediately after the 

 removal of the lamp, all the above described phenomena, which had 

 accompanied the escape of electricity, disappear ; that is, the elec- 

 trical cdor is no longer perceived, the iodine preparation does not turn 

 blue, platinum or gold plates are not polarized. All the phenomena 

 reappear, however, gradually, as the point cools. 



In order to make the experiment distinct, with reference to the odor, 

 it must be made with wires of one of the precious metals, because the 

 easily oxidable metals diffuse a peculiar smell simply by being heated. 

 Very thin wires are not suitable for this experiment ; but as thick 



* See report for 1855, p. 377. 



