RECENT PROGRESS IN PHYSICS. 367 



tions of mj Leiirhuch der PhyHih, the whole parapiiraph was omitted, 

 its matter seeming to me too problematical, and therefore unsuitable 

 for a text book. 



The discovery of Armstrong, and the investigations of Faraday 

 upon the development of electricity by escaping steam, gave a new 

 point of view for tlie interpretation of Pouillet's experiments, which 

 led lleich and Eeiss to repeat them, and thus to discover the true rela- 

 tion of the conditions concerned in the case. 



lleich has published his experiments in the Ahhandlungen hei der 

 Begriindunq der Icoyiigl. scichsischen Gesellschaft der Wissenscha/ten, &c. 

 Leipsic, 1846, p. 191). 



He verified the experiment as described by Pouillet. A clean pla- 

 tinum crucible is insulated and connected with a sensitive electroscope, 

 first heated and then removed from the source of heat ; if then pure 

 water be dropped into it and allowed to evaporate, no electricity is 

 obtained, either with or without the condenser. 



But if a solution of common salt be dropped into the hot crucible, 

 as long as the drop rolls about in the spheroidal state, by reason of 

 the high heat of the crucible, we obtain, as before, no electricity, or, at 

 most, but a mere trace of it ; but as soon as the crucible has cooled 

 enough to allow the liquid to boil away, the electroscope is charged 

 with negative electricity, and pretty strongly, too, if the crucible is a 

 large one. 



The use of the condenser has hardly any advantage, as nearly the 

 same results are obtained without as with it. 



This, and the fact that the development of electricity commenced as 

 suddenly as the boiling, were considered by lleich as decidedly sup- 

 porting the view that the electricity is not owing to evaporation, but 

 has its origin solely in the friction of the particles of water dashed 

 about upon the hot sides of the crucible. 



Now, if friction is the. source of the electricity, it is clear that a 

 powerful development of it can only take place when the particles of 

 water are thrown about with violence. As the liquid is dropped into 

 the vessel, traces of electricity sometimes appear, because, as this is 

 done, a few particles of water are occasionally thrown out. 



The electricity developed by the friction of the drops thrown out 

 upon the sides of the vessel has sufficient tension to cause the diver- 

 gence of tlie gold leaves of the electroscope, but the development is 

 not continuous ; hence the condenser is of no use. 



The non-appearance of electricity when pure water is employed is 

 easily explained ; for, with the solution of salt, the violent boiling 

 commences when the sides of the vessel have a far higher tempera- 

 ture than they have when pure water begins to boil. AVhen the par- 

 ticles of pure water are thrown off they touch the sides of the vessel, 

 already cool enough to be moistened by them ; but when the solution 

 of salt is used, the sides are so hot that the drops roll off. 



In a platinum crucible, properly connected with an electroscope, 

 lleich raised quartz sand, bit of porcelain, rusted iron filings, &c,, to a 

 red heat, removed the lamp, and sprinkled these substances with pure 

 water, tinder such circumstances a very perceptible evolution of elec- 

 tricity took place, while when the crucible was empty not a trace was 

 found . 



