Prof. Volpicelli on Frictional Electricity. 27 



properly adjusted energy, either electric state may be rendered 

 permanent, or seizing the stick by the middle and stroking one 

 extremity more vigorously than the other, the first may be made 

 negative, the second positive; and thus different electric polarities 

 may be developed simultaneously in one and the same body by 

 rubbing its extremities with the same substance, but with differ- 

 ent degrees of energy. 



To the phsenomenon above described, Prof. Volpicelli has 

 given the name of alternate polarity, in order to distinguish it 

 from another, successive polarity, where, by changing the energy 

 of friction, the developed electricity may be changed in one or 

 the other way, but not back again. The three resinous bodies 

 with which the phsenomenon of alternate polarity may be obtained 

 with certainty, are common sealing-wax [cera di Spagna), gum- 

 lac, and jalap resin [resina scialappa). With ten other resins 

 which were examined, as well as with amber, negative electricity 

 alone could be obtained, no matter to what extent the energy of 

 the friction was varied. A cold dry atmosphere is of course 

 favourable to the success of the experiments with the three 

 resins ; but at all times the effects described may be obtained, 

 and that no matter whether the finger and thumb which impart 

 friction be naked or covered ; in the latter case, however, the 

 glove should be well fitted to the hand. Amongst the few kinds 

 of gloves with which the experiments do not succeed are fur and 

 india-rubber ; with the former the electricity developed is always 

 negative, and with the latter almost invariably positive. Another 

 important condition appears to be a compact and polished resi- 

 nous surface. Sulphur, which is not susceptible of such a polish, 

 and whose particles are disintegrated by friction, does not mani- 

 fest the alternate effects. 



If a stick of one of the three resins, after having remained 

 for some time in a neutral state, be gently pressed between 

 the finger and thumb, without intentional longitudinal fric- 

 tion, positive electricity will be developed ; and subsequently 

 this may be cancelled, and even transformed into negative elec- 

 tricity, by gradually increasing the pressui'es. It would appear, 

 however, that this negative electricity cannot be reconverted into 

 positive by the reverse process, that is to say, by gradually di- 

 minishing the applied pressures. 



Under favourable atmospheric conditions, negative electricity 

 may often be changed to positive, without actual friction, by 

 merely passing the fingers covered with an insulating tissue along- 

 side the stick and as close to it as possible without contact. 



On raising the temperature of the three resins by means of the 

 flame of a spirit-lamp (taking care not to render them flexible 

 or plastic), they lose the property of producing positive elcc- 



