28 Prof. Volpicelli on Fridional Electricity. 



tricity, — a fact which suggests the probability that, at a lower 

 temperature, the ten resins before alluded to might exhibit the 

 alternate phenomena. 



On rubbing a stick of sealing-wax, a yard in length, briskly 

 with a linen cloth, it becomes negative throughout its whole 

 length ; whilst in this state, if the same cloth be passed a few 

 times more gently along the surface, the extremities will be found 

 . to possess unlike polarities, and in the middle will be a neutral 

 zone. 



The condensing electrometer of Volta was occasionally used 

 in the foregoing experiments, but more generally the dry-pile 

 electroscope. The latter was even placed under the exhausted 

 receiver of an air-pump, when the effects were, if anything, still 

 more manifest. The disturbing influence of air-currents, there- 

 fore, was beyond all question eliminated. 



On first experimenting with glass rods and cylinders 6 or 8 

 inches in length, the alternate effects obtained with resins did not 

 manifest themselves ; it was only afterwards, when rods a yard in 

 length, and rubbers of fine fur were used, that the experiments 

 succeeded perfectly. Here, again, energetic friction tends to 

 produce negative electricity, and gentler friction positive ; and, 

 as before, the passage from either electric condition through the 

 neutral to the opposite may be brought about by changing solely 

 the quantity of friction. 



The experiments succeed best with a rubber of fine fur, the 

 thicker and finer the better ; nevertheless the phaenomeua may 

 also be obtained with a woollen rubber. Many different kinds 

 of glass were examined, and with none were the experiments 

 unsuccessful; the ordinaiy greenish Roman glass, however, ex- 

 hibited the effects with the greatest facility. The most conve- 

 nient form was found to be that of a solid cylinder, 3| centi- 

 metres in diameter, and not less than 3 decimetres in length ; 

 with shorter bars the developed electricity is almost invariably 

 positive. As in the case of resins, the production of negative 

 electricity is facilitated by previously raising the temperature of 

 the bar over a spirit-lamp ; and under favourable atmospheric 

 conditions negative electricity, once developed, may be changed 

 into positive without contact, by merely passing the rubber 

 alongside of, and close to the bar. Here, too, a single stroke 

 often suffices to develope negative electricity at one end of a bar, 

 and positive at the other. English barometer-tubes in particular 

 often presented this curious phsenomenon. 



Some crystalline bodies also present the phseuomena of alter- 

 nate polarity. For instance, if one of the polished faces of a 

 crystal of Iceland spar or of selenite be rubbed move or less 

 briskly against a piece of flannel, fixed at one end and stretched 



