ELECTRICITY. 



electricity disappear. The electricity 

 developed by the process of cooling was 

 called by Wilke, who first observed it, 

 spontaneous electricity, in contradistinc- 

 tion to that which, originating from 

 friction, he called excited electricity. 

 Van Marum, however, attributes the 

 electricity developed by the separation 

 of the two substances, to a species of 

 friction ; for he remarks, that the electri- 

 city does not manifest itself till the sul- 

 phur begins to contract in thfc act of con- 

 gelation, and that it attains its maximum 

 at the point of the greatest contraction. 



(200.) It is reasonable to suppose 

 that whatever change was produced in 

 the electrical state by congelation, the 

 reverse would be produced by liquefac- 

 tion. We are not aware of any ex- 

 periments which bear directly upon this 

 question. 



(201.) The conversion of bodies into 

 the state of vapour, as well as the con- 

 densation of vapour, is generally attend- 

 ed by some alteration of their electrical 

 condition ; and the bodies in contact 

 with f he vapour are thereby rendered 

 electrical. Thus, if a plate of metal 

 strongly heated be placed upon a gold- 

 leaf electroscope, and water be dropped 

 upon the plate, at the moment the va- 

 pour rises the leaves of the electroscope 

 diverge with negative electricity. The 

 general fact was noticed by Laplace, 

 Lavoisier, and Volta, in the year 1781 ; 

 and was found to extend both to solids 

 and to liquids passing into a gaseous 

 form. De Saussiire made an extensive 

 series of experiments on the ebullition 

 of water and other fluids, with a view 

 to ascertain the degree and kind of elec- 

 tricity developed during this process. 

 But investigations of this kind are at- 

 tended with great difficulty, from the 

 multitude of minute circumstances which 

 are liable to affect the results ; and we 

 accordingly find, that different experi- 

 ments of the same kind often afford the 

 most opposite conclusions. 



(202.) In general it is found, that the 

 vaporization of water by simple ebulli- 

 tion produces negative electricity in the 

 remaining fluid, or vessel which con- 

 tains it : the vapour itself being positive. 

 On the contrary, when aqueous vapour 

 is condensed into water, it becomes ne- 

 gative, leaving the bodies with which it 

 was last in contact in a state of positive 

 electricity. Yet in some of De Saus- 

 sures's experiments, when the heat was 

 communicated to a quantity of water 

 contained in an insulated metallic ves- 

 sel, by throwing into it a mass of red- 



hot iron, the electricity was very strongly 

 positive. This difference in the result 

 was probably owing to the chemical de- 

 composition of the water in the latter 

 experiment, a circumstance which, as 

 we shall presently see, is itself a source 

 of electricity. It is principally on ac- 

 count of the interference of chemical 

 actions with the regular operations of 

 temperature, and of the complications 

 introduced by electric induction, that 

 experiments on this subject have hither- 

 to presented such anomalous, and, appa- 

 rently, discordant results. 



CHAPTER XIV. 



Developement of Electricity by Contact, 

 Compression, and other mechanical 

 Changes in Bodies, and also by their 

 Chemical Action. 



(203.) IT had long been suspected, 

 rather than proved, that a feeble degree 

 of electricity is evolved by the contact 

 or collision of different metals : but this 

 important fact was established in the 

 clearest manner by Volta, about the year 

 1801. The apparatus he employed in 

 his investigations on this subject con- 

 sisted of two discs, the one of zinc, the 

 other of copper, (fig. 41,) rather more 



Fig. 41. 



than two inches in diameter, ground 

 perfectly plane, and having in their cen- 

 tres insulating handles perpendicular to 

 their surfaces, by means of which the 

 plates could be brought into contact, 

 without being actually touched with the 

 hand. With this precaution the discs 

 were made to approach till they touched 

 one another ; they were then separated, 

 by keeping them parallel as they were 

 drawn back. The electricity they pos- 

 sessed after this separation was then 

 examined by means of the condenser ; 

 and, that the effects might be rendered 

 more distinct, : the electricity produced 

 by a number of successive contacts, 

 (taking care to restore the discs to the 

 neutral state after each contact,) was 

 accumulate:! in the same condenser. It 

 was constantly found that the copper 

 disc charged the condenser with nega- 

 tive, and the zinc disc with positive elec- 



