ELECTRICITY. 



'47 



light of the discharge will appear less 

 brilliant in passing that portion ; this 

 must arise from the greater division 

 of the fluid when passing over an ex- 

 tended conductor than over one that is 

 narrow. 



3. Chemical Effects of Electricity. 



(172.) Electricity exerts a most ex- 

 tensive and important influence in effect- 

 ing: changes in the chemical composition 

 of bodies; but as this influence is most 

 conspicuously exerted in that particular 

 mode of agency, which is known by the 

 name of Galvanism, this subject will 

 more properly be considered in the treatise 

 on that branch of electrical science. For 

 the present, we must content ourselves 

 with adducing a few instances, illustra- 

 tive of the chemical effects of electricity 

 in the forms under which it has now 

 been presented to our notice. 



(1 73.) Some of the chemical changes 

 consequent on powerful electrical ex- 

 plosions, appear to be merely the effects 

 of the heat which is evolved in that pro- 

 cess. The surfaces of metallic bodies 

 through which accumulated electricity is 

 made to pass are frequently oxidated ; 

 this is seen more especially in the case 

 of wires that have been fused or vola- 

 tilized by the electric discharge. It is 

 known that metals intensely heated are 

 disposed to combine with the oxygen of 

 the atmosphere, and, consequently, to 

 assume the state of oxides ; it is simpler, 

 therefore, to ascribe this effect in the 

 present case to a cause which is known 

 to be in operation at the same moment, 

 than to any peculiar or determining 

 agency of electricity. A multitude of 

 experiments are on record in which the 

 partial oxidation of metals has been ef- 

 fected by electric explosions. This subject 

 was prosecuted with minute and labori- 

 ous attention by Van Marum, by Cuth- 

 bertson, and more lately by Singer. It 

 is remarked by this last experimentalist, 

 that the oxides of metals produced in 

 this way appear to consist of several 

 distinct portions of different degrees of 

 fineness ; when a wire is exploded in a 

 receiver, part of the oxide immediately 

 falls to the bottom, but another portion 

 remains suspended in the air for a con- 

 siderable time, and is at length gradually 

 deposited. It is probable that this cir- 

 cumstance may in part account for the 

 different colours of oxides produced in 

 close receivers and in the open air, for in 

 the latter case a portion of the oxide is 

 always lost. 



(174.) Under other circumstances, 

 electricity is found to exert a power the 

 reverse of the former ; for it decomposes 

 metallic oxides, extricating their oxygen, 

 and restoring them to the metallic state. 

 This deoxidating power was known to 

 several of the earlier electricians. Bec- 

 caria reduced the oxides of tin and of 

 mercury to their metallic state by elec- 

 tricity. In order to effect this change, a 

 quantity of the oxide may be introduced 

 into a glass tube, and pointed conducting 

 wires inserted through corks at the op- 

 posite ends of the tube, so that a portion 

 of the oxide may lie between them. This 

 apparatus is then to be placed on the 

 table of the universal discharger, (136,) 

 and repeated shocks are to be sent 

 through the oxide until its partial or 

 total reduction is accomplished. Ver- 

 milion, which consists of sulphur and 

 mercury, is very easily decomposed by 

 this process, and by a very moderate 

 charge. 



(175.) When a succession of electric 

 discharges from a powerful electric ma- 

 chine are sent through water, a decom- 

 position of that fluid takes place, and it 

 is resolved into its two elements of oxy- 

 gen and hydrogen, which immediately 

 assume the gaseous form. This fact w r as 

 discovered in 1789, by Messrs. Dieman, 

 Paetz, and Van Troostwyck, who had 

 formed themselves into a society for ex- 

 perimental research in Holland ; and it 

 completed the chain of evidence by which 

 the great discovery of the composition of 

 water, made five years before by Caven- 

 dish, is established. The abovemen- 

 tioned Dutch chemists being occupied, 

 in conjunction with Mr. Cuthbertson, in 

 investigating the effects of electricity 

 when passed through different bodies, 

 were desirous of ascertaining its effect 

 on pure water. They employed for this 

 purpose an apparatus consisting of a 

 glass tube, twelve inches long and one- 

 eighth of an inch in diameter, through 

 one end of which a gold wire was in- 

 serted, projecting about an inch and a 

 half within the tube ; that end was then 

 hermetically sealed. Another wire was 

 introduced at the other end of the tube, 

 which was left open, and passed up- 

 wards, so that its extremity came to a 

 distance of five-eighths of an inch from 

 the end of the first wire. The tube was 

 then filled with distilled water, which 

 had been freed from air by an excellent 

 air-pump, and inverted in a vessel con- 

 taining mercury. A little common air 

 -was let into the top of the tube, in 



