SECT. XXIX. VOLTAIC DISCHARGE. 305 



receiver; and when a scarcely perceptible film has lined the 

 receiver, if it be washed with an acid, it then gives, with 

 ferrocyanide of potassium, the Prussian-blue precipitate. In this 

 case we readily distil iron, a metal by ordinary means fusible 

 only at a very high temperature." 



Another strong evidence that the Voltaic discharge consists of 

 the material itself of which the terminals are composed, is the 

 peculiar rotation which is observed in the light when iron is 

 employed, the magnetic character of this metal causing its parti- 

 cles to rotate by the influence of the Voltaic current. In short, 

 Mr. Grove concludes that, although it would be hasty to assert 

 that the electrical disruptive discharge can in no case take place 

 without the terminals being affected, yet he had met with no 

 instance of such a result, provided the discharge had been suffi- 

 ciently prolonged, and the terminals in such a state as could be 

 expected to render manifest slight changes ! * 



Some years ago Mr. Grove discovered that the electrical dis- 

 charge possesses certain phases or fits of an alternate character, 

 forming rings of alternate oxidation and deoxidation on metallic 

 surfaces. A highly polished silver plate in an air-pump was 

 connected with the pole of a powerful inductive battery, while a 

 fine metallic wire, or even a common sewing needle, was fixed at 

 the other pole, and so arranged as to be perpendicular to the 

 silver plate, and very near, but not touching it. By means of 

 this apparatus the electrical discharge could be sent through any 

 kind of rarefied media. In some of the experiments a series of 

 concentric coloured rings of oxide alternating with rings of 

 polished or unoxidated silver were formed on the plate under the 

 point of the needle or wire. When the plate was previously 

 coated with a film of oxide, the oxide was removed in concentric 

 spaces by the discharge, and increased on the alternate ones, 

 showing an alternate positive and negative electricity, or elec- 

 tricity of an opposite character in the same discharge. 



When the silver plate was polished the centre of the rings 

 formed on it was yellow-green surrounded by blue-green ; then a 

 ring of polished silver, followed by a crimson ring with a slight 

 orange tint on the inner side and deep purple on the outer ; 



* ' Correlation of the Physical Forces, by W. R. Grove, Esq.,' one of the 

 most remarkable and talented works that has appeared, to which the author 

 with pleastxre acknowledges her obligations. 



