VOLTAIC INSTRUMENT. 235 



duction. Under such influence, too, the molecule appears to 

 be indifferently polar. 



Electrical induction at a distance, Mr. Faraday has shewn 

 to be always an action of contiguous particles, chains of par- 

 ticles of air, or some other " dielectric," extending between the 

 excited body which is inducing, and the induced body. His 

 investigation of this subject led to the remarkable discovery 

 that the intensity of electric induction at a constant distance 

 from the inducing body, is not always the same, but varies 

 in different media, the induction through a certain thickness of 

 shell-lac, for instance, being twice as great as through the same 

 thickness of air. Numbers may be attached to different bodies 

 which express their relative inductive capacities : 



Specific inductive capacity of air . 1 



glass . .1.76 



shell- lac . . 2 



n sulphur . . 2.24 



The inductive capacity of all gases is the same as that of air, 

 and this property, it is remarkable, does not alter in these 

 bodies with variations in their density. 



VOLTAIC INSTRUMENTS. 



Voltaic battery. The working forms of the battery are too 

 numerous to be individually noticed. That form only need be 

 particularly described which is the most nearly perfect, and which 

 in experiments of research is likely to supersede all others, the 

 constant battery of Mr. Daniell. A cell of this battery consists 

 of a cylinder of copper 3^ inches in diameter, which experience 

 has proved to the inventor to afford the most advantageous 

 distance between the metallic surfaces, but which may vary 

 in height from 6 to 20 inches, according to the power which it 

 is wished to obtain. A membranous bag formed of the gullet 

 of an ox, is hung in the centre by a collar and circular copper 

 plate, resting upon a rim within and near the top of the 

 cylinder ; and in this is suspended by a wooden cross bar, a 

 cylindrical rod of amalgamated zinc half an inch in diameter. 

 The outer cell is charged with a mixture of 8 measures 01 

 water and one of oil of vitriol, which has been saturated with 



