52 THEORY OF LIGHT. 



without the interference of an acid, or the introduction of coy- 

 er iron, or any other metal which may be in a distinctly opposite 

 state of electricity, the zinc when in contact with the water, 

 having its lower and upper surfaces in opposite states of electri- 

 city, its action in the decomposition of the water commences, 

 the oxygen being attracted to the positive poles of the metallic 

 surface ; by continual interchange elementary electrical currents 

 are produced, and pervade the metallic and fluid media, in a 

 direction exactly conformable with that produced by a pair of 

 plates of the Voltaic pile. 



The power of composition and decomposition are naturally 

 increased by the introduction of a few drops of acid, affording 

 thereby a diversity of polar action, and greater intensity than 

 can naturally be produced by a fluid composed of only two ele- 

 mentary constituents. 



In operations where dilute acidulous menstrua are employed 

 the polar energies are employed in a moderate ratio, and may be 

 exalted to considerable force by a further increase of acidity, 

 but by the introduction of a piece of copper wire placed in 

 direct contact with the disc of zinc, the energy of metallic corro- 

 sion is considerably increased by the metallic combination with 

 oxygen, while the hydrogen of the water is discharged in the 

 gaseous state in great quantity. 



Two peculiarities are observable in reference to the electrical 

 state of metallic surfaces, when one metal alone is used, (say 

 zinc, or iron, in a dilute mixture of distilled water and sulphuric 

 acid,) that is, that fountains of liberated hydrogen are dis- 

 charged from certain points only from the upper surface of the 

 metal, while the intervening spaces appear to be in a dormant 

 or inactive state, and further on introduction and presentation of 

 a wire of another metal (say copper) to within a certain dis- 

 tance of one of those fountains of gas, the discharge from the 

 fountain becomes more feeble, and on its nearer approach 

 totally ceases, the discharge being then usurped by the metallic 

 wire, as observed by Mr. Sturgeon, in his <c Annals of Chemistry, 



