130 Mr. Pollock on the Action 



as water or moisture, be absent, light will frequently result ; 

 hence the electric spark. While, on the contrary, if a solid 

 become a fluid, or a fluid a gas, there is an increase of capa- 

 city, there is expansion of volume, and the form now assumed 

 will, owing to this expansion, demand more of this pervading 

 principle, and cold will be presented to, or their pervading 

 heat abstracted from, surrounding bodies. Moreover, if a gas 

 merely contract or expand without assuming either the fluid 

 or solid form, the same effect will be produced upon the per- 

 vading principle of heat in surrounding bodies. Here, per- 

 haps, the explosion of euchlorine may be objected as an in- 

 stance of expansion attended by heat and light; but this change 

 takes place with rapidity, and the abstraction of the radiant 

 principle from surrounding bodies will likewise be effected 

 with rapidity, producing light and heat. 



There is no necessity for supposing the existence of heat, 

 light, an electric, galvanic, or perhaps a magnetic fluid di- 

 stinct from this pervading principle ; they are all branches 

 from the same root. Electric or galvanic attraction between 

 bodies is the result of their containing different proportions 

 of the pervading principle ; and by the electric spark the ex- 

 cess is given off and an equilibrium is restored. Thus the 

 thunder cloud being formed from vapour in the atmosphere, 

 there is a contraction of volume, an excess of the pervading 

 principle, which, by being transmitted to the earth beneath, 

 produces lightning. 



Previously to making an attempt to explain the action of 

 the Voltaic pile, it may be necessary to inquire upon what 

 conducting power and capacity for heat depend. Conducting 

 power appears to be the reverse of capacity for heat, as the 

 best conductors have generally the worst capacity. In con- 

 ductors, as metals, attraction predominates ; but in bodies pos- 

 sessing the greatest capacity, as gases, repulsion of particles 

 predominates. Metals are the densest bodies known, at least 

 those forming a component part of the pile. Density must 

 depend upon a strong attraction between the particles of the 

 substance. 



Conducting power appears, then, to be the consequence of 

 the strong mutual attraction existing between the particles of 

 matter, as in a metal ; because if a metal, by exposure to a 

 higher temperature, become expanded, yet, when exposed to 

 a lower temperature, it will again contract more suddenly 

 than a non-conducting substance, because the attraction be- 

 tween the particles of a conducting substance must be com- 

 paratively stronger than that of a non-conducting substance, 

 as the superior density of the former demonstrates : and as the 



particles 



