170 COBKELATTON OF PHYSICAL FORCES. 



the action of chemical force, and, therefore, is no creation ol 

 force by contact. 



The force so developed by catalysis may be converted 

 into a voltaic form thus : in a single pair of the gas battery 

 above alluded to, one portion of a strip of platinum is im 

 mersed in a tube of oxygen, the other in one of hydrogen, 

 both the gases and the extremities of the platinum being con 

 nected by water or other electrolyte ; a voltaic combination 

 is thus formed, and electricity, heat, light, magnetism, and 

 motion, produced at the will of the experimenter. 



In this combination we have a striking instance of cor 

 relative expansions and conti actions, analogous, though in a 

 much more refined form, to the expansions and contractions 

 by heat and cold detailed in the early part of this essay, and 

 illustrated by the alternations of two bladders partially filled 

 with air : thus, as by the effect of chemical combination in 

 each pair of tubes of the^gas battery the gases oxygen and 

 hydrogen lose their gaseous character and shrink into water, 

 so at the platinum terminals of the battery, when immersed 

 in water, water is decomposed, and expands into oxygen and 

 hydrogen gases. The correlate of the force which changes 

 gas into liquid at one point of space, changes liquid into gas 

 at another, and the exact volume which disappears in the 

 one place reappears in the other ; so that it would appear to 

 an inexperienced eye as though the gases passed through 

 solid wires. 



Gravitation, inertia, and aggregation, were but cursorily 

 alluded to in my original lectures ; their relation to the other 

 modes of force seemed to be less definitely traceable ; but the 

 phenomenal effects of gravitation and inertia, being motion 

 and resistance to motion, in considering motion I have in 

 some degree included their relations to the other forces. 



To my mind gravitation would only produce other force 

 when the motion caused by it ceases. Thus, if we suppose 

 a meteor to be a mass rotating in an orbit round the earth, 



