154 CORRELATION OF PHYSICAL FORCES. 



tion 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 contractions, 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 alternate actions 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 is diminished or ceases. Thus, 

 if we suppose a meteor to be a mass rotating in an orbit round 

 the earth, and with no resisting medium, then, as long as that 

 rotation continues, the motion of the meteoric mass itself 

 would be the exponent of the force impelling it ; if there be a 

 resisting medium, part of this motion would be arrested and 

 taken up by the medium, either as motion, heat, electricity, 

 or some other mode of force ; if the meteor approach the earth 

 sufficiently to fall upon it, the perceptible motion of the 

 meteor is stopped, but is taken up by the earth which vibrates 

 through its mass ; part also reappears as heat in both earth 

 and meteor, and part in the change in the earth's position 

 consequent on its increase of gravity, and so on. Gravitation 

 is but the subjective idea, and its relation to other modes of 



