CHEMICAL AFFINITY. 153 



because it tends to unite with another body ; when united, 

 its corrosive power, i. e. its tendency to unite, being satiated, 

 it cannot, so to speak, be further attracted, and it necessarily 

 loses its corrosive power. But there are other cases where 

 no such result could d priori be anticipated, as where the 

 attraction or combining tendency of the compound is higher 

 than that of its constituents : thus, who could, by physical 

 reasoning, anticipate a substance like nitric acid from the 

 combination of nitrogen and oxygen ? 



The nearest approach, perhaps, that we can form to a 

 comprehension of chemical action, is by regarding it (vaguely 

 perhaps) as a molecular attraction or motion. It will 

 directly produce motion of definite masses, by the resultant 

 of the molecular changes it induces : thus, the projectile 

 effects of gunpowder may be cited as familiar instances of 

 motion produced by chemical action. It may be a question 

 whether, in this case, the force which occasions the motion 

 of the mass is a conversion of the force of chemical affinity, 

 or whether it is not, rather, a liberation of other forces exist- 

 ing in a state of static equilibrium, and having been brought 

 into such state by previous chemical actions ; but, at all 

 events, through the medium of electricity chemical affinity 

 may be directly and quantitatively converted into the other 

 modes of force. By chemical affinity, then, we can directly 

 produce electricity; this latter force was, indeed, said by 

 Davy to be chemical affinity acting on masses : it appears, 

 rather, to be chemical affinity acting in a definite direction 

 through a chain of particles ; but by no definition can the 

 exact relation of chemical affinity and electricity be expressed ; 

 for the latter, however closely related to the former, yet exists 

 where the former does not, as in a metallic wire, which when 

 electrified, or conducting electricity, is, nevertheless, not 

 chemically altered, or, at least, not known to be chemically 

 altered. 



Volta, the antitype of Prometheus, first enabled us de- 

 7* 



