138 CORRELATION OF PHYSICAL FORCES. 



reasoning, anticipate a substance like nitric acid from the com- 

 bination 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 pro- 

 duced 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 existing 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 par- 

 ticles ; 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 defi- 

 nitely to relate the forces of chemistry and electricity. When 

 two dissimilar metals in contact are immersed in a liquid 

 belonging to a certain class, and capable of acting chemically 

 on one of them, what is termed a voltaic circuit is formed, 

 and, by the chemical action, that peculiar mode of force 

 called an electric current is generated, which circulates from 

 metal to metal, across -the liquid, and through the points of 

 contact. 



Let us take, as an instance of the conversion of chemical 

 force into electrical, the following, which I made known some 

 years ago. If gold be immersed in hydrochloric acid, no 

 chemical action takes place. If gold be immersed in nitric 



