142 CORRELATION OF PHYSICAL FORCES. 



other : thus, water, which consists of one part by weight of 

 hydrogen united to eight parts of oxygen, cannot be formed 

 by the same elements in any other than these proportions ; 

 you can neither add to nor subtract from the normal ratio of 

 the elements, without entirely altering the nature of the com- 

 pound. Further, if any element be selected as unity, the 

 combining ratios of other elements will bear an invariable 

 quantitative relation to that and to each other: thus, if 

 hydrogen be chosen as I, oxygen will be 8, chlorine will 

 be 36 ; that is, oxygen will unite with hydrogen in the pro- 

 portion of 8 parts by weight to I, while chlorine will unite 

 with hydrogen in the proportion of 36 to I, or with oxygen 

 in the proportion of 36 to 8. Numbers expressing their com- 

 bining weights, which are thus relative, not absolute, may, by 

 a conventional assent as to the point of unity, be fixed for all 

 chemical reagents ; and, when so fixed, it will be found that 

 bodies, at least in inorganic compounds, generally unite in 

 those proportions, or in simple multiples of them : these 

 proportions are termed Equivalents. 



Now, a voltaic battery, which consists usually of alterna- 

 tions of two metals, and a liquid capable of acting chemically 

 upon one of them, has, as we have seen, the power of pro- 

 ducing chemical action in a liquid connected with it by 

 metals upon which this liquid is incapable of acting : in such 

 case the constituents of the liquid will be eliminated at the 

 surfaces of the immersed metals, and at a distance one from 

 the other. For example, if the two platinum terminals of a 

 voltaic battery be immersed in water, oxygen will be evolved 

 at one and hydrogen at the other terminal, exactly in the 

 proportions in which they form water ; while, to the most 

 minute examination, no action is perceptible in the inter- 

 vening stratum of liquid. It was known before Faraday's 

 time that, while this chemical action was going on in the 

 subjected liquid, a chemical action was going on in the cells 

 of the voltaic battery ; but it was scarcely if at all known 

 that the amount of chemical action in the one bore a constant 

 relation to the amount of action in the other. Faraday proved 

 that it bore a direct equivalent relation : that is, supposing the 

 battery to be formed of zinc, platinum, and water, the amount 



