250 THE ATOMIC THEOEY. 



was found that certain groups of atoms could l)e transferred from com- 

 pound to compound, almost as if they were veritable elements; and 

 radicles, such as ammonium, cyanogen, and benzoyl, were generally 

 recognized. I say "groups of atoms" advisedly, for as such they were 

 regarded, and they could hardly have been interpreted otherwise. 

 Then came the discovery of isomerism; of the fact that two substances 

 could be strikingly different, and yet composed of the same elements 

 in exactly the same proportions. This was onl}' explicable upon the 

 supposition that the atoms were differently arranged within the iso- 

 meric molecules, and it led investigators more and more to the stud}^ 

 of chemical or molecular structure. Without the atomic theory the 

 phenomena would have been hopelessly bewildering; with its aid the}'^ 

 were easy to understand and fertile in suggestions for research. Still 

 another link in the chain of chemical reasoning was forged b}^ Dumas 

 when he proved that the hydrogen of organic compounds was pften 

 replaceable, atom for atom, b}" chlorine. Sometimes the replacement 

 was complete, sometimes it was only partial, and the latter cases were 

 the most signiffcant. In acetic acid, for example, one, two, or three 

 fourths of the hydrogen could l)e successivel^y replaced, but the last 

 fourth was permanently retained. Hj'^drogen, then, was com])ined in 

 acetic acid in two different ways, one part yielding its place to chlo- 

 rine, the other ])eing unaffected. This behavior was soon found to l)e 

 by no means exceptional; indeed, it was very common, and it opened 

 a new line of attack upon the problems of chemical constitution. The 

 existence of radicles, the formation of isomers, and the substitution 

 of one element by another were facts which strengthened the atomic 

 theory and seemed to be incapable of reasonable interpretation upon 

 other terms. Their connection with one another, however, was not 

 well understood, and wearisome discussions preceded their coordina- 

 tion under one general law. 



With the tedious controversies which distracted chemists between 

 1830 and ]850 we have nothing now to do; they were important in 

 their day, but they do not come within the scope of the present argu- 

 ment. Theor}^ after theory was advanced, prospered for a time, and 

 then deca3"ed, and chemical literature is crowded with their fossil 

 remains. Each one, doubtless, indicated an advance in knowledge; 

 but each one also exaggerated the importance of some special set of 

 relations and so overshot the mark. During this period, however, 

 Farada}" discovered the law of electrolysis which is now known by hi.'^ 

 name, and the chemical equivalents were thereby given another exten • 

 sion of meaning. The electro-chemical theories of Berzelius had fallen 

 to the ground, l)ut Faradaj^'s law came as a permanent addition to the 

 physical side of chemistr3\ 



During the sixth decade of the nineteenth century two importtmt 

 forward steps were taken. The kinetic theory of gases gave new force 



