October 23, 190;).] 



SCIENCE. 



519 



appreciated. ^Mitseherlieh's researches upon 

 isoiiiorphisiu were more fortunate, and 

 pave ill! mediate help in the determination 

 of atomic weights and the settlement of 

 formula>. For the moment we need only 

 note that the chemical atom was the under- 

 lying conception by means of which all 

 these lines of testimony were to be unified. 

 From Dalton and Gay Lussac to Frank- 

 land and Cannizzaro was a time of fermen- 

 tation, discussion and discover}'. In chem- 

 istry, contrary to the saying of the 

 preacher, there were many new things under 

 the sun, and some of the discoveries were 

 most suggestive. First it was found that 

 certain groups of atoms could be trans- 

 ferred from compound to compound, al- 

 most 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 thej' could 

 hardly have been interpreted otherwise. 

 Then came the discovery of isomerism; of 

 the fact that two substances could be strik- 

 ingly different, and yet composed of the 

 same elements in exactly the same propor- 

 tions. This was only explicable upon the 

 supposition that the atoms were differently 

 arranged within the isomeric molecules, 

 and it led investigators more and more to 

 the study of chemical or molecular struc- 

 ture. Without the atomic theory the phe- 

 nomena would have been hopelessly be- 

 wildering; with its aid they were easy to 

 understand, and fertile in suggestions for 

 research. Still another link in the chain 

 of chemical rea.soning was forged by Dumas, 

 when he proved that the hydrogen of or- 

 ganic compounds was often replaceable, 

 atom for atom, by chlorine. Sometimes 

 the replacement was complete, sometimes il 

 was only partial, and the latter cases were 

 the most significant. In acetic acid, for 

 example, one, two or three fourths of the 



hydrogen could be successively replaced, but 

 the last fourth was permanently retained. 

 Hydrogen, then, wa.s combined in acetic 

 acid in two different ways, one part j'ield- 

 ing its place to chlorine, the other being 

 unaffected. This behavior was soon found 

 to be by no means exceptional : indeed, it 

 was very common, and it opened a new 

 line of attack upon the problems of chem- 

 ical constitution. The existence of radicles, 

 the formation of isomers, and the substi- 

 tution of one element by another, were facts 

 which strengthened the atomic theory and 

 .seemed to be incapable of reasonable inter- 

 pretation upon other terms. Their connec- 

 tion with one another, however, was not 

 well understood, and wearisome discussions 

 preceded their coordination under one gen- 

 eral law. 



With the tedious controvei'sies which dis- 

 tracted chemists between 1830 and 1850, 

 we have nothing now to do ; they were im- 

 portant in their day, but they do not come 

 within the scope of the present argument. 

 Theory after theory was advanced, pros- 

 pered for a time, and then decayed; and 

 chemical literature is crowded with their 

 fossil remains. Each one, doubtless, indi- 

 cated an advance in knowledge, but each 

 one also exaggerated the importance of 

 some special set of relations, and so over- 

 shot the mark. During this period, however, 

 Faraday discovered the law of electrolysis 

 which is now known by his name, and 

 the chemical equivalents were thereby given 

 another extension of meaning. The eleetro- 

 chemical theories of Berzelius had fallen to 

 the ground, but Faraday's law came as si 

 I)ermancnt addition to the physical side 

 of chemistry. 



During the sixth decade of the nine- 

 teenth century, two important forward 

 steps were taken. The kinetic theory of 

 gases gave new force to Avogadro's law, 

 and made its eomplete recognition by chem- 



