PROGRESS IN NINETEENTH CENTURY 229 



of assigning a tetrahedral form to the atom of carbon. From the pro- 

 perties of such an atom a new class of structural formulae could be 

 deduced, by means of which the so-called cases of "physical isomer- 

 ism " were simply interpreted. The molecules of tartaric and racemic 

 acids, for example, resemble each other as an object resembles its 

 reflection in a mirror, the one being a reversal of the other. Our 

 science acquired a new province, that of stereochemistry, which in less 

 than thirty years has grown to impressive dimensions. The theory of 

 van't Hoff and Lebel did more than to interpret the troublesome 

 known phenomena, it encouraged additional research and led to many 

 discoveries. At first, the asymmetric carbon atom alone was consid- 

 ered, but its peculiar properties are now shared by other elements, 

 and physical or stereochemical isomers are found even among in- 

 organic compounds. When one atom is combined with four other 

 atoms or groups of as many different kinds, optical asymmetry 

 appears, and physical isomerism becomes possible. 



During the ninth decade of the century the dominating interest in 

 organic chemistry began to wane, for the reason that other subjects 

 were demanding their share of attention. I do not mean by this that 

 the activity of organic chemists diminished, for their output of dis- 

 covery was never greater than now; but the centre of the stage was 

 slowly being filled by other groups of actors. Inorganic chemistry was 

 reviving from its long neglect, and physical chemistry loomed large 

 upon the horizon. In each of these branches journals were started, 

 and no difficulty was found in filling their pages with the records of 

 successful investigations. In theory, physical chemistry has made 

 the greatest advances, inorganic research has been more a matter of 

 detail. Let us briefly consider the two themes separately. 



To the inorganic chemist several duties were apparent. Old work 

 needed revision, the compounds of many elements were almost unde- 

 scribed, there was a lack of system to remedy, and the theories de- 

 rived from organic chemistry were to be tested and applied. A very 

 large part of the work was necessarily descriptive, a preparation for 

 the future, but back of it all lay a fundamental question with which 

 all physical science is connected, for the nature of matter itself was 

 to be determined. In its broadest sense this question demands the 

 cooperation of all science and all philosophy, but to inorganic chem- 

 istry one phase of it may be assigned. What is the nature of the 

 chemical elements? Are they one or many? And how shall an element 

 be defined? To these questions there is as yet no final answer, but 

 clues to follow are many, and some of them are offered by the periodic 

 law. To remedy its imperfections is an obvious duty for inorganic 

 chemists to perform. 



Near the middle of the Mendelejeff table and of the Lothar Meyer 

 curve there is an area which is partly blank and partly filled with the 



