368 THE CHEMICAL PROBLEMS OF TO-DAY. 



tributed among the valencies. Riecke, in virtue of his pyro-electric re- 

 searches, was led to the view that the atoms are surrounded by certain 

 systems of positive and negative electric poles. 



Uniting these results with those of purely chemical experimentation, 

 we arrive at the idea that the valencies do not a])pear as points of attack 

 proper, but as baving linear dimensions. The carbon atom represents 

 itself as a sphere, surrounded by an envelope of sether which contains 

 the valencies. The latter seem to be determined by the presence of two 

 opposite electric j)oles which rest at the ends of a very short straight 

 line. Such a system is called a di-pole. The attachment of two valencies 

 to each other consists in the attraction of tlieir opposed poles. It is 

 evident that in a radial position of the di-poles they form an axis around 

 which the atoms are abU^ to rotate, but that this rotation is upset in 

 case of a tangential position. In what has been said so far and through 

 further considerations in regard to the electrical charge of the atoms 

 and of the di-poles a reason is found for the rei)ulsion of the four 

 valencies and consequently for the tetrahedric groui)ing of the same. 



The fact that the valencies can deviate from this position now becomes 

 intelligible; we perceive why the valencies of one atom can not unite 

 with one another, w^hile those of different atoms can combine; it is 

 clear that there can exist two kinds of simple linking, one of which 

 admits of rotation, while the other does not; finally, that in cases of 

 manifold linking the free rotation must be annulled. Hence this hypoth- 

 esis opens to us an understanding of the most important properties of 

 chemical valency. 



So much may be said of the problems relating to the theory of valency. 



But the doctrine of substitution has likewise experienced a peculiar 

 enlargement. Dumas first showed that the properties of organic com- 

 j)ounds are generally little changed when the hydrogen of the same is 

 replaced by univalent elements or groups. Now it has been learned 

 from later experiments that even much more radical changes in the 

 composition do not materially influence the properties of the substance. 

 If for example we replace in the hydrocarbon benzol — two carbon and 

 two hydrogen atoms by one atom of sulphur, the resulting product, 

 thiophen^ resembles benzol chemically and physically so closely as to 

 be mistaken for it. We learn from this that the sulphur atom is able 

 to take upon itself the functions of four atoms of entirely different 

 nature. Similar facts have been found in regard to oxygen and to the 

 imido group, which is equivalent to it. 



Turning away from these researches to cast a glance upon general 

 chemical studies which lie some years behind us, we must above all 

 consider one of the most far reaching discoveries of our epoch, the rev- 

 elation of the natural system of the chemical elements. We owe this to 

 the far-seeing Demetrius Mendelejetf. By the side of the titanic figure 

 of the Russian scholar we see the Englishman, Newlauds, and our own 

 countryman, Lothar Meyer, successfully co-operating in the foundation 



