368 THE CHEMICAL PROBLEMS OF TO-DAY. 



tribnteil amo!!-]^ the vahMicies. Kiecke, in virtue of liis ])yro-('lectric re- 

 soarcbes, was led to the view that tbe atoms are siirrouiuled by certaiu 

 systems of positive and negative electric poles. 



Unitinfj these results with those of purely chemical experimentation, 

 we arrive at the idea that the vaK'iicies do not aj)pear as poiyifs of attack 

 proper, but as havinj? linear dimensions. The carbon atom represents 

 itself as a s])heri', surrounded by an envelope of a'ther which contains 

 the valen(;ies. The latter seem to be determined by the presence of two 

 op[)osite electric jjoles which rest at the ends of a very short straight 

 line. Such a system is called a cU-poJe. The attachment of two valencies 

 to eacli other consists in the attraction of tiieir opposed ])()les. It is 

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

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

 case of a taii<;ential 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 consecpiently for the tetrahedric grouping of the sauie. 



The fact tliatthe valencies can deviate from this position now becomes 

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

 with one another, while those of ditJerent atoms can combine; it is 

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

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

 manifold linking the free rotation must be annulled. Hence this hyi)otl)- 

 esis opens to ns an nnderstanding of the most important properties of 

 chemical valency. 



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



But the doctrine of substitution has likewise experienced a j)eculiar 

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

 pounds are generally little changed when the hydrogen of the same is 

 rei)laced by univalent elements or groups. Now it has been learned 

 from later experiments that even much more radiiral changes in tiie 

 composition do not materially influence the properties of tbe substance. 

 If for example we rei)lace in the hydrocarbon benzol — two carbon and 

 two hydrogen atoms by one atom of suli)hur, 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 ditferent 

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

 imido gronp, which is equivalent to it. 



Turning away from these researches to cast a glance ni)on general 

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

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

 elation of tbe natural .si/ste 01 of the chemical element!^. We owe this to 

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

 of the llussian scholar we see the Englishman, Newlands, and our own 

 countrynian, Lothar Meyer, successfully co-operating in tbe foundation 



