106 PROGRESS OF SCIENCE IN THE CENTURY. 



encies; the phenomena of right and left handedness 

 led on to ideas of definite geometrical arrangement 

 within the molecule; in these and other ways the 

 atomic theory in its chemical applications has be- 

 come more and more specialised. " The present 

 position of structural chemistry may be summed up 

 in the statement that we have gained an enormous 

 insight into the anatomy of molecules, while our 

 knowledge of their physiology is as yet in a rudi- 

 mentary condition" (Meldola, 1895). 



THE PERIODIC LAW. 



A General Statement by Mendelejeff. " Many 

 natural phenomena," Mendelejeff says, " exhibit a 

 dependence of a periodic character. Thus the phe- 

 nomena of day and night and of the seasons of the 

 year, and vibrations of all kinds, exhibit variations of 

 a periodic character in dependence on time and space. 

 But in ordinary periodic functions one variable varies 

 continuously, while the other increases to a limit, then 

 a period of decrease begins, and having in turn 

 reached its limit, a period of increase again begins. 

 It is otherwise in the periodic function of the ele- 

 ments. Here the mass of the elements does not in- 

 crease continuously, but abruptly, by steps, as from 

 magnesium to aluminium. So also the valency or 

 atomicity leaps directly from 1 to 2 to 3, etc., without 

 intermediate quantities, and in my opinion it is these 

 properties which are the most important, and it is 

 their periodicity which forms the substance of the 

 periodic law. It expresses the properties of the real 

 elements, and not of what may be termed their mani- 

 festations usually known to us. The external proper- 

 ties of elements and compounds are in periodic de- 

 pendence on the atomic weights of the elements only 



