128 CHEMICAL DISCOVERY AND INVENTION 



been for the last forty years the most important guide in the 

 prosecution of modern inorganic chemical research. 



The last version of his scheme of arrangement of the elements 

 as left in 1904 by Mendeleeff, not long before his death, is shown 

 in the preceding table. Many of the so-called rare earths are 

 omitted owing to the uncertainty which still prevails as to 

 the atomic weights and properties of many members of the 

 group. 



A few words of explanation are necessary ; y in the table is, 

 according to Mendeleeff, an analogue of helium with a density of 

 about 0-2 and a molecular weight 0-4. He supposed that it 

 might hereafter be identified with coronium, a hypothetical 

 element existing in the sun's coronal atmosphere ; x is the 

 " ether " of the physicist, for which Mendeleeff, disregarding 

 conventional views, supposed a molecular or atomic structure. 

 It was supposed also to be chemically inert and to have an ex- 

 tremely minute atomic weight. 



The spaces left vacant after hydrogen in Series 1 should be 

 occupied, according to Mendeleeff, by elements, at present un- 

 known, having approximately the atomic weights 14, 1-8, 2-2, 

 2-6, 2-8, 3-0, and 3-4. These would be the first members of the 

 Groups II to VIII respectively. 



This table is interesting for historical reasons, but the principle 

 of periodicity is more clearly displayed when properties and 

 atomic weights are plotted against each other in a system of 

 rectangular co-ordinates so as to reveal a curve. The first scheme 

 of this kind was published by Professor Lothar Meyer immedi- 

 ately after Mendeleeff's table in 1869. It displays the recurrence 

 of maxima in the values of the atomic volumes when the elements 

 are arranged consecutively in the order of their atomic weights. 

 The physical properties of the elements have been the subject of 

 much study since the time of Mendeleeff and Meyer, and it is 

 now possible to show graphically that not only do atomic 

 volumes wax and wane in following up the series, but other 

 properties such as melting-points and coefficients of expansion 

 follow the same order. The following diagram (Fig. 51) is taken 

 from a paper in the Journal of the American Chemical Society 

 (July, 1915) by Professor T. W. Richards, " Concerning the Com- 

 pressibilities of the Elements and their Relations to other 

 Properties." Here it is interesting to observe how closely the 

 configurations of the several curves agree with one another, the 



