1882.] 



President's Address. 



329 



the spectrum at different elevations during a recent visit to Switzer- 

 land. 



The Davy Medal has been awarded to Dimitri Ivanovitch Mendeleeff 

 and Lothar Meyer. 



The attention of chemists had for many years past been directed to 

 the relations between the atomic weights of the elements and their 

 respective physical and chemical properties ; and a considerable num- 

 ber of remarkable facts had been established by previous workers in 

 this field of inquiry. 



The labours of Mendeleeff and Lothar Meyer have generalised and 

 extended our knowledge of those relations, and have laid the founda r 

 tion of a general system of classification of the elements. They 

 arrange the elements in the empirical order of their atomic weights, 

 beginning with the lightest and proceeding step by step to the 

 heaviest known elementary atom. After hydrogen the first fifteen 

 terms of this series are the following, viz : — 



Lithium 7 



Beryllium 9 '4 



Boron 11 



Carbon 12 



Nitrogen 14 



Oxygen 16 



Fluorine .... 19 



Sodium 23 



Magnesium 24 



Aluminium 27 "4 



Silicon 28 



Phosphorus 31 



Sulphur 32 



Chlorine 35 '5 



Potassium 39 



No one who is acquainted with the most fundamental properties of 

 these elements can fail to recognise the marvellous regularity with 

 which the differences of property, distinguishing each of the first 

 seven terms of this series from the next term, are reproduced in the 

 next seven terms. 



Such periodic re-appearance of analogous properties in the series of \ 

 elements has been graphically illustrated in a very striking manner, 

 with respect to their physical properties, such as melting-points 

 and atomic volumes. In the curve which represents the relations of 

 atomic volumes and atomic weights analogous elements occupy very: 

 similar positions, and the same thing holds good in a striking 

 manner with respect to the curve representing the relations of 

 melting-points and atomic weights. 



Like every great step in our knowledge of the order of nature, this j 

 periodic series not only enables us to see clearly much that we could' 

 not see before ; it also raises new difficulties, and points to many 

 problems which need investigation. It is certainly a most important 

 extension of the science of chemistry. 



z 2 



