162 TRAiJSACTIONS OF SECTION B. 



facts and theories of inorganic chemistry, still it appeared to me that I might 

 utilise this as an opportunity of passing in brief review some of the features of 

 the rise and development of the ' Periodic Law.' 



The memoir, made known to the non-Russian reader by the abstract in 

 German, shows the principle of periodicity — viz., the recurrence of similar pro- 

 perties at regular intervals with increase in the magnitude of atomic weights, 

 the possibility of utilising the atomic weights as a basis of the classification of 

 the elements, the necessity for the revision of the values thus assigned to the 

 atomic weights of certain elements, and finally that the .scheme demanded for 

 its completeness the existence of many new elements. 



The later writings of Mendeleeff contain the mode of tabulating the elements 

 in the form usually adopted in chemical text-books, portraying the principle of 

 periodicity and showing the grouping of the elements into natural families. 

 But undoubtedly the clearest demonstration of the association between the 

 atomic weights and the physical properties of the elements is that exhibited 

 by the curve of atomic w~eights and atomic volumes, which is an outcome of the 

 independent studies of these relationships by Lothar Lleyer, and, as is well 

 known, shows the members of the natural families of elements occupying corre- 

 sponding positions on the curve. This curve, with its undulations, corresponding 

 to the series of the elements, has contributed to impress on the mind of the 

 student the relationship between the properties of the elements and their atomic 

 weights, and may have exercised an influence in drawing attention to these 

 relationships which the attempts of the earlier workers in this field were not 

 successful in doing. 



Mendeleeff's Table of the Elements was just beginning to figure in the 

 teaching of chemistry in my undergraduate days, and, together with the specu- 

 lations underlying it, aroused considerable interest and proved an incentive and 

 inspiration for experimental inquiry. Foremost in this counti-y amongst those 

 who by their writings have contributed to spread a knowledge of IMendeleeff's 

 speculations was my fellow-student, Carnelley. His experimental investigations 

 added materially to our knowledge and definition of the physical properties of 

 elements and compounds, which further emphasised the periodicity in the rela- 

 tion of the atomic weights to the properties of the elements, and have provided 

 data from which curves, resembling in contour the atomic volume curve, have 

 been set up. 



A valuable guide in fixing the atomic weights of the elements has been the 

 specific heat which, as the discovery of Dulong and Petit showed a hundred 

 years ago, varies in the case of solid elementary bodies inversely with their 

 atomic weights: or. as is more usually expressed, the solid elements have the 

 same atomic heat. The investigation of the exceptions to this empirical rule 

 brought out the fact that the specific heat is influenced by temperature, and the 

 study of the influence of low temperatures led Sir James Dewar to the discovery 

 that at about 50° Absolute the atomic heats of the elements are a periodic func- 

 tion of the atomic weights. Further, the graphic representation of this relation 

 gives a curve very similar in its course to that of the atomic volume curve. So 

 that the specific heat is another of the physical properties to fit into the periodic 

 scheme. 



The necessity for a revision of the atomic weights of certain elements, as 

 pointed out by Mendeleeff, has induced several workers to direct their energies 

 to the solution of the problems indicated, so that in our present-day tables many 

 of the anomalies of position and sequence which existed in the earlier schemes 

 have disappeared. Tellurium has still resisted all attempts to bring it into 

 order, with an atomic weight less than that of iodine, which its association with 

 sulphur and selenium demands. The interesting attempts to decompound tel- 

 lurium have so far remained unfruitful. 



But undoubtedly the most fascinating feature of the periodic system is that 

 ' it allows the discovery of many new elements to be foreseen.' This and the 

 manner in which Mendeleeff, in full conviction of the truth of the ' Periodic 

 Law,' boldly assigned properties to those elements required to fill the blank 

 spaces_ in the table of elements, and the verification within twenty years in 

 three instances of these prophetic specifications have contributed to the recogni- 

 tion and firm establishment of the 'Periodic Law ' as an article of belief in 



