SKETCH OF DIMITRI IVAKO VICE MENDELEEF. 263 



can be either formed or decomposed. In addition, tlie equilib- 

 rium between tlie quantity of the definite compound and of its 

 products of dissociation is defined by the laws of chemical equi- 

 librium, which require a relation bet"^een equal volumes and 

 their dependence on the mass of the active component parts," 



In 1881 Mendeleef turned his attention to experiments on the 

 elasticity of the gases, which he continued with the aid of several 

 of his pupils. They led to many interesting results, among which 

 was one showing that the deviations from Marriotte's law were 

 in opposite directions at pressures above and below that of the 

 atmosphere ; indicating that air, for instance, as well as carbonic 

 acid and sulphurous acid gases, experience a change of compressi- 

 bility at certain pressures. 



The results of these experiments were used in studies of the 

 physical nature of the rarefied air of the upper atmosphere and 

 the application of aeronautics, and he attempted to organize 

 meteorological observations in the upper atmosphere by means of 

 balloons. 



The principles on which Mendeleef based the periodic law 

 were first explained in a paper read before the Russian Chemical 

 Society in 18G9. As repeated by the author in his Faraday lect- 

 ure to the English Chemical Society, they declare that the ele- 

 ments, if arranged according to their atomic weights, exhibit 

 a periodicity of properties ; that elements which are similar in 

 chemical properties have atomic weights that are nearly of the 

 same value or which increase regularly ; that the arrangement of 

 the elements or groups of elements in the order of their atomic 

 weights corresponds to their so-called valencies, and, to some 

 extent, to their distinctive chemical properties ; that the elements 

 which are the most widely diffused have small atomic weights ; 

 that the magnitude of the atomic weight determines the charac- 

 ter of the element, just as the magnitude of the molecule deter- 

 mines the character of a compound body ; that the discovery of 

 many yet unknown elements may be expected ; that the calcu- 

 lation of the atomic weight of an element may sometimes be 

 amended by a knowledge of those of its contiguous elements ; 

 and that certain characteristic properties of elements can be fore- 

 told from their atomic weights. The theory was founded upon 

 experiment, and assumed the adoption of the definite numerical 

 values of the atomic weights, and the recognition that the rela- 

 tions between the atomic weights of analogous elements were 

 governed by some general law, with a more accurate knowledge 

 of the relations and analogies of the rarer elements as necessary 

 for the completing and proving of it. In accordance with the 

 theory as thus developed, a table was composed by Mendeleef and 

 Victor Meyer, including nearly but not quite all of the elements 



