can be either formed or decomposed. In addition, the equilibrium between the quantity of the definite compound and of its products of dissociation is defined by the laws of chemical equilibrium, which require a relation between 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 compressibility 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 1869. As repeated by the author in his Faraday lecture to the English Chemical Society, they declare that the elements, 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 character of the element, just as the magnitude of the molecule determines the character of a compound body; that the discovery of many yet unknown elements may be expected; that the calculation 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 foretold 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 relations 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
