It may be well here to explain to students that one of the lines of argument which lead to the conclusion that the water molecule, as it ordinarily exists, is really complex and massive, is based upon measurements of the Faraday dielectric constant for water; for this constant, or "specific inductive capacity," is found to be very large, something like 50 times that of air or free ether; whereas for glass it is only 5 or 6 times that of free space. The dielectric constant of a substance generally increases with the density or massiveness of its molecule,—indeed, the value of this constant is one of the methods whereby matter displays its interaction with and loading of the free ether of space,—and any such density as the conventional nine times that of hydrogen for the molecule of water would be wholly unable to explain its immense dielectric constant. The influence of the massiveness of a water molecule is also displayed in its power of tearing asunder or dissociating any salts or other simple chemical substance introduced into it; common salt, for instance, is found always to have a certain percentage of its molecules knocked or torn asunder directly it is dissolved in water, so that, in addition to a number of salt molecules in solution, there are a few positively charged sodium atoms and a few negatively charged chlorine atoms, existing in a state of loose attraction to the water aggregate, and amenable to the smallest electric force; which, when applied, urges the chlorine one way and the sodium the other way, so that they can be removed at an electrode and their place supplied by freshly dissociated molecules of salt, thus bringing about its permanent electro-chemical decomposition, and enabling the water to behave as an electrolytic conductor directly a little salt or acid is dissolved in it. The power of the water molecule to associate itself with molecules of other substances is illustrated by the well-known fact that water is an almost universal solvent. It is its residual affinity which enables it to enter into weak chemical combination with a large number of other substances, and thus to dissolve those substances. The dissolving power usually increases when the temperature is raised, possibly because the self-contained or self-sufficient groupings of the water molecules are then to some extent broken up and the fragments enabled to cling on to the foreign or introduced matter instead of only to each other. The foreign substance is apt to be extruded again when the liquid cools, and when the affinity of the water-aggregates for each other resumes its sway. Very hot water can dissolve not only the substances familiarly known to be soluble in water, but it can dissolve things like glass also; so that glass vessels are unable to retain water kept under high pressure at a very high temperature, approaching a red heat.