(17.) So it is in the present case. Although we are unable by direct observation to prove the existence of constituent material atoms of determinate figure, yet there are many observable phenomena which render their existence in the highest degree probable, if not morally certain. The most remarkable of this class of effects is observed in the crystallisation of salts. When salt is dissolved in a sufficient quantity of pure water, it mixes with the water in such a manner as wholly to disappear to the sight and touch, the mixture being one uniform transparent liquid like the water itself, before its union with the salt. The presence of the salt in the water may, however, be ascertained by weighing the mixture, which will be found to exceed the original weight of the water by the exact amount of the weight of the salt. It is a well-known fact, that a certain degree of heat will convert water into vapour, and that the same degree of heat does not produce the same effect upon salt. The mixture of salt and water being exposed to this temperature, the water will gradually evaporate, disengaging itself from the salt with which it has been combined. When so much of the water has evaporated, that what remains is insufficient to keep in solution the whole of the salt, a part of the latter thus disengaged from the water will return to the solid state. The saline constituent will not in this case collect in irregular solid molecules; but will exhibit itself in particles of regular figure, terminated by plane surfaces, the figure being always the same for the same species of salt, but different for different species. These particles are called _crystals_. There are several circumstances in the formation of these _crystals_ which merit attention. If one of them be detached from the others, and the progress of its formation observed, it will be found gradually to increase, always preserving its original figure. Since its increase must be caused by the continued accession of saline molecules, disengaged by the evaporation of the water, it follows that these molecules must be so formed, that by attaching themselves successively to the crystal, they maintain the regularity of its bounding planes, and preserve their mutual inclinations unvaried. Suppose a crystal to be taken from the liquid during the process of crystallisation, and a piece broken from it so as to destroy the regularity of its form: if the crystal thus broken be restored to the liquid, it will be observed gradually to resume its regular form, the atoms of salt successively dismissed by the vaporising water filling up the irregular cavities produced by the fracture. Hence it follows, that the saline particles which compose the surface of the crystal, and those which form the interior of its mass, are similar, and exert similar attractions on the atoms disengaged by the water.