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(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.