Compound radicles, consisting of carbon and the three elementary gases, have been discovered which enter into combination in definite proportions as simple atoms, and all compound radicles travel in the galvanic circuit as equivalents to the elementary substances. Hitherto they have been regarded as representatives or equivalents of one atom of hydrogen. Now it is generally admitted that each has the property of replacing two, three, or more atoms of hydrogen by equivalent substitution. This multiple equivalency among compound radicals forms the basis of what is called the polyatomic theory, now so much employed by MM. Hofmann, Berthelot, and other great modern chemists. Water is the most common radicle both in the inorganic and organic world. Though a compound of oxygen and hydrogen, it enters, according to the law of definite proportion, into the composition of various amorphous bodies in a dry state, that is in the form and proportion of its gases. It is an essential element in the greater number of crystals, and abounds in organic matter. In certain cases the same substance crystallizes at different temperatures, unites with different quantities of water under the form of oxygen and hydrogen, and assumes corresponding forms. For example, the seleniate of zinc unites with three different portions of water and takes three different forms, according as its temperature is hot, lukewarm, or cold. Thus each particle of water, containing one atom of oxygen and one of hydrogen, combines with one atom of zinc in three different proportions as if it were a simple atom. The water of crystallization may be driven off from many substances by heat, as from the hydrates of lime, iron, copper, &c., but when combined with the oxides of certain metals, potassium for instance, it cannot be driven off by any means whatever. In general a heat of 212° Fahr. is sufficient, but some crystals lose their water of crystallization at the ordinary atmospheric temperature. Crystals whose atoms are in unstable equilibrium, are readily altered both externally and internally by a very moderate degree of heat. Arragonite and calcareous spar are isomeric, that is, they are chemically the same but differ in form and hardness, which shows that their molecules are grouped differently. When the arragonite is heated, the inertia of its atoms is overcome, the crystal explodes with force, and becomes a mass of crystals of calcareous spar. The expansive force of the heat suddenly overcoming the force of cohesion causes the explosion, and at the same time disturbs the unstable repose of the atoms, which immediately obey their natural attractions and assume the stable form of calcareous spar.