Having reason to conclude that atom combines with atom, according to a system most harmoniously arranged, there can be no difficulty in conceiving that molecule unites with molecule, in a manner regulated by some equally well-marked law. It was, indeed, a discovery by Wenzel, of Fribourg, that, in salts which decompose each other, the acid which saturates one base will also saturate the other base; and the subsequent observations of Richter, of Berlin, who attached proportional numbers to the acids and bases, and who remarked that the neutrality of metallic salts does not change during the precipitation of metals by each other, which led the way to the atomic theory of Dr. Dalton, to whom entirely belongs the observation, that the equivalent of a compound body is the sum of the equivalents of its constituents, and the discovery of combination in multiple proportions. The elements of a molecule can take a new arrangement amongst themselves, without any alteration in the number of the atoms or of their weight, and thus give rise to a body of a different form and colour, although possessing the same chemical constitution. This is the case with many of the organic compounds of carbon and hydrogen. The elements of a compound may be disassociated, and thus the dissimilar substances of which it is composed set free. A piece of chalk exposed to heat is, by the disturbance of its molecular arrangement, changed in its nature; a gaseous body, carbonic acid, is liberated, and quick-lime (oxide of calcium) is left behind. If this carbonic acid is passed through red-hot metal tubes, or brought in contact with heated potassium, it is resolved into oxygen and charcoal--the oxygen combining with the metal employed. The oxide of calcium (lime), if subjected to the action of a powerful galvanic current, is converted into oxygen and a metal, calcium. Thus we learn that chalk is a body consisting of two compound molecules,--carbonic acid, which is formed by the combination of an atom of carbon with two atoms of oxygen,--and lime, which results from the union of an atom of calcium with one of oxygen. The condition requisite to the production of chemical action between bodies is that they should be dissimilar. Two elementary atoms are placed within the spheres of each other’s influences, and a compound molecule results. Oxygen and hydrogen form water; oxygen and carbon give rise to carbonic acid; nitrogen and hydrogen unite to form ammonia; and chlorine and hydrogen to produce hydrochloric acid. In all these cases an external force is required to bring the atoms within the range of mutual affinity: flame,--the electrical spark,--actinism,--or the interposition of a third body, is necessary in each case. There are other examples in which no such influence is required. Potassium and oxygen instantly unite: chlorine, iodine, and bromine immediately, and with much violence, combine with the metals to form chlorides, iodides, or bromides.