Substances which are capable of replacing one another in compounds, and which are endowed with qualities mutually analogous, are said to be isomeric. Many isomeric compounds are formed of the same materials, in the same proportions, and yet differ essentially both in their physical and chemical properties; whence M. Daniel observes, that a specific and definite arrangement of the constituent molecules in space appears to be no less essential to the individual constitution of bodies than a certain proportion between their heterogeneous ingredients.
Successive substitution in isomeric bodies does not alter the character of the chemical formulæ of these bodies; thus chlorine, bromine and iodine, are chemically equivalent with an atom of hydrogen, for they may be put for one or more atoms of hydrogen in various compounds without changing the character of the chemical formulæ of these compounds. The peroxide of hydrogen consists of one atom of hydrogen and two of oxygen; hence if 32·5 parts of zinc, 28 of manganese, and 32 of copper be successively put for the atom of hydrogen, the result will be the peroxides of zinc, manganese and copper respectively. Here the character of the chemical formula of the original compound remains the same, and the three metals are chemically equivalent to one another, and to the atom of hydrogen. In many compounds organic and inorganic, one or more atoms of hydrogen may be replaced by an equal number of atoms of sodium, potassium, zinc, &c., without altering the character of the chemical formula of the compound.
Olefiant gas, olefiant oil and paraffin, form an isomeric series of a gas, a liquid and a solid, consisting of carbon and hydrogen. The gas contains 86 parts in 100 of carbon, and forms the most luminous part of coal gas.
M. Dumas has proved it to be a general law, that when three isomeric bodies are arranged in the sequence of their chemical properties, there will also be a sequence in their respective atomic numbers, and that whenever this symmetry of chemical properties and atomic weights obtains, any one of these substances may be substituted for the other without changing the chemical character of the formula.
Sulphur, selenium, and tellurium, form an isomeric group; that is, they form a sequence, with analogous qualities, for sulphur is the most volatile; selenium, a simple substance found in iron pyrites in Sweden, is less volatile; and tellurium is the least volatile and with regard to their atomic sequence, the atomic weight of sulphur is 16, that of tellurium is 64, and half the sum of these numbers is 40, the atomic weight of selenium, the mean term. Hence selenium might be put in any compound for the sulphur, and the tellurium for the selenium, without changing the chemical character of its formula.