3. General remarks regarding substances that possess the same composition, but different properties. [page 326]
To treat these substances easily, one needs a general way to name them; and one would derive best, it seems to me, from the Greek, as the customary root of scientific terminology. I have thought it necessary to choose between the words : homosynthetic and isomeric substances. The former is built from homos, equivalent, and synthetos, put together; the latter from isomeres has the same meaning, although it only properly says put together from the same pieces. The latter has the advantage with respect to shortness and euphony, and thus I have decided to choose it.
By isomeric substances I understand those which possess the same chemical composition and the same atomic [molecular] weight, but differnet properties. There is yet another sort of substances which possess the same percentage compostions but different atomic weights, most often multiples of one another; of this sort is the hydrocarbon CH2, which, assuming that the analyses have the requisite reliability, can appear as: 1) olefiant gas, 2) another gas, which is easy to condense to an oil, with double the atomic weight of the first, and 3) one or several crystalline substances. I do not include these, since they must be better studied, and then apparently will require a special collective name.
Berzelius proceeds to cite many examples of known isomers and proposes that when an isomer of a preexisting compound is discovered, the new form be called by the same name with the prefix para, from the Greek preposition meaning beside.
Since a chemical formula is nothing more than a simple expression of weight relations, isomeric substances of the same type can be labelled by the same formula.