by J. J. Berzelius
(Published in Poggendorf's Annalen der Physik und Chemie, 1830, vol. 19, pp. 305 ff.)
The experimental portion of this paper establishes that the two acids have the same elemental composition. Berzelius analyzed them as their lead salts which he formulates as lead oxide (PbO) plus the anhydride of the diacids. (We would associate all oxygen with the dianion of the diacids and leave the lead alone as Pb+2.) Berzelius burned one gram of the lead salt of racemic acid to obtain 0.101 grams of water and 0.4975 grams of carbon dioxide. For the dehydrated diacid portion of racemic acid he writes the formula H4C4O5, which he says predicts 0.10109 g water and 0.49692 g of carbon dioxide (we would now use subscripts instead of superscripts and would write C4H4O6 for the diacid dianion). He reports the organic percentage as H (3.0045), C (36.8060), O (60.1895). Berzelius's percentages sum to 100%, unlike the values his student Wöhler reported for urea. [It would be instructive to check his calculations.]
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 make the derivation 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 different 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.