The sphere, however, in which the law of nature discovered by Hegel celebrates its most important triumphs is that of chemistry. Chemistry can be termed the science of the qualitative changes of bodies as a result of changed quantitative composition. That was already known to Hegel himself (Logic, Collected Works, III, p. 488). As in the case of oxygen: if three atoms unite into a molecule, instead of the usual two, we get ozone, a body which is very considerably different from ordinary oxygen in its odour and reactions. Again, one can take the various proportions in which oxygen combines with nitrogen or sulphur, each of which produces a substance qualitatively different from any of the others! How different laughing gas (nitrogen monoxide N2O) is from nitric anhydride (nitrogen pentoxide, N2O5) ! The first is a gas, the second at ordinary temperatures a solid crystalline substance. And yet the whole difference in composition is that the second contains five times as much oxygen as the first, and between the two of them are three more oxides of nitrogen (NO, N2O3, NO2), each of which is qualitatively different from the first two and from each other.
This is seen still more strikingly in the homologous series of carbon compounds, especially in the simpler hydrocarbons. Of the normal paraffins, the lowest is methane, CH4; here the four linkages of the carbon atom are saturated by four atoms of hydrogen. The second, ethane, C2H6, has two atoms of carbon joined together and the six free linkages are saturated by six atoms of hydrogen. And so it goes on, with C3H8, C4H10, etc., according to the algebraic formula CnH2n+2, so that by each addition of CH2 a body is formed that is qualitatively distinct from the preceding one. The three lowest members of the series are gases, the highest known, hexadecane, C16H34, is a solid body with a boiling point of 270ºC. Exactly the same holds good for the series of primary alcohols with formula CnH2n+2O, derived (theoretically) from the paraffins, and the series of monobasic fatty acids (formula CnH2nO2). What qualitative difference can be caused by the quantitative addition of C3H6 is taught by experience if we consume ethyl alcohol, C2H12O, in any drinkable form without addition of other alcohols, and on another occasion take the same ethyl alcohol but with a slight addition of amyl alcohol, C5H12O, which forms the main constituent of the notorious fusel oil. One’s head will certainly be aware of it the next morning, much to its detriment; so that one could even say that the intoxication, and subsequent “morning after” feeling, is also quantity transformed into quality, on the one hand of ethyl alcohol and on the other hand of this added C3H6.
In these series we encounter the Hegelian law in yet another form. The lower members permit only of a single mutual arrangement of the atoms. If, however, the number of atoms united into a molecule attains a size definitely fixed for each series, the grouping of the atoms in the molecule can take place in more than one way; so that two or more isomeric substances can be formed, having equal numbers of C, H, and O atoms in the molecule but nevertheless qualitatively distinct from one another. We can even calculate how many such isomers are possible for each member of the series. Thus, in the paraffin series, for C4H10 there are two, for C6H12 there are three; among the higher members the number of possible isomers mounts very rapidly. Hence once again it is the quantitative number of atoms in the molecule that determines the possibility and, in so far as it has been proved, also the actual existence of such qualitatively distinct isomers.
Friedrich Engels 1883 Dialectics of Nature
II. Dialectics (The general nature of dialectics to be developed as the science of interconnections, in contrast to metaphysics.)