upon dilution of its solutions, a fact confirming the spectroscopic evidence of oscillations due to benzol structure in solutions of small concentration. The presence of isorropesis, here brought about by two carbonyl groups in juxtaposition and indicated, as we have seen, by vibratory frequencies in the ether of longer wave-lengths than those due to keto-enol tautomerism, stands out at once as the source of color in chemical compounds. Again we note that the more pronounced this isorropesis the more active chemically are the groups undergoing the disturbance. The additive capacity of benzil is markedly less than that of diacetyl.
Among the compounds which furnish us with examples of this nature, or substances in which two carbonyl groups can come under the influence of each other, we may mention the most important of all, that of para-benzoquinone. This quinone is a derivative of benzol in which two oxygen atoms are located in the para-position to each other. In order to satisfy the bivalence of the oxygen atoms, the para carbon atoms are regarded as having their free affinities absorbed in these oxygen atoms, leaving the remaining carbon atoms to arrange their free affinities in two pairs of double linkings (according to the Kekulé hypothesis). It may be, however, that the second affinity of each oxygen atom will assert itself in a linking between these two atoms and therefore leave the characteristic benzol nucleus undisturbed. The two forms may be graphically shown as follows:
Now it has been proved chemically that para-benzoquinone can exist in each of these two forms. We have then just such an example of making-and-breaking as has been indicated in isorropesis, but in addition a change in the manner of linkings in the molecule accompanies this process. The absorption spectrum of para-quinone gives a band with its head at the oscillation frequency 2,150, one almost identical with that obtained from other ring compounds, e. g., camphorquinone, where two carbonyl groups are adjacent. From a study of the pulsations of the benzol ring the para-positions have been shown to be closely related, and in fact so well brought under the influence of each other as to be considered as practically adjacent. Since the position of the absorption band is in the blue region color must, of course, be present in this substance. Simple quinones usually show a yellow to an orange color. This color is undoubtedly due to isorropesis and whenever we have this class of substances—known as quinones—
