Pleasant Ways in Science

by Richard A. Proctor

It was important to set this question at rest. This was accomplished by the labours of De la Rive and Marignac, who proved that ozone is simply another form of oxygen.

Here we touch on a difficult branch of modern chemical research. The chemical elements being recognized as the simplest forms of matter, it might be supposed that each element would be unchangeable in its nature. That a compound should admit of change, is of course a thing to be expected. If we decompose water, for instance, into its component elements, oxygen and hydrogen, we may look on these gases as exhibiting water to us in another form. And a hundred instances of the sort might be adduced, in which, either by separating the elements of a compound, or by re-arranging them, we obtain new forms of matter without any real change of substance. But with an element, the case, one would suppose, should be different.

However, the physicist must take facts as he finds them; and amongst the most thoroughly recognized chemical facts we have this one, that elementary substances may assume different forms. Chemists call the phenomenon allotropy. A well-known instance of allotropy is seen in red phosphorus. Phosphorus is one of the chemical elements; and, as every one knows, the form in which it is usually obtained is that of a soft, yellow, semi-transparent solid, somewhat resembling bees’ wax in consistence, poisonous, and readily taking fire. Red phosphorus is the same element, yet differs wholly in its properties. It is a powder, it does not readily take fire, and it is not poisonous.

Ozone, then, is another form of oxygen. It is the only instance yet discovered of gaseous allotropy.

And now we have to deal with the difficult and still-vexed questions of the way in which the change from oxygen is brought about, and the actual distinction between the two forms of the same gas. Schönbein held that common oxygen is produced by the combination of two special forms of oxygen—the positive and the negative, or, as he called them, ozone and antozone. He showed that, in certain conditions of the air, the atmospheric oxygen exhibits qualities which are the direct reverse of those which ozone exhibits, and are distinct from those of ordinary oxygen. In oxygen thus negatived or antozonized, animals cannot live any more than they can in nitrogen. The products of decomposition are not only not destroyed as by ozone, but seem subject to preservative influences, and speedily become singularly offensive; dead animal matter rapidly putrefies, and wounds show a tendency to mortification.

But the theory of positive and negative forms of oxygen, though still held by a few physicists, has gradually given way before the advance of new and sounder modes of inquiry. It has been proved, in the first place, that ozone is denser than ordinary oxygen. The production of ozone is always followed by a contraction of the gas’s volume, the contraction being greater or less according to the amount of oxygen which has been ozonized. Regularly as the observers—Messrs. Andrews and Tait—converted a definite proportion of oxygen into ozone, the corresponding contraction followed, and as regularly was the original volume of the gas restored when, by the action of heat, the ozone was reconverted into oxygen.

And now a very singular experiment was made by the observers, with results which proved utterly perplexing to them. Mercury has the power of absorbing ozone; and the experimenters thought that if, after producing a definite contraction by the formation of ozone, they could absorb the ozone by means of mercury, the quantity of oxygen which remained would serve to show them how much ozone had been formed, and thence, of course, they could determine the density of ozone.

Suppose, for instance, that we have one hundred cubic inches of oxygen, and that by any process we reduce it to a combination of oxygen and ozone occupying ninety-five cubic inches. Now, if the mercury absorbed the ozone, and we found, say, that there only remained eighty-five cubic inches of oxygen, we could reason in this way:—Ten cubic inches were occupied by the ozone before the mercury absorbed it; but these correspond to fifteen cubic inches of oxygen; hence, ozone must be denser than oxygen in the proportion of fifteen to ten, or three to two. And whatever result might have followed, a real absorption of the ozone by the mercury would have satisfactorily solved the problem.

But the result actually obtained did not admit of interpretation in this way. The apparent absorption of the ozone by the mercury, that is, the disappearance of the ozone from the mixture, was accompanied by no diminution of volume at all. In other words, returning to our illustrative case, after the absorption of the ozone from the ninety-five cubic inches occupied by the mixture, there still remained ninety-five cubic inches of oxygen; so that it seemed as though an evanescent volume of ozone corresponded in weight to five cubic inches of oxygen. This solution, of course, could not be admitted, since it made the density of ozone infinite.

The explanation of this perplexing experiment is full of interest and instruction. The following is the account given by Mr. C. W. Heaton (Professor of Chemistry at Charing Cross Hospital), slightly modified, however, so that it may be more readily understood.