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. Modern chemists adopt, as a convenient mode of representing the phenomena which gases exhibit, the theory that every gas, whether elementary or compound, consists of minute molecules. They suppose that these molecules are of equal size, and are separated by equal intervals so long as the gas remains unchanged in heat and density. This view serves to account for the features of resemblance presented by all gases. The features in which gases vary are accounted for by the theory that the molecules are differently constituted. The molecules are supposed to be clusters of atoms, and the qualities of a gas are assumed to depend on the nature and arrangement of these ultimate atoms. The molecules of some elements consist but of a single atom; the molecules of others are formed by pairs of atoms; those of others by triplets; and so on. Again, the molecules of compound gases are supposed to consist of combinations of different _kinds_ of atoms.