mond, graphite, and charcoal. Sulphur, in like manner, was found to crystallize in two wholly incompatible forms, and under these different phases showing such marked differences of qualities that they must be regarded as distinct substances. In 1845 Schrötter proved that what had before been known as red phosphorus, and thought to be a lower oxide of the element, was in fact a different condition, an allotropic form, as it was then called, of pure phosphorus—a form which differs as widely from the wax-like, highly combustible material that is so well known as any two substances well could differ. A few years earlier Schönbein had discovered a new condition of oxygen, which he called ozone, differing widely from ordinary oxygen gas. Now, since all the forms of the same element yield the same products, and hence give the same chemical reactions, it became obvious, as such facts multiplied, that we may have different substances consisting wholly of the same chemical element; and hence that the chemical element, whatever it might be, could not be a definite substance, as Lavoisier had defined it.
Meanwhile another class of facts became prominent, chiefly in consequence of the investigations in organic chemistry to which Liebig had given such great impulse in Germany. Groups of compounds, consisting for the most part of carbon, hydrogen, and oxygen, came to be known, which, although having exactly the same composition (that is, formed by the union of the same elements in the same definite proportions), had, nevertheless, utterly different properties and relations. Such compounds are said to be isomeric, and a good example may be found in acetic ether, a very fragrant neutral spirit, and butyric acid, whose offensive odor and acrid taste are only too well known in rancid butter. But if oxygen is the acidifying principle of butyric acid, why does it not produce the same effect as an equal constituent of the ether? Similar phenomena of isomerism soon became very prominent, and forced on chemists the conviction, often against their prejudices, that the nature of the product depended not solely on the nature and proportions of the elements which entered into its composition, but quite as much, and even more, on the manner in which the constituents were combined.
To this phrase—the manner in which the constituents are combined—no definite meaning was at first attached; but the old atomic theory, first applied in chemistry by Dalton, was soon so modified as to give a form to the conception, and on the distinction between atoms and molecules then introduced the whole philosophy of modern chemistry rests.
In the subdivisions of material bodies, the molecules are the smallest masses in which the qualities of a substance inhere. A molecule of sugar or salt is simply a very small lump of sugar or