reverse changes occurring between different members of the system.

The examination of the distribution of the chemically interacting substances in a system free to settle down into equilibrium led us to give a definite meaning to the term affinity as applied to acids and bases. We found it possible to obtain measurements of the relative affinities of various acids and bases, and thus to attach to each acid and base a constant number which conveys much quantitatively accurate information regarding the amounts of various chemical changes in which the acids and bases play important parts. We were able to see that there are definite connexions between the compositions of acids and bases and the values of the affinityconstants of these compounds. The accurate development of this connexion is in the future.

Although we have endeavoured to separate the chemical from the physical parts of the events we have studied, we have found the two classes of phenomena sometimes so inextricably interwoven that it was impossible wholly to ignore the physical aspects of certain chemical occurrences. Chemical changes, we found, are accompanied by changes of energy, and, as the net result, part of the energy of the initial system is always degraded when the system has attained to chemical equilibrium.

That we might form clear mental pictures of the mechanism of chemical changes, we found it almost necessary to adopt the conceptions and the language of the molecular and atomic theory. This theory put before us two definite portions of each kind of matter, the atom and the molecule; it enabled us to form well-defined conceptions of each of these extremely minute masses. Of the two conceptions, we found the atom the more definite; we were obliged to confine ourselves to gases when attempting to reason accurately concerning molecules, and even then we found it necessary to allow some latitude to our notion of the molecule, such latitude as is implied in the physical definition of the gaseous molecule as “that minute portion of a gas which moves about as a whole, so that its parts, if it has any, do not part company during the motion of agitation of the gas.”

As the molecular theory has been developed from the study of gaseous phenomena, and is as yet strictly applicable only to gases, we found it advisable to speak of chemical changes occurring between solid or liquid substances as being interactions between reacting weights,-meaning thereby aggregates or collocations of atoms—rather than between molecules, of the bodies taking part in the changes.

The study of the interactions of gases led to the conception of the gaseous molecule as a structure built up of definite numbers of atoms arranged in a definite manner; chemical facts obliged us to connect the properties of gaseous molecules not only with the nature, and the number, but also with the arrangement, of their parts. It was sometimes necessary to count two or more atoms in a molecule as a single atom, so far as certain chemical changes were concerned; we thus gained the conception of the compound radicle.

As a guide in our attempts to learn something about the arrangement of the parts of molecules, we made use of the hypothesis of atomic valency, which asserts that each atom forming part of a gaseous molecule is capable of directly interacting with a limited number of other atoms. We agreed to measure the maximum number of atoms between which and any specified atom there could be direct intramolecular action by the maximum number of atoms of hydrogen, fluorine, chlorine, bromine, or iodine, with which the specified atom combines to form a gaseous molecule.

The more our study of chemistry advanced the more importance were we led to attach to those constants, the atomic weights of the elements, until at last we arrived at a system of chemical classification, based on the atomic weights of the elements, which as it is developed seems to include in itself all other classificatory schemes.

Chemistry is the daughter of alchemy. The object of both has always been to find the changeless foundation of changing phenomena.

Alchemists dreamt of the philosopher's stone, and worked hard to find it. Chemists have found the elements, and beneath the elements they have found the atoms, and beneath the atoms they sometimes think they perceive the atoms of the one element, of which all the known elements and compounds, it may be, are developed forms.

INDEX.

The numbers refer to paragraphs.

Elements and compounds are referred to in groups. For instance, the heading Strontium
and its compounds refers the reader to Calcium group. Again, Ammonia will be found
under the heading Nitrogen group, hydrides of. Similarly Sulphuretted hydrogen must be
looked for under Sulphur group, hydrides of ; Hydrochloric acid under Chlorine group,
hydrides of