always remembering that this help is given to solve chemical problems, and that with purely physical problems, he, as a chemist, is not concerned'.

Pursuing then an almost purely analytical method the chemist finds that his subject branches off into two main divisions. The properties of bodies are modified :—he studies the relations between the new substances and those by the mutual action of which the new bodies were produced; but changes in the properties of bodies involve a consideration of the relative positions of the changing body and of other bodies, involve the action of force: he endeavours to elucidate the laws of action of this force.

The hypothesis that bodies consist of small parts-called molecules in motion, is one of the lines along which dynamical science pursues its advance into the sphere of chemistry. The study of chemical phenomena is also brought within the pale of dynamical methods by the application to these phenomena of the general principles of the conservation and degradation of energy. The latter (thermodynamic) method is more applicable to the study of the laws of chemical forces than of the properties of the substances depending on the actions of these forces, which properties have been chiefly elucidated by the help of the molecular theory.

We may indeed study relations between forces accompanying changes in the distribution of certain material magnitudes, which we may call molecules, without reference to what is generally known as the molecular theory of matter.

But it seems certain that no chemical phenomenon—and it is well for the student to bear in mind that the chemical part is always but one aspect of any natural occurrence—can be fully explained unless both methods of investigation are applied; unless the relations between the reacting bodies and the products of the reaction, and the relations between the forces exhibited in the phenomenon in question, are considered.

1 Chemistry, being more concrete, is less exact than Physics; mathematical methods can scarcely as yet be applied to chemical data.

2 See Clerk Maxwell: Science Conferences at South Kensington, 1876.

In the following pages an attempt is made to gather together the more important data on which the leading generalisations of chemistry are based, and in the light of this material to discuss these generalisations.

By the use of the terms Chemical Statics and Chemical Kinetics I endeavour to indicate, roughly, that the phenomena included under the first of these headings are on the whole those exhibited by chemical bodies or systems of bodies in equilibrium, while the phenomena classed together as chemical kinetics relate more to bodies or systems of bodies when chemically active.

It may seem pedantic to make use of terms having definite and precise significations when from the very nature of the facts they can be employed only in the broadest and roughest way. I only wish to indicate that the subject-matter of chemical science is considered in this book as divisible into two large parts, of which one comprises the facts and principles concerned, on the whole, with chemical composition, and the other those which, broadly speaking, relate to chemical action.

It will of course be found that chemical occurrences present, I think one may say always present, both statical and kinetical problems; the two sides of any chemical problem can scarcely be regarded apart, in the present state of knowledge at any rate, without danger; it may therefore be that phenomena ranked by one chemist as statical would by another be classed as kinetical.

I begin by considering the facts and principles roughly classed as statical, because although the study of kinetics seems naturally to precede that of statics, yet in chemistry our knowledge of composition is much in advance of our knowledge of action: I then consider the data and generalisations of so-called chemical kinetics; and lastly I endeavour to review some of those phenomena, explanations of which, generally only very partial explanations, can be gained, or hoped for, only by the help of both methods.

BOOK I..

CHEMICAL STATICS.

CHAPTER I.

ATOMS AND MOLECULES.

1. THE foundations of the atomic theory were laid in the later years of last century by the German chemist RICHTER'. The work of BERGMANN, although of earlier date than that of Richter, cannot be regarded of equal importance as concerns the history of the atomic theory.

Richter studied the neutralisation of acids by bases, and of bases by acids, and shewed that a definite amount of acid (or base) always combines with a definite amount of base (or acid) when neutralisation is accomplished. By determining the weights of various bases neutralised by one and the same weight of each acid, and the weights of various acids neutralised by one and the same weight of each base, Richter was able to arrange many acids and bases in order of neutralisation. FISCHER, in 1803, published the first table of chemical equivalents. Richter had given a series of numbers for each base expressing the quantities thereof which would neutralise 1000 parts of sulphuric, hydrochloric, nitric &c. &c. acids; Fischer saw that it was sufficient to attach a single number to each base and a single number to each acid, 1000 parts of sulphuric acid being adopted as the unit of neutralisation. Fischer's numbers expressed quantities of bases, or

1 Ueber die neueren Gegenstände der Chemie, 1791-1802: and Anfangsgründe der Stöchiometrie oder Messkunst chemischer Elemente, 1822.

2 Chemische Werke, 2. 25 et seq.

3 In a note to Berthollet's Essai de Statique Chimique.

acids, which were of equal value so far as power to neutralise a constant weight of a certain acid or base was concerned1.

Foreshadowings of the atomic theory are to be found in a work by W. HIGGINS entitled A comparative view of the Phlogistic and Antiphlogistic Theories, with Inductions (1791) [see Henry's Life of Dalton p. 75 et seq.] but to DALTON is undoubtedly due the signal honour of introducing a clear and self-consistent theory regarding the composition and structure of chemical substances, a theory which in its essential points has stood the test of rigorous experimental verification, and has adapted itself to the wants of each successive school of chemical thought.

2. Dalton, and others, found that elements were united in many compounds in fixed proportions by weight, and moreover that in certain compounds of one element with others the amount by weight of this element could be expressed by whole multiples of one fundamental number3. To account for these facts Dalton recalled the atomic theory of the Greek philosophers; but he introduced accuracy where there had been vagueness. From an interesting intellectual plaything Dalton's genius produced an exact scientific theory capable of experimental application.

Every chemical substance, simple or compound, is made up of atoms, or small undivided parts1;—the old hypothesis had gone nearly as far as this: Dalton added, the atom of every chemical substance has a definite weight, and although this weight cannot be determined, we nevertheless can determine the relative weights of the atoms of all bodies. It

1 For more details regarding the work of Richter and Fischer, see Wurtz, The Atomic Theory, pp. 12-22.

It is important to note that the atomic theory was conceived by Dalton in 1802 from considering the results of physical experiments: he distinctly states in a paper on the absorption of gases in liquids read to the Manchester Philosophical Society in that year that he had lately been prosecuting with remarkable 'success,' 'an inquiry into the relative weights of the ultimate particles of bodies.' 3 For examples of this law of multiple proportions see Roscoe and Schorlemmer's Treatise on Chemistry, 1. pp. 60-65.

Dalton's application of the term atom to the small chemically indivisible parts of compounds, seems to shew that he did not regard his atoms as absolutely indivisible; see Life by Henry, p. 88.

is only necessary to choose some substance as a standard, then the weight of the smallest quantity of any other substance which combines with the unit weight of the standard substance represents the weight of the atom of the combining substance in terms of the unit chosen.

As this point is of supreme importance it may be well that we should have Dalton's own words before us. In the New System of Chemical Philosophy (1808) after discussing the constitution of mixed gases, Dalton proceeds: 'When 'any body exists in the elastic state its ultimate particles are 'separated from each other to a much greater distance than 'in any other state; each particle occupies the centre of a 'comparatively large sphere, and supports its dignity by 'keeping all the rest, which by their gravity or otherwise are 'disposed to encroach upon it, at a respectful distance. When 'we attempt to conceive the number of particles in an atmo'sphere, it is somewhat like attempting to conceive the number 'of stars in the universe; we are confounded by the thought. 'But if we limit the subject, by taking a given volume of any 'gas, we seem persuaded that, let the divisions be ever so 'minute, the number of particles must be finite; just as in a 'given space of the universe the number of stars and planets 'cannot be infinite.

'Chemical analysis and synthesis go no further than to 'the separation of particles one from another, and to their ' reunion. No new creation or destruction of matter is within 'the reach of chemical agency. We might as well attempt to 'introduce a new planet into the solar system, or to annihi'late one already in existence, as to create or destroy a particle of hydrogen. All the changes we can produce 'consist in separating particles that are in a state of cohesion 'or combination, and joining those that were previously at a 'distance.

'In all chemical investigations it has justly been consider'ed an important object to ascertain the relative weights of 'the simples which constitute a compound. But unfortunately 'the inquiry has terminated here; whereas from the relative 'weights in the mass, the relative weights of the ultimate