"L'action chimique est réciproque son effet est le résultat d'une tendance mutuelle à la combinaison; on ne peut pas, à la rigeur, dire plutôt qu'un liquide agit sur un solide, qu'on ne peut dire que le solide agit sur le liquide: la commodité de l'expression fait transporter sans inconvénient toute l'action dans l'une des deux substances, quand on veut examiner l'effet de cette action plutôt que l'action elle-même." BERthollet.

INTRODUCTORY.

CHEMISTRY is preeminently the science which concerns itself with the changes presented in material phenomena; it is the science which attempts to classify the mutations that matter undergoes.

In the chemical examination of any kind of matter two questions have always pressed for answers:-What can this substance do? Of what is this substance composed? While attempting to answer these questions separately, and while thus more or less adopting two schemes of classification, chemists have for the most part recognised that no complete answer could be given to either question considered wholly apart from the other; hence the two methods of chemical investigation, and the two lines of chemical advance have generally been closely interwoven.

In older times a substance was said to be capable of doing this or that because it contained certain elements or essences; substances were classed together because of similarity of actions, but the points of resemblance on which classification was based were uncertain and undefined:-the conception of element was paramount. The substances in a class shewed many or a few points of resemblance because each member of the class contained the same element, and hence was a more or less perfect means for exhibiting the properties of that element. The ideas of composition and properties, as we now use these expressions, were both implied in the older conception of element.

M. C.

I

If it be granted that the various forms of matter are vehicles for displaying the properties of a few elements, it follows that the addition or withdrawal of this or that element will probably suffice to change one into another form of matter. Hence arose the art of alchemy and the pursuit of the philosopher's stone. This pursuit resulted in the accumulation of many facts most of which could for some time be explained by aid of the one underlying general conception of element. But as facts accumulated the foundation was found to be too narrow to bear the structure raised upon it; a need was felt for minor explanations and for partial hypotheses. Observers began to contrast sour, acid substances with mild, tasteless, non-corrosive substances; hence arose the division of a large class of bodies into two minor classes, acids and alkalis. This classification when carried to completion produced the school of iatro-chemists, in whose hands chemical science became a branch of the art of medicine. But once again facts were observed which could not be explained by the theories of the medical chemists the experimental method was recognised as alone leading to definite and trustworthy results in the examination of natural phenomena; but the experimental method, it was found, to be of value must be accurate, and to be accurate must be quantitative. Advance became rapid. The conception of element remained but in modified form, the distinction between alkali and acid remained, but proved to mean at once less and more than its originators supposed. Bodies were compared as to their actions and as to their compositions; the comparison led on one hand to the recognition of force exerted by one body on another, called affinity, and on the other hand to the recognition of ultimate forms of matter, called elements, of which all bodies are composed.

From this point the two broad paths of advance become more easily distinguished; advance is made by seeking answers to questions such as these:-What is the nature of the elements? What is the composition of compounds? Can the facts regarding elementary combinations be generalised? Advance is also made by pursuing inquiries indicated

by such questions as these:-What connection, if any, exists between the properties of elements and of compounds of these elements? What actions are these compounds capable of performing? And advance is also made by combining both methods of inquiry in seeking answers to such a question as this: Why are the properties of these compounds such as they are observed to be?

At one time those chemists for whom the composition of compounds was all-important have been supreme; at another time the place of authority has been occupied by those who regarded function, or power of doing, as the essential subject of study. The greatest outcome of the work of the former school is the atomic hypothesis, now merged in the wider molecular theory of matter; the most important result of the studies of the latter school is the conception of chemical affinity; both have taken part in the development of the modern views regarding molecular structure and rational formulæ.

While assigning the credit of special advances to one of the two great schools of chemistry, we cannot but recognise that these advances have been made by the help of suggestions borrowed from the other; recent developments of the atomic theory cannot be separated from the rise of the unitary system, the latest hypotheses regarding the structure of molecules are connected with the subject of chemical affinity.

Eighty years ago Berthollet attempted to arrange the facts of chemical action under a general conception which should serve to connect chemical with physical changes; but the attempt was only partially successful because of the scanty supply of purely chemical data. General views of chemical action were soon abandoned for a study of the properties of the products of this action, but of late years many chemists have resumed the investigation of the general conditions of chemical action, and have obtained results which give good grounds for hoping that this study will throw light on the masses of facts already accumulated concerning compounds, and groups of compounds, and taken along with that method

of investigation which is based on a study of the composition of compounds, will lead to the establishment of chemistry as a branch of the science of dynamics.

The study of the motions of material bodies considered as accompanying mutual actions between these bodies belongs to the general science of dynamics. Phenomena presented by mutually acting bodies wherein the properties of these bodies are not profoundly modified, belong to the domain of physical science. Chemistry deals with those reactions between bodies wherein profound modifications in the properties of the bodies occur. Or, we may say that whereas physical science is concerned with the properties of this or that kind of matter considered for the most part apart from the action on it of other kinds of matter, chemistry is concerned with the mutual actions which occur between matter of different kinds whereby persistent changes in the properties of the reacting kinds of matter occur.

Chemistry furnishes problems for the solution of which physical and dynamical methods are applicable. Chemical science is ever tending toward abstract truths, i. e. truths involved in many phenomena although actually seen in none: but before she gains abstract truths chemistry amasses many general truths, i.e. 'truths which sum up many facts'.'

The chemist is set to solve the problem,-Why are the properties of bodies profoundly modified under certain conditions? In attempting to find a solution, he must divide the phenomena which he observes into their factors, and study each of these as far as possible independently of the others.

The chemist need not regard the methods pursued by those sciences which are more concrete than his own, although he may furnish them with subject-matter for investigation; inasmuch however as the science of matter and motion is a more abstract science than that of chemistry, he must seek help for his work in the methods of that science,

1 The abstract and the general truths of chemistry are scarcely yet so differentiated as to allow of each class being considered separately. I do not purpose attempting more than a very rough separation in this book.