If in this book I have shewn any just appreciation of the scientific study of nature, I owe it to the teaching of

PROFESSOR SIR WILLIAM THOMSON.

To him, therefore, I dedicate my work.

PREFACE.

THIS book is intended to give a fairly complete account of the present state of knowledge regarding the principles and general laws of chemistry.

It is addressed to students of this science who have already a considerable acquaintance with descriptive chemistry, but it is hoped that by such students the book will be found complete in itself; so that while it certainly deals with chemical principles and theories on the supposition that its readers have some knowledge of chemical facts, yet the book may fairly claim to rank as a systematic treatise on chemical philosophy.

While I have tried to supply full information regarding those points which appear to me of most importance, I have also sought to avoid great detail, and to present a sketch of the principles of chemistry the parts of which shall hang together as being mutually dependent.

To know what to omit has been one of the most difficult parts of my undertaking. The chemical student is too often subjected to a shower-bath of facts; he is made to feel that

to sit as a passive bucket and be pumped into...can in the long-run be exhilarating to no creature.'

An attempt is made in this book to treat the principal theories of modern chemistry to some extent from an historical point of view, and to trace the connections between the older theories and those which now prevail in the science. It is hoped that the student may thus gain a firmer grasp of those theories than he is able to do when they are put before him as entirely creations of recent times.

I have tried to deal with chemical facts and generalisations so as to shew their reality. This can best be done, I believe, by following in the very foot-prints of the great discoverers, by watching them as they make their footing sure, and as they feel their way up the heights. That the student may be able to verify the accounts I have given of the more important investigations, and more especially that he may fill in the details which I have necessarily omitted, I have given copious references to original memoirs and papers; these references will, I believe, be found correct, at least I have spared no pains to make them so. I have also endeavoured to make the index full and complete.

So far as I am aware, no sufficiently comprehensive guide to the study of the principles of chemistry exists, in an English form, although we have many excellent works dealing with descriptive chemistry, with the materials, that is to say, from which chemical science is being constructed. Professor Lothar, Meyer's Die Modernen Theorien der Chemie, to a considerable extent meets the wants of the German student. I have made free use of that book in preparing my own; but I venture to think I have incorporated in my general plan many important facts and principles which do not find a