PREFACE.

THE "Class-book of Chemistry," first published in 1852, was rewritten in 1863, and has now been again thoroughly revised, so as to bring it into harmony with the latest views, and adapt it more perfectly to the wants of those for whom it was prepared. The first edition represented the state of chemistry as it prevailed at the time of publication, and had been long established; but the revised edition, though adhering to the old theories, recognized that they were undergoing important modifications. These modifications have been long in progress, and having at length issued in a new system of chemical doctrine, which has been generally accepted by chemists, it has been adopted in the present volume, and explained and applied as fully as the plan of the work will allow. The present position of the science is, therefore, of special importance in relation to its exposition.

There can be no question that the new theories mark an important step in the progress of chemistry. They harmonize a wider range of facts, and give us a more consistent philosophy of the subject, than the theories they supersede. Yet they are far from being complete. The present situation is the proverbially

uncomfortable one of transition; the old house having ceased to be habitable, while the new one is unfinished. Prof. A. Crum Brown, of the Edinburgh University, in a late address before the British Association, well stated the present attitude of chemical theory in the following words:

"It is impossible to make a certain forecast: looking back, we see a logical sequence in the history of chemical speculation; and no doubt the next step will appear, after it has been taken, to follow as naturally from the present position. One thing we can distinctly see-we are struggling toward a theory of chemistry. Such a theory we do not possess. What we are sometimes pleased to dignify with that name is a collection of generalizations of various degrees of imperfection. We cannot attain to a real theory of chemistry until we are able to connect the science by some hypothesis with the general theory of dynamics."

This view of chemical science, as a body of thought in process of development, more perfect at present than ever before, but still imperfect in relation to the future, should not now be lost sight of. It shows both the reason and necessity of change, reconciles difficulties, and enables us rightly to estimate the value of preceding systems, which, although now displaced, were essential conditions of chemical advancement. We are not to regard past theories as mere exploded errors, nor present theories as final. The living and growing body of truth has only moulted its old integuments in the progress to a higher and more vigorous state. It is certainly desirable that this complexion of the subject should be recognized in its presentation to ordinary students. Practical text-books, intended for mastering the subject experimentally, must, of course, be much confined to existing facts and the principles by which they are now interpreted; but books designed to present