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the science in its general relations for popular educational uses should not overlook the considerations suggested in the above-quoted passage. In this volume, therefore, I have aimed to preserve somewhat the transitional aspect of the subject, so that the “New Chemistry” may neither be regarded as an ingenious device of yesterday, nor as a finality to be acquired with no expectation of further improvement.

To prevent misconception respecting the claims of this class-book, it is necessary to repeat what was said in the Preface to the preceding edition. It is not designed as a manual for special chemical students. It aims to meet the wants of that considerable class, both in and out of school, who would like to know something of the science, but who are without the opportunity or the desire to pursue it in a thorough experimental way. Some acquaintance with the subject is now required as a part of every good education; but books designed for laboratory use, and abounding in technical details, are ill-suited to those who do not give special and thorough attention to the subject. I have here attempted to furnish such an outline of the leading principles and most important facts of the science as shall meet the needs of the mass of students in our high schools, seminaries, and academies, who go no further with the subject than to study a brief text-book, with the assistance perhaps of a few lectures, and the observation of some accompanying experiments.

The present edition has been much reduced in compass, both by the use of larger type and fewer pages, and it has thus been brought into more manageable limits for school-use. Much new matter has, however, been introduced under various heads. The rapid development of spectrum analysis since the former edition

was published, and the great interest of the subject, have led to considerable expansion of that topic. The treatment of the chemistry of light is also amplified, and the chapter on theoretical chemistry explaining the new system is made as full as the proportions of the volume will allow. Tables of the French system of weights and measures are appended for the use of those who desire to employ it. As the progress of investigation is constantly bringing physics and chemistry into closer relations, the division of chemical physics has been retained, although the text has been much reduced.

Such a class-book can, of course, have little value for the usual purposes of reference. It must be but a brief compend of general principles and descriptions of some of the most important substances, and is not to be judged by the fullness of its details. Such are already the vast proportions of the science, and such the enormous rapidity of its growth, that nothing less than works of encyclopedic scope have value for general consultation. Watts's invaluable “Dictionary of Chemistry," with its five volumes averaging a thousand closely-printed pages, has already a thousand-paged supplement; and it would require such a volume every year adequately to report the progress of the science. The class-book should be supplemented by some such ample treatises in every school-library.

I have to acknowledge especial indebtedness in preparing the chapter on theoretical chemistry to the admirable volume of Professor J. P: Cooke, entitled “ The New Chemistry "-one of the finest pieces of exposition in the language. It is a book that every chemical teacher should study, and I would moreover earnestly recommend them to place it in the hands of their classes, and have them go carefully through it. No other work that I know of can put them in such thorough possession of the later stand-points of chemical study.

To many teachers and superintendents of schools who have been anxious for the appearance of this revised edition of the class-book, my apologies are due for broken promises and a delay in publication that may well have seemed without excuse. I have only to plead that the volume would have been issued long since but for the failure of my eyesight from overwork. I have been greatly aided in this revision by my excellent friend Professor Charles Froebel; and I have also to thank another friend, Miss Mary E. Shaw, for efficient assistance in seeing the book through the press. That errors may have crept in is probable, but I think they will not be found serious, and shall be glad to have any inaccuracies pointed out for correction in future editions.

E. L. Y. New York, June, 1876.

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