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VIII ADVERTISEMENT TO THE FOURTH EDITION.

is increasing yearly. There is, therefore, a probability that the metrical system will be generally used by men of science, and this can be said of no other system of weights and measures. I have, therefore, thought it right to introduce it side by side with our own, in order to familiarize the student with its units.

The volume has also received various additions of greater or less importance, such as the progress of science demanded. A considerable portion of the introductory chapter has been re-written. Among the larger additions may be mentioned an account of the researches of Deville and of Graham upon the permeation of metallic septa by gases, at elevated temperatures; and the results obtained by Gladstone and Dale, and by Landolt upon the connexion of the optical properties of bodies with their chemical composition. Considerable additions have been made to the Section on Spectrum Analysis; and the Chapter on the Photographic Actions of Light has been transferred from the second volume, in order that it may be considered along with the other phenomena of light described in the present part. Some account of the British Association standard of electrical resistance, as well as of the machines of Holmes and of Wilde for producing light by magneto-electric currents, will also be found in their appropriate places.

King's College, London, June 5 th, 1867.

ADVERTISEMENT

TO THE FIRST EDITION.

THE work, of which the first part is now presented to

the Reader, was originally designed to supply the Students who were attending the Course of Lectures on Chemistry at King's College with a text-book to guide them in their studies.

The present Part, on Chemical Physics, is devoted to a subject upon which no elementary work has appeared in this country since the publication of the excellent Treatise of the late Professor Daniell, and in attempting to supply what the Author, in his own experience, has felt to be a want, he ventures to hope that the result of his labours may be found useful to persons beyond the circle of his own immediate Class. Much new matter, which has never yet been reduced to a systematic form, is now presented to the Student, particularly in the chapters on Adhesion, on Heat, and on Voltaic Electricity.

It is proposed to complete the work in Three Parts. The Second Part, which will be devoted to Inorganic Chemistry, is expected to be ready by the end of the present year; and the Third Part, which will embrace Organic Chemistry, in the spring of next year.

As the Author was originally a pupil of Professor Daniell, and was subsequently, for several years, associated with him as Lecturer on Chemistry, it has happened that, in some of the subjects treated of in this volume, the

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PREFACE TO THE FIRST EDITION.

thoughts and mode of arrangement resemble those adopted by that distinguished philosopher in his Introduction to the Study of Chemical Philosophy.

The second Edition of that work was published so far back as 1843; and even if the work itself had not been long out of print, the progress of science would have detracted greatly from its utility as a text-book. The adaptation of that work to the systematic teaching of the present day would have involved changes of an extensive character: moreover, every teacher who takes an interest in the progress of his class has his own views and methods. The Author, therefore, judged it better, after much consideration, to bring out a new work, leaving untouched that of his late Master as the true exponent of his views upon some of those branches of science which his researches had contributed to advance and adorn.

The Author cannot omit to avail himself of the present opportunity of expressing his obligations to his friend, Mr. C. Tomlinson, for many valuable suggestions, and for the warm interest which he has taken in the progress of the work, but more especially for the devotion of no inconsiderable portion of time and labour to the revision of the proof-sheets.

King's College, London, March, 1855.

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