Transcriber Note Text emphasis denoted as _Italics_ and =Bold=. Whole and fractional parts of numbers as 123-4/5 and some fractions as 1/50th. A MANUAL OF PHOTOGRAPHIC CHEMISTRY. A MANUAL OF PHOTOGRAPHIC CHEMISTRY, INCLUDING THE PRACTICE OF THE COLLODION PROCESS. By T. FREDERICK HARDWICH, LECTURER ON PHOTOGRAPHY IN KING'S COLLEGE, LONDON; LATE DEMONSTRATOR OF CHEMISTRY IN KING'S COLLEGE. Fourth Edition. [Illustration] LONDON: JOHN CHURCHILL, NEW BURLINGTON STREET. MDCCCLVII. [_The Author reserves to himself the right of translating this Edition._] PRINTED BY JOHN EDWARD TAYLOR, LITTLE QUEEN STREET, LINCOLN'S INN FIELDS. PREFACE TO THE THIRD EDITION. [Illustration] It is a source of much, gratification to the Author to find himself called upon to prepare a Third Edition of his Manual in less than fourteen months from the date of its first publication. No greater proof could have been afforded of the rapid advance which the Photographic Art is now making in this country. On once more entering upon the task of revision, the Writer has been led to reflect in what way the utility of the Work may be promoted; and from numerous inquiries he believes that this result will best be attained by carefully omitting everything which does not possess _practical_ as well as scientific interest. The majority of Photographers look to the Art to furnish them with amusement as well as instruction, and they are deterred from entering upon a study which seems to involve a great amount of technical detail: these remarks however are not intended to discourage a habit of perseverance and careful observation, but simply to distinguish between the essential and the non-essential in the theory of the subject. The present Edition differs in many important particulars from those which have preceded it. It has undergone a fresh arrangement throughout. In some parts it is condensed, in others enlarged. The Chapters on Photographic Printing are entirely re-written, and include the whole of the Author's investigations, as published in the Society's Journal. The minute directions given in this part of the Work will show how much success in Photography is thought to depend upon a careful attention to minor particulars.