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The constantly increasing interest in the beautiful Art of Photography, has naturally led all who practise it to desire some knowledge of the agency by which their pictorial results are produced. The chemistry of the solar radiations, or, as it has been appropriately termed, Actino-chemistry, is essentially a new science. The breaking up of strong chemical combinations by a power so subtile as that of the sunbeam, is amongst the most striking of the facts which claim the attention of the natural philosopher. While the physical and chemical phenomena of the solar radiations require for their investigation the aid of all the refinements of modern science.

For the purpose of placing in the hands of all who are interested in this science, a clear and satisfactory record of the progress which has been made since the publication of the First Edition of these “ Researches,” it has been found advisable to entirely recast the work. More than two-thirds of it has therefore been re-written, and the remaining third materially modified.

The former Edition was spoken of by some reviewers as a compilation. Since the rewards to the original investigator are but few, I feel it necessary to claim for my labours a higher position.

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