Photography with Emulsions
by William De Wiveleslie Abney (9781458841551)

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER III. THE CAUSE AND CURE OF FOG IN EMULSIONS. Every student in emulsion work has found, and will find, that the chief obstacle that he has to overcome to obtain success is the tendency for the plates prepared with an mulsion to fog, or veil over on development, and it has taken a great deal of experimental work to enable it to be overcome. The writer ventures to think that the researches he has made on the subject have explained in a great measure, if not entirely, its raison d'etre. Setting aside the collodion or gelatine from the question, and merely taking into consideration the sensitive salts mployed, we may arrive at very definite results. It has been asserted that a neutral combination between two substances can never take place; for example, if we mix potassium chloride with silver nitrate we shall never be able to get pure silver chloride, however much we may wash it that cither the soluble potassium or silver salt -will always be in excess, though probably in the minutest quantities. This certainly is the case theoretically, because do what you will, and wash as long as you like, there still must be some infinitely small part of the soluble salt left behind. Now, in ordinary chemical analysis, where products have to be weighed, the residual impurity may be inappreciable, being so infinitesimal that no balance yet constructed can show them. Though a balance may be inoperative, yet, as is well known, light is able to show us impurities in a substance which may not be one-millionth part of a grain in weight. By passing the light emitted from heated vapours of the substance and its impurity through a prism, and noting its spectrum, we may be able to detect the latter. The spectroscope will not tell us at present, however, whether the silver or potassium is...

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