Читать книгу A Practical Manual of the Collodion Process. Giving in Detail a Method For Producing Positive and Negative Pictures on Glass and Paper онлайн

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"Of all the substances used in this process, the gun-cotton is usually the only one actually prepared by the operator himself; in this he cannot fail to have observed the great variations in the solubility, and, when dissolved, the transparency and tenacity of the films, to which it is liable; the various processes also that are given appear at first sight unaccountably different, some directing ten minutes, others a few seconds immersion. In consequence of this I have specially examined into the cause of all these variations, with a view to obtain certainty, and also have endeavored to discover how far they affect the sensitiveness of the prepared surface. If we take a mixture of the strongest nitric and sulphuric acids and immerse as much cotton as can be wetted, after some minutes squeeze out the acid as far as possible, then immerse a second portion of cotton, and again express the acids for a third portion of cotton, and so on until the liquid is exhausted, we shall find, on comparing the cottons thus treated, after washing and drying, that there is a gradual alteration in their properties, the first being highly and perfectly explosive, and each succeeding portion less so, until the portion last immersed will be found hardly explosive, leaving distinct traces of charcoal or soot when burned. This may not appear surprising at first sight, as it may be imagined that the latter portions are only a mixture of gun-cotton and common cotton; this is, however, not the case, for if each quantity be immersed sufficiently long, it will not contain a fibre of common cotton, and may yet become charred on burning like unaltered cotton. The most remarkable difference, however, is discovered on treating them with ether containing a little alcohol, when, contrary to what might have been anticipated, the first or strongest gun-cotton remains untouched, while the latter portions dissolve with the utmost ease, without leaving a trace behind, which alone is sufficient proof that no unaltered cotton remains. This difference in properties is owing to the gradual weakening of the acid mixture, in consequence of the nitric acid being removed by the cotton, with which it becomes intimately combined, at the same time that the latter gives out a proportionate quantity of water. In consequence of these experiments, a great many mixtures of these acids were prepared of various strengths, each being accurately known, both to determine whether there were more than one kind of soluble gun-cotton, and, if there were, to ascertain exactly the mixture required to produce the most suitable to photographic purposes. By this means, and by, what I believe has not been pointed out, varying the temperature, at least five varieties were obtained;—first, gun-cotton, properly so called, as before stated, quite insoluble in any mixture of alcohol and sulphuric ether. Secondly, an explosive cotton, likewise insoluble, but differing chemically from the first, obtained by a mixture of certain strength when used cold. If warm, however, either from the heat produced spontaneously on mixing the two acids; or by raising the temperature artificially to about 130°, the cotton then immersed becomes perfectly soluble, producing a third variety; if, however, it be thoroughly dried, it becomes in a great measure insoluble. The fourth is obtained by the use of weaker acids used cold, and the fifth when the mixture has been warmed to 130° previous to the immersion of the cotton; in either of the two last cases the product is perfectly soluble, but there is a remarkable difference between their properties, for on dissolving 6 grains of each in 1 ounce of ether, the cotton treated with warm acids gives a perfectly fluid solution (which is likewise the case with the third variety produced by acids something stronger), while that obtained by the use of cold acids makes a mixture as thick as castor-oil.