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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"This difference of condition is owing to the very sparing solubility of ether in water, which in the first case prevents the entrance of the nitrate of silver into the film, consequently the iodide and silver solutions meet on the surface; but on addition of alcohol, its solubility enables the two to interchange places, and thus the iodide of silver is precipitated throughout the substance in a state of the utmost division. This difference is clearly seen under the microscope, the precipitate being clotted in the one case, while in the other the particles are hardly discoverable from their fineness.

"The presence of a little water considerably modifies these results, since it in some degree supplies the place of alcohol, and is so far useful; but in other respects it is injurious, for, accumulating in quantity, if the collodion is often used, it makes the film weak and gelatinous, and what is worse, full of minute cracks on drying, which is never the case when pure ether and alcohol are used. Since the ether of the shops almost always contains alcohol, and frequently water, it is important to ascertain their amount before employing it for the preparation of collodion; the quantity of alcohol may be easily ascertained by agitating the ether in a graduated measure glass (a minim glass does very well) with half its bulk of a saturated solution of chloride of calcium; this should be poured in first, its height noted, and the ether then poured on its surface, the thumb then placed on the top, and the two agitated together; when separated, the increase of bulk acquired by the chloride of calcium indicates the quantity of alcohol present, and for this allowance should be made, in the addition of alcohol afterwards to the collodion.