"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. "Water is readily detected, either in ether or alcohol by allowing a drop to fall into spirits of turpentine, with which they ought to mix without turbidity; this is immediately produced if they contain water: for detecting water in _alcohol_, benzole is a more delicate re-agent than spirits of turpentine (Chemist, xxix, 203). It is also necessary that ether should be free from a remarkable property it acquires by long keeping, of decomposing iodides and setting free iodine, which thus gives the collodion a brown color; the same property may be developed in any ether, as Schonbein discovered by introducing a red hot wire into the vapor in the upper portion of a bottle containing a little ether and water; if it be then shaken up and a solution of iodide poured in, the whole rapidly becomes brown; this reaction is very remarkable and difficult to explain for even a mixture of the ether and nitric acid fails to produce a color _immediately_. Ether thus affected can only be deprived of this property by rectification with caustic potash." Bromo-Iodized Collodion for Positives.--No. 1. [Sidenote: IODIZED COLLODION FOR POSITIVES.] One very important object in connection with this part of the collodion process is to have chemicals of a good quality, and always employ those of a fixed standard. Plain collodion, 10 ounces. Solution of bromide, and iodide of potassium and silver, (page 61) 3 drachms. Iodide of ammonium, 10 grains. Hydro-bromic acid 6 drops. Double iodide of potassium and silver (see page 62) enough so that when the plate comes from the nitrate of silver bath, it will have an opaque cream color.