[33] Observe that the Ether be pure; if it contains too much water and Alcohol, it will not dissolve the Pyroxyline, or will yield an opalescent solution.
There is a third condition of Pyroxyline, different from either of the above, which may be puzzling:—the fibres of the Cotton mat together very slightly or not at all on immersion, and the washing proceeds tolerably well; the compound formed is scarcely explosive, and dissolves imperfectly in Ether, leaving little nodules or hard lumps. The ethereal solution yields, on evaporation, a film which is opaque instead of transparent. In this case (presuming the Ether to be good) the acid mixture is slightly too weak, or the temperature is too low, being probably about 90°, instead of 130° to 140° (?).
When the acid mixture has been brought to the proper strength by a few preliminary trials, proceed according to the directions given at the next page.
PREPARATION OF NITRO-SULPHURIC ACID BY OIL OF VITRIOL AND NITRE.
This process is recommended, in preference to the other, to the amateur who is unable to obtain Nitric Acid of convenient strength. The common Oil of Vitriol sold in the shops is often very good for Photographic purposes; but it is best, if possible, to take the specific gravity, when any doubt exists of its genuineness. At a temperature of 58° to 60°, specific gravity 1·833 is the usual strength, and if it falls below this, it should be rejected. (See Part III. for 'Impurities of Commercial Sulphuric Acid.')
The Nitre must be the purest sample which can be obtained. Commercial Nitre often contains a large quantity of Chloride of Potassium, detected on dissolving the Nitre in distilled water, and adding a drop or two of solution of Nitrate of Silver. If a milkiness and subsequent curdy deposit is formed. Chlorides are present. These Chlorides are injurious; after the Oil of Vitriol is added, they destroy a portion of Nitric Acid by converting it into brown fumes of Peroxide of Nitrogen, and so alter the strength of the solution.
Nitrate of Potash is an anhydrous salt,—it contains simply Nitric Acid and Potash, without any water of crystallization; still, in many cases, a little water is retained mechanically between the interstices of the crystals, and therefore it is better to dry it before use. This may be done by laying it in a state of fine powder upon blotting-paper, close to a fire, or upon a heated metallic plate.
The sample must also be reduced to a fine powder before adding the Oil of Vitriol; otherwise portions of the salt escape decomposition.
These preliminaries having been properly observed, weigh out
Pure Nitre, powdered and dried, 600 grains.