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. This quantity is equivalent to 1-1/4 ounce Troy or Apothecaries' weight;--and to 1-1/4 ounce Avoirdupois weight _plus_ 54 grains. Place this in a teacup or any other convenient vessel, and pour upon it. Water 1-1/2 fluid drachms mixed with Oil of Vitriol 12 " Stir well with a glass rod for two or three minutes, until all effervescence has ceased, and an even, pasty mixture, free from lumps, is obtained. During the whole process, abundance of dense fumes of Nitric Acid will be given off, which must be allowed to escape up the flue or into the open air. _A modification of the formula._--The above formula will invariably succeed with a good sample of acid and pure Nitre. When tried however with Oil of Vitriol rather weaker than ordinary, and _commercial_ Nitre, it may fail, the cotton being gelatinized and dissolved. When such is the case, the addition of water must be omitted or the quantity reduced from one drachm and a half to half a drachm. GENERAL DIRECTIONS FOR IMMERSING, WASHING, AND DRYING THE PYROXYLINE.