The mixture of Sulphuric Acid and Nitre requires to be used immediately after its preparation, as it solidifies into a stiff mass on cooling; but the mixed acids may be kept for any length of time in a stoppered bottle. When Cotton is used, the fibres should be well pulled out, and small tufts added one by one to the acid mixture, stirring with a glass rod in order to keep up a constant change of particles. The Paper is cut into squares or strips, which are introduced singly. In either case the quantity must not be too great, or some portions will be imperfectly acted upon; about 20 grains to each fluid ounce of the mixture will be sufficient. The _time of immersion required_ varies from ten minutes with Cotton, to twenty minutes or even half an hour with the Paper. When an unusually large proportion of Sulphuric Acid is used, as in the case of a weak sample of Nitric Acid, the Cotton should be removed at the expiration of six or seven minutes, as there is a tendency to partial solution of the Pyroxyline in the acid mixture under those circumstances. It is an advantage in some cases to prepare the material at a high temperature, but unless the proportions of the Acids are strictly according to Mr. Hadow's formula, solution of the Cotton may take place if the thermometer indicates more than 140°. After the action is complete, the Nitro-Sulphuric Acid is left weaker than before, from addition of various atoms of water necessarily formed during the change. Hence, if the same portion be used more than once, an addition of Sulphuric Acid will be required. _Directions for Washing._--In removing the Pyroxyline from the Nitro-Sulphuric Acid, press out as much of the liquid as possible, and wash it rapidly in a large quantity of cold water, using a glass rod to preserve the fingers from injury. If it were simply thrown into a small quantity of water and allowed to remain, the rise in temperature and weakening of the acid mixture might do mischief. The washing should be continued for at least a quarter of an hour, or longer in the case of Paper, as it is essential to get rid of every trace of acid. When the Nitre plan has been adopted, a portion of the _Bisulphate of Potash_ formed adheres to the fibres, and if not carefully washed out, an opalescent appearance is seen in the Collodion, resulting from the insolubility of this salt in the ethereal mixture. If no acid taste can be perceived, and a piece of blue litmus-paper remains in contact with the fibres for five minutes without changing in colour, the product is thoroughly washed. It is however a safe plan to place the Pyroxyline in running water and allow it to remain for several hours. Lastly, wring it out in a cloth, pull out the fibres, and dry slowly, by a moderate heat. After drying, it may be kept for any length of time in a stoppered bottle. RECAPITULATION OF THE GENERAL CHARACTERS OF PYROXYLINE PREPARED IN NITRO-SULPHURIC ACID OF VARIOUS DEGREES OF CONCENTRATION.