As already indicated, cellulose is insoluble in all simple solvents; it is, however, dissolved by certain reagents, but only by virtue of a preceding chemical modification. An exception to this is to be found, perhaps, in the ammoniacal solution of cupric oxide (Schweitzer’s reagent), in which it dissolves without essential modification, being recovered by precipitation, in a form which is chemically identical with the original, though differing, of course, in being structureless, or amorphous. This reagent may be employed in a variety of forms, but the following method of using it is to be recommended as the most certain in its results. The substance to be operated upon is intimately mixed with copper turnings in a tube which is narrowed below and provided with a stopcock. Strong ammonia is poured upon the contents of the tube and, after being allowed to stand for some minutes, is drawn off and returned to the tube; the operation is several times repeated until the solution of the substance is effected. In order to facilitate the oxidation of the copper by the atmospheric oxygen, a current of air may be aspirated through the apparatus. The solution of the oxide prepared in this way is more effective in its action on cellulose than that obtained by dissolving the precipitated hydrate in ammonia. Cellulosic tissues in contact with this reagent are seen to undergo a disaggregation of their fibres, which swell up, become gelatinous, and disappear in solution. On adding an acid to the viscous solution, a precipitate of the amorphous cellulose is obtained in the form of a jelly resembling hydrated alumina; after washing and drying, it forms a brownish, brittle, horny mass. The cellulose is also precipitated upon simply diluting the viscous solution with water and allowing it to stand {6} 8–10 days in a closed vessel. From this observation it was inferred by Erdmann that the cellulose could not be considered as dissolved in the strict sense of the word, but the experiments of Cramer upon the osmotic properties of the solution proved this inference to be unfounded, and that cellulose is actually dissolved by the ammoniacal solution of copper oxide. On treating the ammonio-cupric solution of cellulose with metallic zinc, this metal precipitates the copper, replacing it in the solution, and producing the corresponding ammonio-zincic solution of cellulose, which is colourless. Some of these solutions are lævo-gyrate.