_The acid mixture too strong._--The appearance of the cotton is not much altered on its first immersion in the mixture. It washes well, without any disintegration. On drying, it is found to be strong in texture, and produces a peculiar crackling sensation between the fingers, like starch. It explodes on the application of flame, without leaving any ash. It is insoluble in the mixture of Ether and Alcohol, but dissolves if treated with Acetic Ether. _The acid mixture of the proper strength._--No agglutination of the fibres of the cotton on immersion, and the product washes well; soluble in the ethereal mixture, and yields a _transparent_ film on evaporation. _The acid mixture too weak._--The fibres of the cotton agglutinate, and the Pyroxyline is washed with difficulty. On drying, the texture is found to be short and rotten. It does not explode on being heated, but either burns quietly with a flame, leaving behind a black ash--in which case it consists simply of unaltered cotton,--or is only slightly combustible, and not explosive. It dissolves more or less perfectly in glacial Acetic Acid. When treated with the ethereal mixture, it is acted on _partially_, leaving behind lumps of unchanged cotton; the solution does not form an even transparent layer on evaporation, but becomes opaque and cloudy as it dries. This opacity however may be seen to a small extent with any sample of Pyroxyline, if the solvents contain too much water. In using Swedish Paper in place of Cotton, the Pyroxyline formed in too weak a Nitro-Sulphuric Acid is usually insoluble in Ether and Alcohol, and burns slowly like unchanged paper. By studying these characters, and at the same time bearing in mind that _a drachm and a half of water_ in the quantities of acid given in the formula (p. 188) will suffice to cause the difference, the operator will overcome all difficulties. PURIFICATION OF THE SOLVENTS REQUIRED FOR COLLODION. The purity of the Ether employed is a matter of as much importance in the manufacture of a good Collodion as that of any other ingredient; this point must be attended to in order to secure a good result. There are four kinds of Ether sold by manufacturing chemists; first, ordinary rectified Sulphuric Ether, containing a certain percentage of Alcohol and of water; specific gravity about ·750. Second, the washed Ether, which is the same agitated with an equal bulk of water, to remove the Alcohol: by this proceeding the specific gravity of the fluid is reduced considerably. Third, Ether both washed and re-rectified from a caustic alkali, so as to contain neither Alcohol nor water; in this case the specific gravity should not be higher than ·720. Fourth, "Methylated" Ether, manufactured at a lower price than the others.