Care must be taken that the solution thus withdrawn is perfectly clear.  About 10 to 15 c.c. of colloid solution and a film of insoluble matter remain at the bottom of the cup; these are stirred up well, the stirrer is rinsed with ether-alcohol, about 50 c.c. of fresh ether-alcohol are added; the mixture is again treated in the centrifugal apparatus for about eight minutes; the whole washing process is then repeated until all soluble matter has been removed.  This may require about seven or eight (or for samples with much insoluble matter ten or twelve or more) washings, but as the extraction proceeds, the period of rotation may be somewhat reduced.  After extraction is completed, the insoluble matter is transferred to a Gooch crucible with the usual asbestos pad, dried at 100 deg.  C., and weighed.  The residue may, if wished, be dried and weighed in the aluminium cup, but then it cannot be ignited.  The whole time for an analysis exclusive of that required for drying, is from one to two hours—­average time, 1-1/4 hour.  The results are satisfactory both as to accuracy and rapidity.  Acetone-soluble nitro-cellulose may be determined by the same method.

The Unconverted or Non-nitrated Cotton.—­However well the cotton has been nitrated, it is almost certain to contain a small quantity of non-nitrated or unconverted cotton.  This can be determined thus:—­Five grms. of the sample are boiled with a saturated solution of sodium sulphide, and then allowed to stand for forty-eight hours, and afterwards filtered or decanted, and again boiled with fresh solutions of sulphide, and again filtered, washed first with dilute HCl and then with water, dried, and weighed.  The residue is the cellulose that was not nitrated, plus ash, &c.  It should be ignited, and the weight of the ash deducted from the previous weight.

Acetone, and acetic-ether (ethyl-acetate) may also be used as solvents for the nitro-cellulose.  Another process is to boil the gun-cotton, &c., in a solution of sodium stannate made by adding caustic soda to a solution of stannous chloride, until the precipitate first formed is just re-dissolved.  This solution dissolves the cellulose nitrates, but does not affect the cellulose.  Dr Lunge found the following process more satisfactory in the case of the more highly nitrated products:—­The reagent is an alcoholic solution of sodium-ethylate prepared by dissolving 2 to 3 grms. of sodium in 100 c.c. of 95 per cent. alcohol, and mixing the filtered solution with 100 c.c. of acetone.  It has no effect upon cellulose, but decomposes nitro-cellulose with the formation of a reddish brown compound, which is soluble in water.  In the determination, 5 grms. of gun-cotton are heated to 40 deg. or 50 deg.  C. on the water bath with 150 c.c. of the reagent, the liquid being shaken at intervals for twenty to thirty minutes; or the mixture may be allowed to stand for a few hours at the ordinary temperature.  The brown-red solution is decanted from the undissolved