The Solubility Test.—­The object of this test is to ascertain, in the case of gun-cotton, the percentage of soluble (penta and lower nitrates) cotton that it contains, or in the case of soluble cotton, the quantity of gun-cotton.  The method of procedure is as follows:—­Five grms. of the sample which has been previously dried at 100 deg.  C., and afterwards exposed to the air for two hours, is transferred to a conical flask, and 250 c.c. ether-alcohol added (2 ether to 1 alcohol).  The flask is then corked and allowed to digest, with repeated shaking, for two or three hours.  The whole is then transferred to a linen filter, and when the solution has passed through the filter, is washed with a little ether, and pressed in a hand-screw press between folds of filter paper.  The sample is then returned to the flask, and the previous treatment repeated, but it will be sufficient for it to digest for one hour the second time.  The filter is then again pressed first gently by hand, then in the press, and afterwards opened up and the ether allowed to evaporate.  The gun-cotton is then removed from the filter and transferred to a watch-glass, and dried in the water oven at 100 deg.  C. When dry it is exposed to the air for two hours and weighed.  It equals the amount of gun-cotton and unconverted cotton in the 5 grms.  The unconverted cotton must be determined in a separate 5 grms. and deducted.

The method of determining the soluble cotton now used in the Government laboratories is as follows:—­Fifty grains of the nitro-cotton are dissolved in 150 c.c. of ether-alcohol, and allowed to stand, with frequent shakings, in a 200 c.c. stoppered measure for six hours; 75 c.c. of the clear solution are then drawn off by the aid of a pipette and evaporated in a dish on the water bath, and finally in the water oven at 120 deg.  F. (49 deg.  C.), until constant in weight.  The weight found equals the quantity of soluble cotton in the 75 c.c., which, multiplied by 4, equals the percentage, thus:  Suppose that 2.30 grains was the weight found, then

(2.3 x 150)/75 = 4.6 in 50 = 9.20 per cent.

A method for the determination of soluble nitro-cellulose in gun-cotton and smokeless powder has been published by K.B.  Quinan (Jour.  Amer.  Chem.  Soc., 23 [4], 258).  In this method about 1 grm. of the finely divided dry sample to be analysed is placed in an aluminium cup 1.9 inch in diameter and 4-1/8 inch deep.  It is then covered and well stirred with 50 c.c. of alcohol, 100 c.c. of ether are then added, and the mixture is stirred for several minutes.  After removing the stirrer, the cup is lightly covered with an aluminium lid, and is then placed in the steel cup of a centrifugal machine, which is gradually got up to a speed of 2,000 revolutions per minute, the total centrifugal force at the position occupied by the cups (which become horizontal when in rapid rotation) is about 450 lbs.  They are rotated at the full speed for ten to twelve minutes, and the machine is then gradually stopped.  By this time the whole of the insoluble matter will be at the bottom of the cup, and the supernatant solution will be clear.  It is drawn off to within a quarter of an inch of the bottom (without disturbing the sediment), with the aid of a pipette.