Page:Popular Science Monthly Volume 56.djvu/318

304 cotton. The covered crock is now placed with others in wooden troughs containing running water so as to keep the temperature uniform, where the cotton is allowed to digest for about twenty-four hours. The acid is then wrung out in a steel centrifugal, and the wrung gun cotton is thrown in small lots into an immersion tank containing a large volume of flowing water, in which a paddle wheel is revolving so as to rapidly dilute and wash away the residual



acid in the gun cotton without permitting any considerable rise of temperature from the reaction of the water with the acid.

Even these severe means are not enough, for, as the cotton fiber is in the form of hairlike tubes, traces of the acid sufficient to bring about the subsequent decomposition of the gun cotton are retained by capillarity. Therefore, after boiling with a dilute solution of sodium carbonate, the gun cotton is pulped and washed in a beater or rag engine until the fiber is reduced to the fineness of corn meal, and a sample of it will pass the "heat test." This is a test of the resistance of gun cotton to decomposition, and requires that when the air-dried sample of gun cotton is heated to 65.5° C in a closed tube in which a moistened strip of potassium iodide and starch paper is suspended, the paper should not become discolored in less than fifteen minutes' exposure.

This pulping of the gun cotton not only enables one to more