1864.] 



Gun-cotton and Gunpowder. 



205 



from 1 to 8 in inches of mercury) the gun-cotton, when ignited by means 

 of the platinum wire, burned very slowly, presenting by daylight an appear- 

 ance as if it smouldered, with little or no flame attending the combustion. 

 I was at first led by these results to conjecture that this peculiar kind of 

 combustion of the gun-cotton was determined solely by its ignition in 

 atmospheres rarefied beyond a certain limit ; and I was induced, in con- 

 sequence, to institute a number of experiments with the view of ascertain- 

 ing what was the most highly rarefied atmosphere in which gun-cotton 

 would burn as in the open air — with a flash, accompanied by a body of 

 bright flame. In order to ensure uniformity in the degree of heat applied 

 to the cotton in these experiments, the platinum wire employed was suffi- 

 ciently thin to be instantaneously melted on the passage of the voltaic 

 current. About fifty difl'erent experiments were made with equal quanti- 

 ties of gun-cotton (0*2 grain), placed always in the same position, so that 

 the platinum wire rested upon the material. A tolerably definite limit of 

 the degree of rarefaction was arrived at, within which the gun-cotton was 

 exploded instantaneously, as in the open air. When the pressure of air 

 in the apparatus was reduced to 8*2 in inches of mercury, the gun-cotton 

 still exploded with a flash, but not quite instantaneously ; on reducing the 

 pressure to 8 inches, the cotton underwent the slow kind of combustion in 

 the majority of cases ; on a few occasions it exploded with a flash of flame. 

 The same occurred in a succession of experiments, until the pressure was 

 reduced gradually to 7*7 inches, when instances of the rapid explosion of 

 gun-cotton were no longer obtained. 



Although these results were moderately definite when the conditions of 

 the experiments were as nearly as possible uniform, it was found that they 

 could be altered by slight modifications of any one particular condition 

 (such as the quantity of gun-cotton, its mechanical condition, its position 

 with reference to the source of heat, the quantity of heat applied, and the 

 duration of its application). In illustration of this, the following results 

 may be quoted. 



If the gun-cotton was wrapped round, instead of being simply placed 

 across the wire, its instantaneous combustion was efi^ected in atmospheres 

 considerably more rarefied than with the above experiments. 



In employing a small piece of gun-cotton (0'3 of an inch long and 

 weighing 0"3 to 0*4 of a grain) loosely twisted, laid across the wire, or 

 upon a support immediately beneath the latter so that the wire rested 

 upon it, the slow combustion established in it by the heated wire, under 

 greatly diminished atmospheric pressure (amounting to 0*6 inch in this and 

 the following experiments), proceeded uniformly towards each end of the 

 piece of twist, until the whole was transformed into gas. But if a piece 

 of the same twist, of considerably greater length (say 4 inches long and 

 weighing about 2 grains), was exposed to heat in an atmosphere of the 

 same rarefaction, the gun-cotton being laid over the wire and hanging 

 down on either side, it was cut through by the passage of the current, and 



