208 



Mr. Abel on the Combustion of 



[April 21, 



material in a less compact form. In these cases the maintenance of the 

 chemical change is favoured either by radiation of heat to the cotton, and 

 provision of additional heat, from an external source, to the gases as they 

 escape and expand, or by establishing the change in a greater mass of the 

 material, and thus reducing the rapidity with which the heat will be con- 

 veyed away by the escaping gases, or, finally, by allowing the gases, as they 

 escape, to pass to some extent between the fibres of the cotton, and thus 

 favouring the transmission of heat to individual particles of the material. 



In the description of the two experiments last referred to above, I have 

 stated that some peculiar phenomena were observed to attend the imperfect 

 kind of combustion induced in the gun-cotton in rarefied atmospheres. 



In order to examine these phenomena more closely, I instituted a series of 

 experiments, in a darkened room, with equal quantities of gun-cotton 

 inch of twist=0-3 gr.) placed always in the same position, across the 

 platinum wire, the only varying element in the experiment being the 

 pressure of the atmosphere in the vessel, which was gradually increased. 

 The following were the results observed : — 



Experiment I. — Pressure=0'62 inch. The wire was heated just suffi- 

 ciently to ignite the material ; the current was then interrupted. The 

 gun-cotton burned very slowly in both directions, emitting only the small 

 green phosphorescent flame, or brush, already described. 



Exp. II. — Pressure=l inch. In addition to the green glow which sur- 

 rounded the burning ends, a very faint yellowish flame was observed hover- 

 ing over the gun-cotton. 



Exp. III. — Pressure= 1*5 inch. The cotton burned a little faster, and 

 the faint yellowish flame was of a more decided character ; indeed two sepa- 

 rate flames were observed, each following up the green light as the cotton 

 burned in the two directions. 



Exp. IV. — Pressure=2 inches. The results were the same as in the 

 preceding experiment, excepting that the yellowish flames became more 

 marked. 



Exp. V. — Pressure=2'5 inches. The same phenomena, the cotton 

 burning considerably faster. 



Exp. VI. — Pressure =3 inches. The same phenomena, the yellow flames 

 increasing in size. 



Exp. VII. — Pressure=4 inches. The rapidity of combustion of the 

 cotton increased again considerably ; the other phenomena observed were as 

 before. 



Exp. VIII. — Pressure = 6 inches. The pale yellow flame had increased 

 in size considerably, no longer forming a tongue, as in the preceding experi- 

 ments, but completely enveloping the burning ends of the gun-cotton. 

 The green glow, though much reduced, was still observed immediately 

 round the burning surfaces. 



Exp. IX. — Pressure = 8 inches. The green glow was only just percep- 

 tible in this instance, and the cotton burned very rapidly, almost with the 



