ISO 
MINUTES OF PROCEEDINGS OF 
gun-cotton -which is actually in contact with, or in close proximity to it; 
and the heat resulting from this combustion, which is contained in the 
products of the change, will suffice to cause the transformation of the 
explosion to proceed from particle to particle. But if the pressure of the 
atmosphere in which the gun-cotton is submitted to the action of heat be 
reduced, the gases resulting from the combustion of the particles nearest to 
the source of heat will have a tendency, proportionate to the degree of 
rarefaction of the air, to pass away into space, and thus to convey away from 
proximity to the cotton, more or less rapidly and completely, the heat 
necessary to carry on the combustion established in the first particles. 
Thus, when the heated wire is enveloped in a considerable body of gun¬ 
cotton, the ignition of the entire mass is apparently not instantaneous, if 
attempted in a highly rarefied atmosphere, because the products of the 
combustion first established in the centre of the mass of gun-cotton escape 
rapidly into space, conveying away from the point of combustion the heat 
essential for its full maintenance; the gun-cotton therefore undergoes at first 
an imperfect form of combustion, or a kind of metamorphosis different from 
the normal result of the action of heat upon this material. But the effects 
of the gradual generation of heated gases from the interior of the mass of 
cotton are, to impart some of their heat to the material through which they 
have to escape, as well as gradually to increase the pressure of the atmosphere 
in the vessel, and thus to diminish the rapidity of their escape; hence a 
condition of things is in time arrived at when the remainder of the gun¬ 
cotton undergoes the ordinary metamorphosis, a result which is accelerated 
by maintaining the original source of heat. If, however, the gun-cotton be 
employed in a compact form (in the form of twist or thread), and placed 
only in contact with the source of heat at one point, the heat will be so 
effectually conveyed away by the escaping gases, that the material will 
undergo even what may be termed the secondary combustion or metamor¬ 
phosis for a limited period only; so that, if a sufficient length of gun-cotton 
be employed, it will after a short time cease to burn, even imperfectly, 
because the heat essential for the maintenance of any chemical activity is 
soon completely abstracted by the escaping gases. These results may 
obviously be modified in various ways, as shown in the experiments 
described: thus, by increasing and maintaining the source of heat in¬ 
dependent of the burning cotton, the slow combustion may be maintained 
through a much greater length of the material until the pressure of the 
atmosphere is increased, by the products disengaged, to an extent sufficient 
to admit of a more rapid and perfect metamorphosis being established in the 
remainder of the material; or the same result may be attained, independently 
of the continued application of external heat, by employing a thicker mass 
of cotton, or by using the 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 conveyed 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. 
