Restraint may be offered by an enclosing solid material, or by the inertia of the gases themselves, and the surrounding air. If a block of compressed gun cotton is ignited in the open air by a flame of moderate temperature, it will often consume away very gradually; but if ignited by an electric spark, or the impact of a bullet, it will explode with great violence; the probable explanation being that in the former case the first ignition at lower temperature permitted the gases to expand without producing a very high tension, this relation continuing to the end, while in the latter case the first ignition was violent, and the relief too slow to prevent a self-multiplying tension.