Cool Flame

It has long been known that certain flames are capable of existence at relatively low temperatures. Thus, a century ago, Davy observed that when a hot platinum wire was introduced into a mixture of ether-vapour and air, not only did the wire become red hot in consequence of active surface combustion, but a pale phosphorescent light could be detected above the wire, particularly when the latter ceased to glow, if the experiment were conducted in a dark room. Doebereiner observed the same phenomenon, and mentioned that when ether is dropped into a retort at 100° C. it may assume the spheroidal state accompanied by a pale blue flame, visible only in the dark, and too cool to set fire to other bodies. Perkin suggests several ways by which this phenomenon may be exhibited at lectures. Of these, perhaps the best is to heat an iron or copper ball to dull redness, allow it to cool to such a temperature that it is just invisible in the dark, and, by means of a wire, suspend it over a dish containing several filter papers saturated with ether. "As the ball approaches the ether, a beautiful blue flame will form, passing over its heated surface upwards for several inches. The ball may be let right down into the ether without causing ordinary combustion."

The blue flame is characterised by its low temperature. The fingers may be placed in it without discomfort; paper is not charred by it, and even carbon disulphide is not ignited by it.

In studying the limits of flame propagation both in ether-air and in acetaldehyde-air mixtures, White observed that, upon ignition with a hot platinum spiral at the bottom of a vertical glass tube, "a ray of glowing gas often seemed to extend upwards from the spiral for any distance up to 80 cm. The ray often remained in this position for many seconds before the top opened into a 'cool' flame travelling up the tube." The cool flame was found to readily yield the ordinary hot flame, particularly in the case of acetaldehyde. The author points out that the existence of this type of cool flame may serve to explain some of the hitherto inexplicable explosions with combustible vapours. On one occasion he was attempting to demonstrate in the usual way in daylight the flow of heavy ether vapour down an open wooden gutter 4 metres long, the vapour being obtained from a sponge saturated with liquid ether and held just above the top of the gutter. The inclination of the gutter was 1 in 4. After a time the attempt to ignite the ether-air mixture at the bottom of the gutter by means of a taper was apparently unsuccessful, although the characteristic odour of the so-called lampic acid was readily detectable. A few seconds later, however, the sponge burst into flame. It would appear, therefore, that a cool flame, practically invisible in the daylight and unobserved by the experimenters, had travelled up the gutter and given rise to ordinary combustion at the top.