“Danger In Hair Washes”

By Lord Kelvin

Communication originally sent to the London Times,
reprinted in the Brooklyn Daily Eagle, Sept. 5, 1897, pg. 18.

As a warning that may possibly be useful in preventing the recurrence of any such terrible accident as that which recently happened through the ignition of an inflammable hair wash, it ought to be generally known that the faintest electric spark suffices to ignite an inflammable mixture of a combustible gas with air. This is illustrated in elementary lectures on electricity by “Volta’s cannon,” a little varnished brass gun, mounted on a glass pillar and having a wide touch hole plugged with sealing wax, in the center of which is mounted a brass wire, carrying a little brass knob outside and projecting inside to within one-twentieth of an inch of the end of another brass wire fixed to the metal of the gun. The gun is filled with an explosive mixture of oxygen and hydrogen and its muzzle is plugged with a cork. The varnished outside is struck with a piece of catskin and, thus electrified, the gun is left insulated on its glass pillar. To fire it, all that is necessary is to touch the projecting knob with the finger. This causes discharge of the electricity by two exceedingly faint sparks, one barely, if at all, perceptible by the finger before contact with the knob outside, the other in the one-twentieth of an inch air space within the explosive mixture inside. A loud explosion is heard and the cork is projected with sufficient violence to tear a canvas picture if it chances to touch one. Ignition of vapor of benzine by electric spark is well known to dyers in their process for cleaning silks and other fabrics by boiling in large cauldrons of liquid benzine. When the goods are taken out of the cauldron and spread out to dry on a table, explosions have often taken place, and I believe it is quite certain that an electric spark, caused by some slight friction between dried or partially dried portions of the fabrics, is the incendiary. We all know how readily electric sparks, visible in the dark, and perceptible to the ear by slight crackling sounds, are produced by drawing a hand over very dry hair, or the teeth of a comb through it. In the recent inquest, it was stated that the merit of the hair wash was that it dried so readily. The hairdresser said he felt the hair warm in his hand, and immediately after that all was enveloped in flames. The fact that the hair seemed warm to the hand was due not to the beginning of some kind of spontaneous combustion, as must, I believe, have been imagined by many readers of the report. It showed merely that the part of the hair touched had quickly become dry. Very slight friction of the hand on the dry hair would suffice to produce an electric spark, and the explosive atmosphere of air, mixed with combustible vapor from the portions of the hair not yet dry, was there. The conclusion of the inquest shows that no gas was burning in the neighborhood, and that no lucifer matches were lying about on the floor which could have been ignited by being trodden on. A gas flame within a yard or two of the place would certainly have been dangerous, but far less apt to produce the disaster than an electric spark from the hair in the very place of greatest liability to the presence of an explosive mixture.