Of the several contrivances which have been proposed for safely
lighting coal-mines subject to the visitation of fire-damp, or
carburetted hydrogen, the Safety-Lamp of Sir Humphry Davy is the only
one which has ever been judged safe, and been extensively employed. The
inventor first turned his attention to the subject in 1815, when Davy
began a minute chemical examination of fire-damp, and found that it
required an admixture of a large quantity of atmospheric air to render
it explosive. He then ascertained that explosions of inflammable gases
were incapable of being passed through long narrow metallic tubes, and
that this principle of security was still obtained by diminishing their
length and increasing their number. This fact led to trials upon sieves
made of wire-gauze; when Davy found that if a piece of wire-gauze was
held over the flame of a lamp, or of coal-gas, it prevented the flame
from passing; and he ascertained that a flame confined in a cylinder of
very fine wire-gauze did not explode even in a mixture of oxygen and
hydrogen, but that the gases burnt in it with great vivacity.
These experiments served as the basis of the Safety-Lamp. The apertures
in the gauze, Davy tells us in his work on the subject, should not
be more than 1/22d of an inch square. The lamp is screwed on to the
bottom of the wire-gauze cylinder. When it is lighted, and gradually
introduced into an atmosphere mixed with fire-damp, the size and length
of the flame are first increased. When the inflammable gas forms as
much as 1/12th of the volume of air, the cylinder becomes filled with a
feeble blue flame, within which the flame of the wick burns brightly,
and the light of the wick continues till the fire-damp increases to
1/6th or 1/5th; it is then lost in the flame of the fire-damp, which
now fills the cylinder with a pretty strong light; and when the foul
air constitutes one-third of the atmosphere it is no longer fit for
respiration,--and this ought to be a signal to the miner to leave that
part of the workings.
Sir Humphry Davy presented his first communication respecting his
discovery of the Safety-Lamp to the Royal Society in 1815. This was
followed by a series of papers remarkable for their simplicity and
clearness, crowned by that read on the 11th of January 1816, when the
principle of the Safety-Lamp was announced, and Sir Humphry presented
to the Society a model made by his own hands, which is to this day
preserved in the collection of the Royal Society at Burlington House.
From this interesting memorial the Vignette has been sketched.
There have been several modifications of the Safety-Lamp, and the merit
of the discovery has been claimed by others, among whom was Mr. George
Stephenson; but the question was set at rest forty-one years since by
an examination,--attested by Sir Joseph Banks, P.R.S., Mr. Brande, Mr.
Hatchett, and Dr. Wollaston,--and awarding the independent merit to
Davy.
A more substantial, though not a more honourable, testimony of approval
was given by the coal-owners, who subscribed 2500_l._ to purchase a
superb service of plate, which was suitably inscribed and presented to
Davy.[2]
Meanwhile the Report by the Parliamentary Committee “cannot admit that
the experiments (made with the Lamp) have any tendency to detract from
the character of Sir Humphry Davy, or to disparage the fair value
placed by himself upon his invention. The improvements are probably
those which longer life and additional facts would have induced him to
contemplate as desirable, and of which, had he not been the inventor,
he might have become the patron.”
The principle of the invention may be thus summed up. In the
Safety-Lamp, the mixture of the fire-damp and atmospheric air within
the cage of wire-gauze explodes upon coming in contact with the flame;
but the combustion cannot pass through the wire-gauze, and being there
imprisoned, cannot impart to the explosive atmosphere of the mine any
of its force. This effect has been erroneously attributed to a cooling
influence of the metal.
Professor Playfair has eloquently described the Safety-Lamp of Davy as
a present from philosophy to the arts; a discovery in no degree the
effect of accident or chance, but the result of patient and enlightened
research, and strongly exemplifying the great use of an immediate and
constant appeal to experiment. After characterising the invention as
the _shutting-up in a net of the most slender texture_ a most violent
and irresistible force, and a power that in its tremendous effects
seems to emulate the lightning and the earthquake, Professor Playfair
thus concludes: “When to this we add the beneficial consequences, and
the saving of the lives of men, and consider that the effects are to
remain as long as coal continues to be dug from the bowels of the
earth, it may be fairly said that there is hardly in the whole compass
of art or science a single invention of which one would rather wish
to be the author.... This,” says Professor Playfair, “is exactly such
a case as we should choose to place before Bacon, were he to revisit
the earth; in order to give him, in a small compass, an idea of the
advancement which philosophy has made since the time when he had
pointed out to her the route which she ought to pursue.”