HUMPHRY AND HIS “WONDERFUL LAMP.”
Humphry found it impossible to quit the subject of Combustion without
some inquiry as to the nature of flame.
“What,” said he to himself, “are those brilliant sheets of light that
dart from burning substances? And why is it that certain bodies burn
with the evolution of flame, and that others are merely capable of being
rendered incandescent at the highest heat?”
Now the boy had before reflected upon the production of _heat_ by the
_slow combination_ of oxygen with certain combustible substances, so he
immediately passed to the consideration of the production of _light_ by
the same means.
He knew well that a stick of phosphorus always appears luminous in the
dark, and that lines of fire can be written with it on a wall: the
reason of this being simply that the air combines with it _slowly_ at
ordinary temperatures. Again, the lad was aware that decayed wood emits
a faint light, which is visible by night, the light being due to the
same cause, viz. the _slow_ combustion which is continually going on in
it; for chemists have discovered that the rotting vegetable substance is
constantly absorbing oxygen from the atmosphere, and evolving carbonic
acid gas—the same as if it were really burning—though at a _less rapid_
rate. Moreover, the decay of many animal substances, Humphry had read, is
attended with like phenomena—the flesh of many fresh and saltwater fish
becoming luminous previous to putrefaction.
But, in all such cases, the light emitted is more like a halo, or feeble
_glow_, than the character of _flame_; or, rather, it seems as if the
substance had become _incandescent in the cold_, and acquired the
property of ignition at ordinary temperatures, so that it was capable of
giving out light without being sensibly heated.[44]
Humphry had noticed, too, that a green wax taper—the colour of which he
knew to be produced by “verdigris” (acetate of copper)—on being lighted
and blown out shortly afterwards, will continue glowing in the wick, or
burning without flame, until the whole of the taper be consumed.
Now this, the boy was well aware, was another case of combustion, going
on at a temperature _below_ that of flame, though it was a state of
continuous incandescence, requiring a higher heat for its production than
the phosphorescence of decaying vegetable and animal matter.
Having thought the matter well over for some little time, the boy-Chemist
eventually conceived that it might be possible, by some such means, to
produce a lamp which would burn continuously, and give light, without
flame; and this he imagined might be found of great service in the
coal-mines, since the fire-damp was not explosible at the highest _red
heat_, and required positive _flame_ to cause it to enter into combustion.
In the case of the glowing wick of a green taper, the youth saw that
it was necessary for the wick itself to be made red hot before the
ignition could be maintained; and he concluded that it was merely the
incandescence of the wick which caused the gases given off from the taper
to combine with the air, and so to keep it continually red hot. It struck
him, therefore, that if he were to use some combustible liquid that was
highly vaporizable, like ether, and to suspend above this a spiral coil
of fine platinum wire, which had been previously heated to redness, the
same continuous state of ignition might be made to go on, and that in a
much simpler manner.
Accordingly, Humphry procured a tall ale-glass, and having poured into it
a tea-spoonful of ether, he suspended within it a coil of platinum-wire,
which had been made red hot in the flame of a spirit-lamp.
The arrangement of the little apparatus is represented in the annexed
engraving.
[Illustration]
Immediately the red-hot coil was placed in the vapour from the liquid,
the excited boy was overjoyed to see it glow with a bright red heat. He
stood watching it for a long time, and was rejoiced to find that there
was not the least diminution of the incandescence till the whole of the
liquid had disappeared, and the vapour been made by the heat of the wire
to combine with the surrounding air.
[Illustration: HUMPHRY’S FIRST ATTEMPT AT THE SAFETY LAMP.—Page 291.]
Still it struck Humphry that a more convenient mode of attaining the same
end might be devised; so he took an ordinary spirit-lamp (for spirits of
wine he was aware had the same tendency to give off vapour as the liquid
he had previously employed), and wound a coil of fine platinum wire round
the wick—thus. Then, having lighted the lamp, he suffered it to burn
a few seconds, after which he put an extinguisher over the flame, and
instantaneously removed it. The consequence was, that the coil retained
heat enough to carry on the _slow combustion_ of the vapour from the
spirit, so that the ignition was kept up, and the wire continued glowing,
till the whole of the spirit had evaporated from the lamp.
[Illustration]
Overjoyed at the discovery he had made, Humphry hastened to exhibit his
lamp without a flame to Mr. Borlase and Mrs. Foxell, and his old friend
Mr. Tonkin: and as he did so, the boy descanted fervently upon the
superiority of such a means of obtaining light in coal-mines over the
“_steel-mills_” which were then used for the purpose.
Mrs. Foxell was as pleased as the boy himself at what she considered the
successful termination of his labours; and when Humphry ran over to her
the long train of investigation he had pursued in order to arrive at the
object, she was warm in her praises of his perseverance and genius, and
assured him that, by such patient inquiry, he must ultimately gain the
honours and the universal esteem which he so much desired.
Mr. Borlase was surprised at the talent of his young pupil, and went
with him to Mr. Tonkin to talk over the matter; and when Humphry’s
foster-father saw what he had achieved, and was informed of the
benevolent spirit which had stirred him to the discovery, the old
gentleman hugged the lad to him, and told him he was well repaid for all
the care and affection he had bestowed upon him.
* * * * *
When, however, the first impressions had subsided, and Humphry’s
exultation at the discovery he had made had been toned down by
continually reflecting upon the subject, he began to think his little
safety-lamp might be much improved if he could only increase the light
from it. In its present form he thought it would be available merely in
such cases as the steel-mill was used, viz. to explore those mines which
were _known_ to be charged with fire-damp. The miners, nevertheless,
needed some light at their work, and if candles or lanterns were
employed, the flame would be sure to ignite any fire-damp that might
accidentally become mixed with the atmosphere, and so to cause the whole
to explode.
What was required, therefore, was a light which would be as bright as
those the miners employed at their work, but which would, at the same
time, be incapable of inflaming the explosive gases evolved from the
coals.
Such a light Humphry knew could be obtained only from flame itself, for
he was now satisfied that it was impossible for any one to see to work by
the rays from a red-hot wire. Still the difficulty was, that the presence
of flame would be sure to cause the fire-damp to explode whenever it
became mixed with the air in the mines, and it was impossible to make a
lamp burn without air.
For a time the difficulties involved in the construction of such a lamp
appeared to the lad to be insurmountable. At length, however, nothing
daunted, he set to work to discover the nature of flame itself, and
so to find out what conditions were necessary for the production and
maintenance of it.
“_Flame_,” said Humphry, while reflecting upon the subject, “may be
regarded merely as a _sheet, or film, of gas, in a high state of
ignition_. The temperature of flame is always very intense, since it will
ignite substances that cannot be lighted, even with the highest heat of
incandescence. The _light_, however, emitted by different flames is by no
means equally vivid, since there are gases which burn with the production
of a flame so feeble as to be scarcely visible in broad daylight.”
Accordingly it struck the boy, that the best way to proceed would be to
produce a feeble light first, and then to ascertain by what means he
could increase the brilliance of it. So Humphry made a large bladder full
of hydrogen gas, and then another bladder of oxygen, and proceeded to
burn the gases together by means of two blow-pipes.
The flame produced was of a very faint blue colour, and though the boy
closed the shutters, he could barely see to read by the light, and yet
the heat of it was almost as intense as any that could be artificially
generated;[45] for, on holding a platinum wire in the flame, it
immediately became white hot, and the light was considerably augmented.
“So, then,” cried Humphry, “the light emitted by a flame seems to be
increased by solid substances introduced into it, and ignited by it.”
This set the boy wondering what would be the effect if he were to cause
some fine dust to pass continually through the flame, and whether the
brightness of the light would be as much increased then as it was when
the platinum wire was introduced into it.
The idea had no sooner struck him than he ran down to the shop, and
procuring a little magnesia in fine powder, proceeded to sift this into
the flame produced by the oxygen and hydrogen gases.
The result was almost magical.
No sooner did the fine white particles fall into the flame, than the
light was changed from a faint blue into a brilliant white; while the
little solid specks shone like so many gem-points in the sunshine,
producing a glare that it pained the eyes to look at, and that instantly
lighted up the little dark chamber almost with the vividness of daylight.
“It _is_ as I expected!” shouted Humphry, as he gazed with admiration at
the beautiful white light; “the luminosity of a flame depends chiefly
upon the particles of _solid_ matter diffused through it, and rendered
incandescent by the burning gases. The flame of a candle or oil-lamp,”
the boy continued, “owes its brightness merely to the same cause; for
there the oil, or grease, is converted into gas by the heat of the wick,
and this gas consists chiefly of carburetted hydrogen—that is to say, of
charcoal and hydrogen combined together: so that in burning, the hydrogen
unites with the oxygen of the air and forms water, while the charcoal
is set free, and passes up through the flame in the form of very minute
particles, which being burnt there, and so rendered nearly white hot,
cause the light to shine with great vividness.”
To prove that the light of an ordinary candle, or oil-lamp, is due merely
to the brilliancy of the incandescent particles of charcoal continually
passing through the flame, the lad was not satisfied merely with holding
a burning candle against the ceiling, and so causing the unburnt charcoal
to be deposited upon it in the form of “lamp-black,” as it is called;
but he passed some hydrogen gas through a small reservoir of naphtha,
and found that the brilliancy of the light was greatly increased by it:
for, the naphtha being merely a liquid form of charcoal and hydrogen in
combination, the gas, on traversing the fluid, took up a certain portion
of the naphtha in vapour, and this being very rich in charcoal, was the
cause of the increased illuminating power given to the flame.
The same principle has been applied since Humphry’s time to the
production of some of the most vivid of our artificial lights. The
“Drummond light,” for instance—which consists merely of a jet of oxygen
and hydrogen gases projected against a cylinder of lime—owes its intense
brilliancy solely to the particles of lime that are rendered white hot by
the flame of the burning gases; and that the lime passes off in vapour is
proved by the fact, that, during the combustion, the roof of the lantern
becomes covered with the sublimed particles.
Again, the brilliancy of the electric light itself is known to arise
from the particles of white-hot charcoal that pass over in vapour
from one pole of the battery to the other; for it is found that the
charcoal point in connexion with the _positive pole_ becomes decreased
and burnt into a cavity, while that in connexion with the _negative
pole_ is proportionately increased, having a small knob or protuberance
of charcoal deposited upon it: so that, strange as it may appear, the
brilliancy of the flame of a common tallow-candle depends upon the same
cause as that of the electric light itself. In the one case, however,
the combustion being imperfect, the points of charcoal are less vividly
ignited; whereas in the other, the heat being the most intense that can
be produced by artificial means, the incandescence is proportionately
higher, and the brilliancy of the light, therefore, increased to the
greatest point. In a word, the particles of charcoal in the flame of a
candle are only at an _orange_ heat, whereas those in the voltaic flame
are at the highest _white_ heat that art can generate.
Accordingly, _artificial light_ would appear to be due solely to
_artificial heat_, and the illuminating power of even flame itself to
depend upon the incandescence of the particles of solid matter diffused
through the burning gases.
* * * * *
Humphry’s next object was to discover whether flame really consisted, as
he before said, of a sheet or film of gas in a state of combustion.
“The flame of a common candle,” he mused, “appears to be a solid cone of
luminous matter. _Is_ it, however, really alight in the middle?” the boy
mentally inquired; “or is it burning at the outside only, so that the
thin film of white-hot vapour incloses a portion of combustible matter
within it, which cannot burn for want of air? How can I ascertain this?”
Humphry thought for a while, and then it struck him, that if he were
to place a piece of thin glass upon the flame itself, so as to press
it down as it were, he could then see whether or not it were alight in
the middle; for if it were alight round the edge only, he would behold
through the glass a bright ring of flame, with a dark spot in the centre,
indicating the portion where the combustion was not going on.
[Illustration]
The experiment was soon tried, and the result proved as Humphry had
anticipated. A luminous circle was seen through the glass, at the point
where the flame touched the surface, and in the centre of this there was
a dark spot, like a black wafer, showing where the decomposed charcoal
gas existed in the interior of the flame in an unburnt state. The
appearance, however, is represented in the subjoined illustration. Indeed
Humphry perceived, that as air was necessary for combustion, and it
was impossible for the atmosphere to get to the interior of the flame,
the outside only of the cone of combustible gases, evolved from the
decomposed tallow, could be alight; while the vapour in the centre could
be burnt merely as it passed off into the air at the upper part of the
flame.
The next step was to see whether the interior dark part of the flame
really consisted of the same inflammable gases as were in a state of
combustion at the exterior, or surface of it.
Accordingly, Humphry placed a small glass tube within the centre of the
flame from an ordinary candle, and found that the unburnt gas from the
interior readily made its way up the tube, and escaped at the top of it,
and that on applying a light to this it was immediately ignited; so that
two flames were thus produced from one candle, as here shown.
[Illustration]
Another proof that the gases in the interior of the flame were not in a
state of combustion was afforded by lighting some spirits of wine in a
large spoon; for Humphry then found that he could introduce some grains
of gunpowder, and even some pieces of phosphorus, by means of a tube,
into the middle of the flame, without igniting them.
Humphry, moreover, could now see that the reason why the blow-pipe so
much increased the heat of an ordinary flame is simply because a current
of air is made by such means to traverse the interior part of it, so that
the combustion being rendered more perfect, the temperature, of course,
becomes proportionately higher.
Again, the wick of the “Argand Lamp” is constructed circular, so that
a current of air may be supplied to the interior of the flame; and
hence the superior brilliancy of this kind of burner. Table-lamps are
now generally made upon the same principle, and so, indeed, are the
gas-burners in shops. Sometimes, however, a jet is used, with the holes
so arranged as to spread the gas into a thin wide sheet, and thus allow
the air to act upon a large surface of combustible matter at each side.
These are called “_bat’s wing_” and “_fish-tail_” burners, &c., according
to the shape of the flat broad flames produced by them.
The next point in the inquiry was to discover what would be the effect of
cooling down a flame, since it was evident that it required a _very high
temperature for its existence_.
Humphry was well aware that, by projecting a current of cold air
upon substances in a state of combustion, the flames were generally
extinguished. Still he wished to see whether it was not possible to lower
the temperature of a flame by some other means.
Accordingly he procured a piece of wax-candle, and proceeded to pull
out all the wick, except one thread, so that when it was lighted the
flame from it should be as small as possible. Then the boy made a little
ring of iron wire, about the eighth of an inch in diameter, and this
he affixed to a wooden handle. Passing the wire ring over the lighted
wick he found, that on bringing it a little below the flame the light
was immediately extinguished, for the metal, being a good conductor
of caloric, served to carry off the heat which was necessary for the
maintenance of the flame.
It then struck Humphry that the pins which housewives stuck in rushlights
that they wished to be extinguished at a certain time, acted merely upon
the same principle—the metal carrying off the heat, and so lowering the
temperature requisite for the existence of the light.
The youth thought for a long time as to whether he could avail himself of
this principle in any way for the construction of his safety-lamp, for,
as he had now discovered that it was possible to extinguish flame merely
by reducing its temperature—and that a piece of wire, placed in connexion
with it, did this most effectually—he fancied he might take advantage
of some such means for preventing the passage of flame to fire-damp.
But though he racked his brain long he could hit upon no plan for
accomplishing his object. So he contented himself with merely making a
memorandum of the fact in his note-book, saying to himself, that he knew
it would prove serviceable some day—and then passed on to another part of
his inquiry.
Fire-damp, he was well aware, is explosible only when mixed with
the atmosphere; for the air is necessary for the combustion of all
substances, and explosion, as before observed, is merely the result of
_instantaneous combustion_. Still, certain liquids, Humphry knew, are
attracted by the sides of fine tubes, so that the fluid in which they are
immersed is maintained within them above the level of that without. He
knew, too, that by the same principle a certain quantity of water might
be retained in a fine sieve without running through the holes; and that a
vessel full of small holes might, for the like reason, be immersed to a
certain depth in water without sinking, or any of the liquid entering it.
So the lad was anxious to see what would be the effect of fine tubes upon
gases, and whether, upon passing an explosive mixture of carburetted
hydrogen and air along a narrow glass pipe, the flame would pass down it,
or be arrested in its progress.
Accordingly, having prepared some carburetted hydrogen gas, and mixed it
with eight parts of air—for these proportions he had found to be the most
explosive—he passed the mixture along a glass tube, the bore of which was
only ⅛th of an inch in diameter, and discovered, on lighting the gas that
issued from the end of it, that the flame, instead of travelling down
the narrow channel, and being instantly communicated to the whole of the
explosive mixture, remained stationary at the orifice of the tube, where
the gases burnt as slowly and quietly as with an ordinary candle.
Humphry was half-bewildered with joy at the discovery he had made.
Fire-damp he had now ascertained was not explosible within narrow tubes,
and instantly a hundred and one plans flashed across his mind for
rendering the fact serviceable to those engaged in the coal-mines.
Still there was much to work out. It was necessary to know through how
large a tube flame _was_ capable of being transmitted; so he procured
another glass pipe, the bore of which was a quarter of an inch in
diameter, or double that of the one he had first employed, and passing
the same explosive mixture along it, he found that, on lighting the
gases at the end of this, the flame no longer remained stationary at the
orifice, but travelled slowly down the channel, taking more than a second
before it reached the other end, though the tube itself was only a foot
long.
Then pursuing the same course of experiments, Humphry subsequently
discovered that by diminishing the diameter of the tube he could shorten
the length of it without any danger of the flame travelling along it, and
so causing the gases without to explode. Moreover, the lad ascertained
that when fine metal tubes were substituted for glass ones, the security
was even more perfect.
To insure greater safety, however, by making the channel through which
the explosive gases passed as fine as possible, he ultimately inserted
some short pieces of metal tubing, one within the other; so that the
bores being of different diameters, they formed a series of broad
concentric rings, each enclosing another, and having a _fine channel_
between them—thus.
[Illustration]
These Humphry proved to be perfectly secure against all explosion, and
then proceeded to fix one set at the bottom of a lamp, and another at the
top of a glass chimney that was made to fit air-tight round the flame.
The concentric tubes at the bottom were for the supply of air to the
light, and those at the top for the issue of the smoke; so that thus, if
any fire-damp were present in the mine, and entered with the atmosphere
through the air-tubes at the bottom, the flame would not be communicated
to the explosive gases outside the lamp—and thus the safety of the miner
would be thoroughly ensured.
For rendering the arrangement adopted more intelligible, a design of
Davy’s first Safety-Lamp is here given.
[Illustration]
The principle upon which this lamp depended for its security had been
so carefully worked out that it scarcely required testing. To assure
himself, however, of its efficacy, the boy immersed the lamp in the most
explosive mixture of carburetted hydrogen and air that he could possibly
form, and was well repaid for all his labours by seeing that the flame
was incapable of being transmitted to the combustible gases without; so
that, upon their entering the lamp, the light itself was extinguished.
In a word, he found that his lamp was “absolutely safe,” and that the
fire-damp had been disarmed by him of its terrors.
Humphry’s friends were as delighted as the boy himself at the successful
termination of his labours. Mrs. Foxell, who had, in the first instance,
encouraged him to proceed, was, perhaps, the most gratified of all;
nor did she refrain from lecturing her brother, Mr. Borlase, upon his
previous want of faith in the accomplishment of the result, telling him
that it was cruel, where so many lives were at stake, to damp the ardour
of any one who could believe in the possibility of rendering them secure
for the future.
Mr. Borlase was sufficiently generous to confess his error, and frankly
acknowledged that he never thought Humphry would be able to accomplish
half as much as he had.
The boy’s old friend, Mr. Tonkin, too, felt prouder than ever of his
foster-son, when he saw the lad test the powers of the lamp in his
presence; and the old gentleman made Humphry’s heart swell again with
the praises he heaped upon him, and the tears start to his eyes with the
confession of the long love he had borne the boy.
At length, however, Mr. Tonkin said that he thought one improvement still
was required, though how it was to be effected he must leave Humphry
to find out, for it was more than he could manage. On the admission of
the fire-damp into the lamp the light was extinguished. “This,” the old
gentleman observed, “appeared to be a great defect, for the extinction
of the flame was a sure indication that danger surrounded those who
carried the lamp; and to leave the miner in darkness at a time when
immediate flight was necessary, struck him as being a serious drawback to
its utility. If Humphry, therefore,” he said, “could, by some means or
other, arrange it so that the light should continue burning, even in the
presence of the fire-damp itself, the ingenuity of the apparatus would
not only be much greater, but its value be considerably enhanced.”
Humphry saw the force of Mr. Tonkin’s objection, and it struck him that
the defect was almost irremediable. He was, therefore, not a little
vexed to find that an instrument which he had fancied perfect, was not
altogether faultless.
The lad pondered over the matter for many days, and tried a number of
experiments to overcome the difficulty which had been raised. It proved,
however, beyond his powers; so, exhausted by his long study, and vexed
at the idea of his comparative failure, Humphry, at length, dismissed
the subject from his mind, and locked the safety-lamp in his cupboard,
determined that it never should be made public until it was perfect in
all its arrangements.
It was in vain that Mrs. Foxell urged him to make known the invention,
even in an unfinished state; but the boy was firm in his resolve. “_Some
day_,” said he, “it shall be completed, and then the world may have it,
and welcome; but as it is now, it is hardly worth either the giving or
accepting, and no honours can come of it. When I know more,” he added,
“I can do better: nor do I intend, because I have been baffled in this
my first project, to give over studying; for I find such delight in
the perception of the new truths which are daily unfolded to me, that
I would rather forego any source of pleasure than that which comes from
scientific discovery.”
* * * * *
It was many years before the Safety-Lamp was perfected; and when it was,
the alteration was so slight, that it has been well observed—“The history
of this elaborate inquiry affords a striking proof of the inability
of the human mind to apprehend simplicities without a long process of
previous complication.”[46]
Humphry Davy, as we have narrated, had already discovered, while
experimenting upon the inflaming of explosive gases in narrow tubes,
that, provided he diminished the bore, he could shorten the length of
them in a corresponding ratio. Now a sheet of wire-gauze, it is obvious,
consists merely of a series of fine tubes, of very short lengths. It
appears strange, therefore, that our hero was some time before he
conceived the happy idea of constructing his lamp entirely of wire-gauze,
instead of feeding it with air through narrow channels, or “safety
canals,” as he called them; for, as flame cannot be transmitted along
small tubes, it is manifest that, on surrounding a lamp with a wire-gauze
cylinder, or cage, the fire-damp, as it enters with the air, will burn
within the cage itself, and the flame be incapable of passing through
the small apertures in the gauze, and thus exploding the gas outside of
it.
To render the action of wire-gauze in this respect more intelligible, a
few of Humphry Davy’s after-experiments may be cited in connexion with
this part of the subject.
If a small piece of wire-gauze (say about 9 inches square, and having
about 30 meshes to the square inch) be gradually brought down upon the
flame of a candle, the flame itself will be cut off where it touches the
gauze, and merely a dark spot be there observed, encircled by a ring
of light, while the combustible matter of the flame will issue through
the small apertures in the form of smoke; for during the passage of the
burning gases through the metallic meshes they will be so far cooled
down that the flame will be extinguished, and thus rendered incapable of
traversing the gauze itself.
If, however, a lighted taper be held above the wire-gauze, the
inflammable gas or smoke from the candle will be immediately rekindled,
and the flame continue to burn then, both on the upper and under side of
the tissue.
But the impermeability of metallic gauze to flame may be rendered
still more evident, by placing a piece of fine wire web above a jet of
gas previous to lighting it, when it will be found that, if a burning
taper be held over the wire web, the gas will be kindled above it, and
continue to burn upon the gauze itself, while the combustible matter
beneath will remain unignited; for the metallic threads, being good
conductors of heat, will so cool the flame as to prevent it passing
through to the gas on the lower side.
[Illustration]
Again, if a small piece of camphor be laid in the centre of a sheet of
wire-gauze, and a light held beneath it, the vapour from the camphor,
which is very inflammable, will be made to burn with a bright flame
_downwards_—instead of upwards, as flames usually do—while the camphor
itself will lie upon the upper side of the tissue in an uninflamed state.
[Illustration]
The power of metallic webbing thus to intercept or extinguish flame,
depends simply _upon the cooling effect_ it produces; so that as flame
requires a high temperature for its existence, it is plain that it no
sooner becomes cooled, by being passed through a good conductor of heat,
than it is extinguished.
If, however, the meshes of the wire-gauze be not sufficiently small, or
if the wire of the gauze itself becomes intensely heated, the flame will
traverse it in either instance; because the cooling power is reduced, in
the one case, by the largeness of the apertures, and in the other by the
high temperature of the wire.
For the knowledge of these facts, we repeat, we are indebted to the
after-investigations of Davy himself.
Now it is evident from the above experiments, that if a lamp be covered
with a cylindrical cage of wire-gauze, no flame will be able to pass
from the interior of the lamp to the exterior of it, in consequence of
the cooling power of the metallic web encompassing it; so that, if the
surrounding air be charged with an explosive gas, it will enter the
lamp through the meshes of the web and burn within the cage, while that
without will remain unkindled.
Such, then, is the principle of the “Safe-Lamp” as perfected by Davy, and
which is here shown:
[Illustration]
The safety of this lamp may be exhibited by immersing it in a large
jar, at the bottom of which is a little Ether; for the vapour from this
liquid, on mingling with the air, forms a highly inflammable atmosphere.
On introducing the lamp into this, the flame first becomes enlarged and
is then extinguished, while the whole of the cage remains filled with a
lambent blue light. On withdrawing the lamp, however, and bringing it
into the open air, the wick is suddenly rekindled, and the flame returns
to its natural size and colour.
For perfect safety, it is necessary that the wire-gauze of these lamps
should contain about 30 wires, or 900 apertures in every square inch, and
that the heat of the wire itself should never rise above redness; for
“if,” as Davy said, “the iron wire become white hot, the lamp will be no
longer safe. This, however,” he adds, “need never happen in a colliery;
for if a workman finds the temperature of the wire increasing rapidly
in an explosive atmosphere, he can easily diminish the heat by turning
his back upon the current, and keeping it from playing upon the gauze by
means of his clothes.”
Of this wonderful lamp it has been well said, “that it is a present from
Philosophy to the Arts, and to the class of men farthest removed from
the interests of science. We know of no discovery in which the admirer
of Science and the lover of mankind have greater reason to congratulate
one another. The discovery,” adds the late Professor Playfair, “is in
no degree the effect of accident; and chance, which comes in for so
large a share in the credit of human inventions, has no claims on this,
which is altogether the result of patient and enlightened research. The
great use of an immediate and constant appeal to experiment cannot be
better evinced than in this example. The result is as wonderful as it is
important. An invisible and impalpable barrier, made effectual against a
force the most violent and irresistible in its operations, and a power
that, in its tremendous effects, seemed to emulate the lightning and the
earthquake, confined within a narrow space, _and shut up in a net of
the most slender texture_; these are facts which must excite a degree
of wonder and astonishment, from which neither ignorance nor wisdom can
defend the beholder. 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 fairly be 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.”[47]
It should be borne in mind, moreover, that Davy in the accomplishment
of his noble task, not only did not seek, but positively rejected all
pecuniary reward; so that it becomes difficult which to admire the
most—the benevolence which prompted him to undertake the long train of
laborious investigations, the genius which carried him to so successful
an issue, or the noble disinterestedness which bade him refuse to traffic
in an invention that was destined to mitigate the sufferings of some of
the poorest and least educated of his fellow-creatures.[48]