We were engaged in our last lecture in considering the various methods
that have been adopted from early times for obtaining fire, and we left
off at the invention of the lucifer match. I ventured to hint at the
conclusion of my last lecture, that the tinder-box had something to say
to the lucifer match, by way of suggestion, that just as the lucifer
match had ousted it, so it was not impossible that something some day
might oust the lucifer match. Electricians have unlimited confidence (I
can assure you) in the unlimited applications of electricity:--they
believe in their science. Now one of the effects of electricity is to
cause a considerable rise of temperature in certain substances through
which the electrical current is passed. Here is a piece of platinum
wire, for example, and if I pass an electrical current through it, you
see how the wire glows (Fig. 14). If we were to pass more current
through it, which I can easily do, we should be able to make the
platinum wire white hot, in which condition it would give out a
considerable amount of light. There is the secret of those beautiful
incandescent glow lamps that you so often see now-a-days (Fig. 15).
Instead of a platinum wire, a fine thread of carbon is brought to a very
high temperature by the passage through it of the electrical current,
in which condition it gives out light. All that you have to do to light
up is to connect your lamp with the battery. The reign of the match, as
you see, so far as incandescent electric lamps are concerned, is a thing
of the past. We need no match to fire it. Here are various forms of
these beautiful little lamps. This is, as you see, a little rosette for
the coat. Notice how I can turn the minute incandescent lamp, placed in
the centre of the rose, off or on at my pleasure. If I disconnect it
with the battery, which is in my pocket, the lamp goes out; if I connect
it with my battery the lamp shines brilliantly. This all comes by
"switching it on" or "switching it off," as we commonly express the act
of connecting or disconnecting the lamp with the source of electricity.
[Illustration: Fig. 14.]
[Illustration: Fig. 15.]
Here is another apparatus to which I desire to call your attention. If I
take a battery such as I have here--a small galvanic battery of some ten
cells--you will see a very little spark when I make and break contact of
the two poles. This is what is called an electrical torch, in which I
utilize this small spark as a gas-lighter (Fig. 16). This instrument
contains at its lower part a source of electricity, and if I connect the
two wires that run through this long tube with the apparatus which
generates the current, which I do by pressing on this button, you see a
little spark is at once produced which readily sets fire to my gas-lamp.
We have in this electrical torch a substitute--partial substitute, I
ought to say--for the lucifer match. I think you will admit that it was
with some show of reason I suggested that after all it is possible the
lucifer match may not have quite so long an innings as the tinder-box.
But there is another curious thing to note in these days of great
scientific progress, viz. that there are signs of the old tinder-box
coming to the front again. Men, I have often noticed, find it a very
difficult thing to light their pipes with a match on the top of an
omnibus on a windy day, and inventors are always trying to find out
something that will enable them to do so without the trouble and
difficulty of striking a match, and keeping the flame a-going long
enough to light their cigars. And so we have various forms of
pipe-lighting apparatus, of which here is one--which is nothing more
than a tinder-box with its flint and steel (Fig. 17). You set to work
somewhat in this way: placing the tinder (_a_) on the flint (_b_), you
strike the flint with the steel (_c_), and--there, I have done it!--my
tinder is fired by the spark. So you see there are signs, not only of
the lucifer match being ousted by the applications of electricity, but
of the old tinder-box coming amongst us once again in a new form.
[Illustration: Fig. 16.]
[Illustration: Fig. 17.]
I am now going to ask you to travel with me step by step through the
operation of getting fire out of the tinder-box. The first thing I have
to do is to prepare my tinder, and I told you, if you remember, that the
way we made tinder was by charring pieces of linen (see Fig. 4). I told
you last time what a dear old friend told me, who from practical
experience is far more familiar with tinder-boxes and their working than
I am, that no material was better for making tinder than an old cambric
handkerchief. However, as I have no cambric handkerchief to operate
upon, I must use a piece of common linen rag. I want you to see
precisely what takes place. I set fire to my linen (which, by the bye, I
have taken care to wash carefully so that there should be no dirt nor
starch left in it), and while it is burning shut it down in my
tinder-box. That is my tinder. Let us now call this charred linen by its
proper name--my tinder is carbon in a state of somewhat fine
subdivision. Carbon is an elementary body. An element--I do not say this
is a very good definition, but it is sufficiently good for my
purpose--an element is a thing from which nothing can be obtained but
the element itself. Iron is an element. You cannot get anything out of
iron but iron; you cannot decompose iron. Carbon is an element; you can
get nothing out of carbon but carbon. You can combine it with other
things, but if you have only carbon you can get nothing out of the
carbon but carbon. But this carbon is found to exist in very different
states or conditions. For instance, it is found in the form of the
diamond. (Fig. 18 _a_). Diamonds consist of nothing more nor less than
this simple elementary body--carbon. It is a very different form of
carbon, no doubt you think, to tinder. Just let me tell you, to use a
very hard word, that we call the diamond an "allotropic" form of carbon.
Allotropic means an element with another _form_ to it--the diamond is
simply an allotropic form of carbon. Now the diamond is a very hard
substance indeed. You know perfectly well that when the glass-cutter
wants to cut glass he employs a diamond for the purpose, and the reason
why glass can be cut with a diamond is because the diamond is harder
than the glass. I dare say you have often seen the names of people
scratched on the windows of railway-carriages, with the object I suppose
that it may be known to all future occupants of these carriages that
persons of a certain name wore diamond rings. Well, in addition to the
diamond there is another form of carbon, which is called black-lead.
Black-lead--or, as we term it, graphite--of which I have several
specimens here--is simply carbon--an allotrope of carbon--the same
elementary substance, notwithstanding, as the diamond. This black-lead
(understand black-lead, as it is called, contains no metallic lead) is
used largely for making lead-pencils. The manufacture of lead-pencils,
by the bye, is a very interesting subject. Formerly they cut little
pieces of black-lead out of lumps of the natural black-lead such as you
see there; but now-a-days they powder the black-lead, and then compress
the very fine powder into a block. There is a block of graphite or black
lead, for instance, prepared by simple pressure (Fig. 18 _b_). The great
pressure to which the powder is subjected brings these fine particles
very close together, when they cohere, and form a substantial block. I
will show you an experiment to illustrate what I mean. Here are two
pieces of common metallic lead. No ordinary pressure would make these
two pieces stick together; but if I push them together very
energetically--boys would call it giving them "a shove" together--that
is to say, employing considerable pressure to bring them into close
contact--I have no doubt that I can make these two pieces of lead stick
together--in other words, make them cohere. To cohere is not to adhere.
Cohesion is the union of similar particles--like to like; adhesion is
the union of dissimilar particles. Now that is exactly what is done in
the preparation of the black-lead for lead-pencils. The black-lead
powder is submitted to great pressure, and then all these fine particles
cohere into one solid lump. The pencil maker now cuts these blocks with
a saw into very thin pieces (Fig. 19 _b_). The next thing is to prepare
the wood to receive the black-lead strips. To do this they take a piece
of flat cedar wood and cut a number of grooves in it, placing one of
these little strips of black-lead into each of the grooves (Fig. 19 _a_,
which represents one of the grooves). Then having glued on the cover
(Fig. 19 _c_), they cut it into strips, and plane each little strip into
a round lead-pencil (Fig. 19 _d_). But what you have there as black-lead
in the pencil (for this is what I more particularly wish you to
remember) is simply carbon, being just the same chemical substance as
the diamond. To a chemist diamond and black-lead have the same
composition, being indeed the same substance. As to their money value,
of course there is some difference; still, so far as chemical
composition is concerned, diamonds and black-lead are both absolutely
true varieties of the element carbon.
[Illustration: Fig. 18.]
[Illustration: Fig. 19.]
Well now, I come to another form of carbon, called charcoal (Fig. 18
_c_). You all know what charcoal is. There is a lump of wood charcoal.
It is, as you see, very soft,--so soft indeed is it that one can cut it
easily with a knife. Graphite is not porous, but this charcoal is very
porous. But mind, whether it be diamond, or black-lead, or this porous
charcoal, each and all have the same chemical composition; they are what
we call the elementary undecomposable substance carbon. The tinder I
made a little while ago (Fig. 4), and which I have securely shut down in
my tinder-box, is carbon. It is not a diamond. It is not black-lead, but
all the same it is _carbon_--that form of porous carbon which we
generally call charcoal. Now I hope you understand the meaning of that
learned word _allotropic_. Diamond, black-lead, and tinder are
allotropic forms of carbon, just as I explained to you in my last
lecture, that the elementary body phosphorus was also known to exist in
two forms, the red and the yellow variety, each having very different
properties.
[Illustration: Fig. 20.]
Now it has been noticed when substances are in a very finely-divided
state that they often possess greater chemical activity than they have
in lump. Let me try and illustrate what I mean. Here I have a metal
called antimony, which is easily acted upon by chlorine. I will place
this lump of antimony in a jar of chlorine, and so far as you can see
very little action takes place between the metal and the chlorine. There
is an action taking place, but it is rather slow (Fig. 20 A). Now I will
introduce into the chlorine some of the same metal which I have finely
powdered. See! it catches fire immediately (Fig. 20 B). What I want you
to understand is, that although I have in both these cases precisely the
same chlorine and the same metal, nevertheless, that whilst the action
of the chlorine on the _lump_ of antimony was not very apparent, in the
case of the _powdered_ antimony the action was very energetic. Again,
there is a lump of lead (Fig. 21 _a_). You would be very much astonished
if the lead pipe that conveys the water through your houses caught fire
spontaneously; but let me tell you that, if your lead water-pipes were
reduced to a sufficiently fine powder, they would catch fire when
exposed to the air. I have some finely-powdered lead in this tube (Fig.
21 _b_), which you will notice catches fire directly it is exposed to
the atmosphere (Fig. 21 _c_). There it is! Only powder the lead
sufficiently fine,--that is to say, bring it into a state of minute
subdivision,--and it fires by contact with the oxygen of the air. And
now apply this. We have in our diamond the element carbon, but
diamond-carbon is a hard substance, and not in a finely-divided state.
We have in this tinder the same substance as the diamond, but
tinder-carbon is finely divided, and it is because it is in a
finely-divided condition that the carbon in our tinder-box catches fire
so readily. I hope I have made that part of my subject quite clear to
you. I should wish you to note that this very finely-divided carbon has
rather an inclination to attract moisture. That is the reason why our
tinder is so disposed to get damp, as I told you; and, as damp tinder is
very difficult to light, this explains the meaning of those
disrespectful words that I suggested our tinder-box had often had
addressed to it in the course of its active life of service.
[Illustration: Fig. 21.]
But to proceed. What do I want now? I want a spark to fire my tinder. A
spark is enough. Do you remember the motto of the Royal Humane Society?
Some of my young friends can no doubt translate it, "Lateat scintilla
forsan"--perchance a spark may lie hid. If a person rescued from
drowning has but a spark of life remaining, try and get the spark to
burst into activity. That is what the motto of that excellent society
means. How am I to get this spark from the flint and steel to set fire
to my tinder? I take the steel in one hand, as you see, and I set to
work to strike it as vehemently as I can with the flint which I hold in
the other (Fig. 3 A B). Spark follows spark. See how brilliant they are!
But I want one spark at least to fall on my tinder. There, I have
succeeded, and it has set fire to my tinder. One spark was enough. The
spark was obtained by the collision of the steel and flint. The sparks
produced by this striking of flint against steel were formerly the only
safe light the coal-miner had to light him in his dark dreary work of
procuring coal. Here is the flint and steel lamp which originally
belonged to Sir Humphry Davy (Fig. 22). The miners could not use candles
in coal-mines because that would have been dangerous, and they were
driven to employ an apparatus consisting of an iron wheel revolving
against a piece of flint for the purpose of getting as much light as the
sparks would yield. This instrument has been very kindly lent to me by
Professor Dewar. I will project a picture of the apparatus on the
screen, so that those at a distance may be better able to see the
construction of the instrument.
[Illustration: Fig. 22.]
And now follow me carefully. I take the steel and the flint, and
striking them together I get sparks. I want you to ask yourselves, Where
do the sparks come from? Each spark is due to a minute piece of _iron_
being knocked off the steel by the blow of flint with steel. Note the
precise character of the spark. Let me sprinkle some iron filings into
this large gas flame. You will notice that the sparks of burning iron
filings are very similar in appearance to the spark I produce by the
collision of my flint and steel.
[Illustration: Fig. 23.]
But now I want to carry you somewhat further in our story. It would not
do for me simply to knock off a small piece of iron; I want when I knock
it off that it should be red-hot. Stay for a moment and think of
this--iron particles knocked off--iron particles made red-hot. All
mechanical force generates heat.[A] You remember, in my last lecture, I
rubbed together some pieces of wood, and they became sufficiently hot to
fire phosphorus. On a cold day you rub your hands together to warm them,
and the cabmen buffet themselves. It is the same story--mechanical force
generating heat! The bather knows perfectly well that a rough sea is
warmer than a smooth sea. Why?--because the mechanical dash of the waves
has been converted into heat. Let me remind you of the familiar phrase,
"striking a light," when I rub the match on the match-box. "Forgive me
urging such simple facts by such simple illustrations and such simple
experiments. The facts I am endeavouring to bring before you are
illustrations of principles that determine the polity of the whole
material universe." Friction produces heat. Here is a little toy
(cracker) that you may have seen before (Fig. 23). It is scientific in
its way. A small quantity of fulminating material is placed between two
pieces of card on which a few fragments of sand have been sprinkled
(Fig. 23 _a_). The two ends of the paper (_b b_) are pulled asunder. The
friction produces heat, the heat fires the fulminate, and off it goes
with a crack. And now put this question to yourselves, What produced the
friction? Force. What is more, the amount of heat produced is the exact
measure of the amount of force used. Heat is a form of force. I must
urge you to realize precisely this energy of force. When you sharpen a
knife you put oil upon the hone. Why?--When the carpenter saws a piece
of wood he greases the saw. Why?--When you travel by train you see the
railway-porter running up and down the platform with a box of yellow
grease with which he greases the wheels. Why?--The answer to these
questions is not far to seek--it is because you want your knife
sharpened; it is because you want the saw to cut; it is because you want
the train to travel. The carpenter finds sawing hard work, and he does
not want the force of the muscles of his arm--his labour, in short--to
be converted into heat, and so he greases the saw, knowing that the more
completely he prevents friction, the more wood he will cut. It is the
force of steam that makes the engine travel. Steam costs money. The
engine-driver does not want that steam-force to be converted into heat,
because every degree of heat produced means diminished speed of his
train; and so the porter greases the wheels. But as you approach the
station the train must be stopped. The steam is turned off, and the
guard puts on what he calls "the brake." What is the brake? It is a
piece of wood so constructed and placed that it can be made to press
upon the wheel. Considerable friction results between the wheel and the
brake;--heat is produced;--the train gradually comes to a stop. Why? We
have now the conversion of that force into heat which a minute ago was
being used for the purpose of keeping the train a-going. Given a certain
force you can have heat _or_ motion; but you cannot have heat _and_
motion with the same force in the same amount as if you had them singly.
In every-day life, you cannot have your pudding and eat it.
[A] I need scarcely say, that whatever is of any value in the
following remarks is derived from that charming book of Professor
Tyndall's, _Heat a Mode of Motion_.
Heat then is generated by mechanical force; it is a mode of motion.
There was an old theory that heat was material. There was heat, for
instance, you were told, in this nail. Suppose I hammer it, it will get
hot, and at the same time I shall reduce by hammering the bulk of the
iron nail. A pint pot will not hold so much as a quart pot. The nail
(you were told) cannot hold so much heat when it occupies a less bulk as
it did when it occupied a larger bulk. Therefore if I reduce the bulk of
the nail I squeeze out some of the heat. That was the old theory. One
single experiment knocked it on the head. It was certain, that in water
there is a great deal more entrapped heat--"latent heat" it was
called--than there is in ice. If you take two pieces of ice and rub them
together, you will find the ice melts--the solid ice changes (that is to
say) into liquid water. Where did the heat come from to melt the ice?
You could not get the heat _from_ the ice, because it was not there,
there being admittedly more latent heat in the water than in the ice.
The explanation is certain--the heat was the result of the friction. And
now let me go to my hammer and nail. I wish to see whether I can make
this nail hot by hammering. It is quite cold at the present time. I hope
to make the nail hot enough by hammering it to fire that piece of
phosphorus (Fig. 24). One or two sharp blows with the hammer suffice,
and as you see the thing is done--_I_ have fired the phosphorus. But
follow the precise details of the experiment. It was _I_ who gave motion
to the hammer. _I_ brought that hammer on to that nail. Where did the
motion go to that I gave the hammer? It went into the nail, and it is
that very motion that made the nail hot, and it was that heat which
lighted the phosphorus. It was _I_ who fired the phosphorus: do not be
mistaken, _I_ fired the phosphorus. It was my arm that gave motion to
the hammer. It was my force that was communicated to the hammer. It was
_I_ who made the hammer give the motion to the nail. It was _I_ myself
that fired the phosphorus.
[Illustration: Fig. 24.]
I want you then to realize this great fact, that when I hold the steel
and strike it with the flint, and get sparks, I first of all knock off a
minute fragment of iron by the blow that I impart to it, whilst the
force I use in striking the blow actually renders the little piece of
detached iron red-hot. What a wonderful thought this is! Look at the
sun, the great centre of heat! It looks as if it were a blazing ball of
fire in the heavens. Where does the heat of the sun come from? It seems
bold to suggest that the heat is produced by the impact of meteorites on
the sun. Just as I, for instance, take a hammer and heat the nail by the
dash of the hammer on it, so the dash of these meteorites on the sun are
supposed to produce the heat so essential to our life and comfort.
[Illustration: Fig. 25.]
[Illustration: Fig. 26.]
[Illustration: Fig. 27.]
Let us take another step forward in the story of our tinder-box. Having
produced a red-hot spark and set fire to my tinder, I want you to see
what I do next. I set to work to blow upon my lighted tinder. You
remember, by the bye, that Latin motto of our school-books--_al[)e]re
flammam_, nourish the flame. When I blow on the tinder my object is to
nourish the flame. Here is a pair of common kitchen bellows (Fig. 25);
when the fire is low the cook blows the fire to make it burn up. What is
the object of this blowing operation? It is to supply a larger quantity
of atmospheric oxygen to the almost lifeless fire than it would
otherwise obtain. Oxygen is the spark's nourishment and life, and the
more it gets the better it thrives. Oxygen is an extremely active agent
in nourishing flame. If, for instance, I take a little piece of carbon
and merely set fire to one small corner of it, and then introduce it
into this jar of oxygen, see how brilliantly it burns; you notice how
rapidly the carbon is becoming consumed (Fig. 26). In the tinder-box I
blow on the tinder to supply a larger amount of oxygen to my spark. A
thing to burn under ordinary conditions must have oxygen, and the more
oxygen it gets the better it burns. It does not follow that the supply
of oxygen to a burning body must necessarily come directly from the air.
Here, for instance, I have a squib. I will fire it and put it under
water (Fig. 27). You see it goes on burning whether it is in the water
or out of it, because one of the materials of which the squib is
composed supplies the oxygen. The oxygen is actually locked up inside
the squib. When then I blow upon my tinder, my object is to supply more
oxygen to it than it would get under ordinary conditions. And, as you
see, the more I blow, within certain limits, the more the spark
spreads, until now the whole of my tinder has become red-hot. But my
time is gone, and we must leave the rest of our story for the next
lecture.