Recall for a few minutes the facts I brought before you in my last
lecture. The first point we discussed was the preparation of the tinder.
I explained to you that tinder was nothing more than carbon in a
finely-divided state. The second point was, that I had to strike the
steel with the flint in such manner that a minute particle of the iron
should be detached; the force used in knocking it off being sufficient
to make the small particle of iron red-hot. This spark falling upon the
tinder set fire to it. The next stage of the operation was to blow upon
the tinder, in order, as I said, to nourish the flame; in other words,
to promote combustion by an increased supply of oxygen, just as we use
an ordinary pair of bellows for the purpose of fanning a fire which has
nearly gone out into a blaze.
And now comes the next point in my story of a tinder-box. Having ignited
the tinder I want to set fire to the match. Now I have here some of the
old tinder-box matches, and you will see that they are simply wooden
splints with a little sulphur at the end. Why (you say) use sulphur? For
this reason--the wood is not combustible enough to be fired by the
red-hot tinder. We put therefore upon the wood a substance which is more
combustible than the wood. This sulphur--which most people call
brimstone--has been known from very early times. In the middle ages it
was regarded as the "principle of fire." It is referred to by Moses and
Homer and Pliny. A very distinguished chemist, Geber, describes it as
one of "the principles of nature." Having fired my tinder, as you see,
and blown upon it, I place my sulphur match in contact with the red-hot
tinder. And now I want you to notice that the sulphur match does not
catch fire immediately. It wants, in fact, a little time, and as you see
a little coaxing. Now I have got it alight. But note, it is the sulphur
that at the present moment is burning. The burning sulphur is now
beginning to set fire to the wood. The whole match is well alight now!
But it was the sulphur that caught fire first, and it was the sulphur
that set fire to the wood. A little time was occupied, we said, in
making the sulphur catch fire. Ask yourselves this question--Why was it
that the sulphur took a little time to catch fire? This was the
reason--because before the sulphur could catch fire it was necessary to
change the _solid_ sulphur (the condition in which it was upon the match
end) into _gaseous_ sulphur. The solid sulphur could not catch fire.
Therefore the heat of my tinder during the interval that I was coaxing
the match (as I called it) was being exerted in converting my solid into
gaseous sulphur. When the solid sulphur had had sufficient heat applied
to it to vapourize it, the sulphur gas immediately caught fire. Now
understand, that in order to convert a solid into a liquid, or a liquid
into a gas, heat is always a necessity. I must have heat to produce a
gas out of a solid or a liquid. I will endeavour to make this clear to
you by an experiment. I have here, as you see, a wooden stool, and I am
about to pour a little water on this stool. I place a glass beaker on
the stool, the liquid water only intervening between the stool and the
bottom of the glass. You see the glass is perfectly loose, and easily
lifted off the stool notwithstanding the layer of water. I will now pour
into the beaker a little of a very volatile liquid--_i. e._ a liquid
that is easily converted into a gas--(bisulphide of carbon). I wish
somewhat rapidly to effect the change of this liquid bisulphide of
carbon into gaseous bisulphide of carbon, and in order to accomplish
this object I must have heat. So I take this tube which, as you see, is
connected with a pair of bellows, and simply blow on my bisulphide of
carbon. This effects the change of the liquid into a gas with great
rapidity. Just as I converted my solid sulphur into a gas by the heat of
the tinder, so here I am converting this liquid bisulphide of carbon
into a gas by the wind from my bellows. But my liquid bisulphide of
carbon must get heat somewhere or another in order that the change of
the liquid into a gas, that I desire should take place, may be effected;
and so, seeing that the water that I have placed between the glass and
the stool is the most convenient place from which the liquid can derive
the necessary heat, it says, "I will take the heat out of the water." It
does so, but in removing the heat from the water it changes the liquid
water into solid ice. And see, already the beaker is frozen to the
stool, so that I can actually lift up the stool by the beaker (Fig. 28).
Understand then why my sulphur match wanted some time and some coaxing
before it caught fire, viz. to change this solid sulphur into gaseous
sulphur.
[Illustration: Fig. 28.]
But let us go a step further: why must the solid sulphur be converted
into a gas? We want a flame, and whenever we have flame it is absolutely
necessary that we should have a gas to burn. You cannot have flame
without you have gas. Let me endeavour to illustrate what I mean. I pour
into this flask a small quantity of ether, a liquid easily converted
into a gas. If I apply a lighted taper to the mouth of the flask, no
gas, or practically none, being evolved at the moment, nothing happens.
But I will heat the ether so as to convert it into a gas. And now that I
have evolved a large quantity of ether gas, when I apply a lighted
taper to the mouth of the flask I get a large flame (Fig. 29). There it
is! The more gas I evolve (that is, the more actively I apply the heat)
the larger is the flame. You see it is a very large flame now. If I take
the spirit lamp away, the production of gas grows less and less, until
my flame almost dies out; but you see if I again apply my heat and set
more gas free, I revive my flame. I want you to grasp this very
important fact, upon which I cannot enlarge further now, that given
flame, I must have a gas to burn, and therefore heat as a power is
needed before I can obtain flame.
[Illustration: Fig. 29.]
Well, you ask me, is that true of all flame? Where is the gas, you say,
in that candle flame? Think for a moment of the science involved in
lighting a candle. What am I doing when I apply a lighted match to this
candle? The first thing I do is to melt the tallow, the melted tallow
being drawn up by the capillarity of the wick. The next thing I do is to
convert the liquid tallow into a gas. This done, I set fire to the gas.
I don't suppose you ever thought so much was involved in lighting a
candle. My candle is nothing more than a portable gas-works, similar in
principle to the gas-works from which the gas that I am burning here is
supplied. Whether it is a lamp, or a gas-burner, or a candle, they are
all in a true sense gas-works, and they all pre-suppose the application
of heat to some material or another for the purpose of forming a gas
which will burn.
[Illustration: Fig. 30.]
Before I pass on, I want to refer to the beautiful burner that I have
here. It is the burner used by the Whitechapel stall-keepers on a
Saturday night (Fig. 30). (Fig. _a_ is an enlarged drawing of the
burner.) Just let me explain the science of the Whitechapel burner.
First of all you will see the man with a funnel filling this top portion
with naphtha (_c_). Here is a stop-cock, by turning which he lets a
little naphtha run down the tube through a very minute orifice into this
small cup at the bottom of the burner (_a_). This cup he heats in a
friend's lamp, thereby converting the liquid naphtha, which runs into
the cup, into a gas. So soon as the gas is formed--in other words, so
soon as the naphtha has been sufficiently heated--the naphtha gas
catches fire, the heat being then sufficient to maintain that little cup
hot enough to keep up a regular supply of naphtha gas. When the lamp
does not burn very well, you will often see the man poking it with a
pin. The carbon given off from the naphtha is very disposed to choke up
the little hole through which the naphtha runs into the cup, and the
costermonger pushes a pin into the little hole to allow the free passage
of the naphtha. That, then, is the mechanism of this beautiful lamp of
the Whitechapel traders, known as Halliday's lamp.
Now I go to another point: having obtained the gas, I must set fire to
it. It is important to note that the temperature required to set fire
to different gases varies with the gas. For instance, I will set free in
this bottle a small quantity of gas, which fires at a very low
temperature. It is the vapour of carbon disulphide. See, I merely place
a hot rod into the bottle, and the gas fires at once. If I put a hot rod
into this bottle of coal gas, no such effect results, since coal gas
requires a very much higher temperature to ignite it than bisulphide of
carbon gas. I want almost--not quite--actual flame to fire coal gas. But
here is another gas, about which I may have to say something directly,
called marsh gas (the gas of coal-mines). This requires a much higher
temperature than even coal gas to fire it. I want you to understand that
although all gases require heat to fire them, different gases ignite at
very different temperatures. Bisulphide of carbon gas, _e. g._, ignites
at a very low temperature, whilst marsh gas requires a very high
temperature indeed for its ignition. You will see directly that this is
a very important fact. Sulphur gas ignites fortunately at a fairly low
temperature, and that is why sulphur is so useful an addition to the
wood splint by which to get fire out of the tinder-box.
[Illustration: Fig. 31.]
And here I wish to make a slight digression in my story. I will show you
an experiment preparatory to bringing before you the fact I am anxious
now to make clear. I have before me a tube, one half of which is brass
and the other half wood. I have covered the tube, as you see, with a
tightly-fitting piece of white paper. The whole tube, wood and brass,
has been treated in exactly the same manner. Now I will set fire to some
spirit in the trough I have here, and expose the entire tube to the
action of the flame. Notice this very curious result, viz. that the
paper covering the brass portion of the tube does not catch fire,
whereas the paper covering the wood is rapidly consumed (Fig. 31). You
see the exact line that divides wood from brass by the burning of the
paper. Well, why is that? Now all of you know that some things conduct
heat (_i. e._ carry away heat) better than other substances. For
instance, if you were to put a copper rod and a glass rod into the fire,
allowing a part of each to project, the copper rod that projects out of
the fire would soon become so very hot that you dare not touch it, owing
to the copper conducting the heat from the fire, whereas you would be
able to take hold of the projecting end of the glass rod long after the
end of the glass exposed to the fire had melted. The fact is, the copper
carries heat well, and the glass carries heat badly. Now with the
teaching of that experiment before you, you will understand, I hope, the
exact object of one or two experiments I am about to show you. Here is a
piece of coarse wire gauze--I am about to place it over the flame of
this Argand burner. You will notice that it lowers the flame for a
moment, but almost immediately the flame dashes through the gauze (Fig.
32 A). Here is another piece of gauze, not quite so coarse as the last.
I place this over the flame, and for a moment the flame cannot get
through it. There, you see it is through now, but it did not pass with
the same readiness that it did in the case of the other piece of gauze,
which was coarser. Now, when I take a piece of fine gauze, the flame
does not pass through at all until the gauze is nearly red-hot. There
is plenty of gas passing all the time. If I take a still finer gauze, I
shall find that the flame won't pass even when it is almost red-hot
(Fig. 32 B). Plenty of gas is passing through, remember, all the time,
but the flame does not pass through. Now why is it that the flame is
unable to pass? The reason is this--because the metal gauze has so
cooled the flame that the heat on one side is not sufficient to set fire
to the gas on the other side. I must have, you see, a certain
temperature to fire my gas. When therefore I experiment with a very fine
piece of gauze, where I have a good deal of metal and a large conducting
surface, there is no possibility of the flame passing. In fact, I have
so cooled the flame by the metal gauze that it is no longer hot enough
to set fire to the gas on the opposite side. I will give you one or two
more illustrations of the same fact. Suppose I put upon this gauze a
piece of camphor (camphor being a substance that gives off a heavy
combustible vapour when heated), and then heat it, you see the camphor
gas burning on the under side of the gauze, but the camphor gas on the
upper side is not fired (Fig. 33). Plenty of camphor gas is being given
off, but the flame of the burning camphor on the under side is not high
enough to set fire to the camphor gas on the upper side, owing to the
conducting power of the metal between the flame and the upper gas.
There is one other experiment I should like to show you. Upon this
piece of metal gauze I have piled up a small heap of gunpowder. I will
place a spirit-lamp underneath the gunpowder, as you see I am now doing,
and I don't suppose the gunpowder will catch fire. I see the sulphur of
the gunpowder at the present moment volatilizing, but the flame, cooled
by the action of the metal, is not hot enough to set fire to the
gunpowder.
[Illustration: Fig. 32.]
[Illustration: Fig. 33.]
I showed you the steel and flint lamp--if I may call it a lamp--used by
coal-miners at the time of Davy (Fig. 22). Davy set to work to invent a
more satisfactory lamp than that, and the result of his experiments was
the beautiful miner's lamp which I have here (Fig. 34). I regard this
lamp with considerable affection, because I have been down many a
coal-mine with it. This is the coal-miner's safety-lamp. The
old-fashioned form of it that I have here has been much improved, but it
illustrates the principle as well as, if not better than, more elaborate
varieties. It is simply an oil flame covered with a gauze shade, exactly
like that gauze with which I have been experimenting. I will allow a jet
of coal gas to play upon this lamp, but the gas, as you see, does not
catch fire. You will notice the oil flame in the lamp elongates in a
curious manner. The flame of the lamp cooled by the gauze is not hot
enough to set fire to the coal gas, but the appearance of the flame
warns the miner, and tells him when there is danger. And that is the
explanation of the beautiful miner's safety-lamp invented by Sir Humphry
Davy.
[Illustration: Fig. 34.]
Now let me once more put this fact clearly before you, that whether it
is the gas flame or our farthing rushlight, whether it is our lamp or
our lucifer match, if we have a flame we must have a gas to burn, and
having a gas, we must heat it to, and maintain it at, a certain
temperature. We have now reached a point where our tinder-box has
presented us with flame. A flame is indeed the consummated work of the
tinder-box.
[Illustration: Fig. 35.]
[Illustration: Fig. 36.]
Just let me say a few words about the grand result--the consummated work
of the tinder-box. A flame is a very remarkable thing. It looks solid,
but it is not solid. You will find that the inside of a flame consists
of unburnt gas--gas, that is to say, not in a state of combustion at
all. The only spot where true combustion takes place is the outer
covering of the flame. I will try to show you some experiments
illustrating this. I will take a large flame for this purpose. Here is a
piece of glass tube which I have covered with ordinary white paper.
Holding the covered glass tube in our large flame for a minute or two,
you observe I get two rings of charred paper, corresponding to the outer
envelope of the flame, whilst that portion of the paper between the
black rings has not even been scorched, showing you that it is only the
outer part of the flame that is burning (Fig. 35). The heat of the flame
is at that part where, as I said before, the combustible gases come
into contact--into collision with the atmosphere. So completely is this
true, that if I take a tube, such as I have here, I can easily convey
the unburnt gas in the centre of the flame away from the flame, and set
fire to it, as you see, at the end of the glass tube a long distance
from the flame (Fig. 36). I will place in the centre of my flame some
phosphorus which is at the present moment in a state of active burning,
and observe how instantly the combustion of the phosphorus ceases so
soon as it gets into the centre of the flame. The crucible which
contains it is cooled down immediately, and presents an entirely
different appearance within the flame to what it did outside the flame.
It is a curious way, perhaps you think, to stop a substance burning by
putting it into a flame. Indeed I can put a heap of gunpowder inside a
flame so that the outer envelope of burning gas does not ignite it (Fig.
37). There you see a heap of gunpowder in the centre of our large flame.
The flame is so completely hollow that even it cannot explode the
powder.
[Illustration: Fig. 37.]
[Illustration: Fig. 38.]
I want you, if you will, to go a step further The heat of the flame is
due, as I explained in my last lecture, to the clashing of molecules.
But what is the light of my candle and gas due to? The light is due to
the solid matter in the flame, brought to a state of white heat or
incandescence by the heat of the flame. The heat is due to the clashing
of the particles, the light is due to the heated solid matter in the
flame. Let me see if I can show you that. I am setting free in this
bottle some hydrogen, which I am about to ignite at the end of this
piece of glass tube (Fig. 38 A). I shall be a little cautious, because
there is danger if my hydrogen gets mixed with air. There is my hydrogen
burning; but see, it gives little or no light. But this candle flame
gives light. Why? The light of the candle is due to the intensely heated
solid matter in the flame; the absence of light in the hydrogen flame
depends on the absence of solid matter. Let me hold clean white plates
over both these flames. See the quantity of black solid matter that I am
able to collect from this candle flame (Fig. 38 B). But my hydrogen
yields me no soot or solid matter whatsoever (Fig. 38 A). The plate
remains perfectly clean, and only a little moisture collects upon it.
The light that candle gives depends upon the solid matter in the flame
becoming intensely heated. If what I say be true, it follows that if I
take a flame which gives no light, like this hydrogen flame (Fig. 39 A),
and give it solid particles, I ought to change the non-luminous flame
into a luminous one. Let us see whether this be so or not. I have here a
glass tube containing a little cotton wadding (Fig. 39 B _a_), and I am
about to pour on the wadding a little ether, and to make the hydrogen
gas pass through the cotton wadding soaked with ether before I fire it.
And now if what I have said is correct, the hydrogen flame to which I
have imparted a large quantity of solid matter ought to produce a good
light, and so it does! See, I have converted the flame which gave no
light (Fig. 39 A) into a flame which gives an excellent light merely by
incorporating solid matter with the flame (Fig. 39 B). What is more, the
amount of light that a flame gives depends upon the amount or rather the
number of solid particles that it contains. The more solid particles
there are in the flame, the greater is the light. Let me give you an
illustration of this. Here is an interesting little piece of apparatus
given to my predecessor in the chair of chemistry at the London Hospital
by the Augustus Harris of that day. It is one of the torches formerly
used by the pantomime fairies as they descended from the realms of the
carpenters. I have an alcohol flame at the top of the torch which gives
me very little light. Here, you see, is an arrangement by which I can
shake a quantity of solid matter (lycopodium) into the non-luminous
alcohol flame. You will observe what a magnificently luminous flame I
produce (Fig. 40).
[Illustration: Fig. 39.]
[Illustration: Fig. 40.]
I have told you that the light of a flame is due to solid matter in the
flame;[B] further, that the amount of light is due to the amount of
solid matter. And now I want to show you that the kind of light is due
to the kind of solid matter in the flame. Here are some pieces of cotton
wadding, which I am about to saturate with alcoholic solutions of
different kinds of solid matter. For instance, I have in one bottle an
alcoholic solution of a lithium salt, in another of a barium, in a third
of a strontium, and so on. I will set fire to all these solutions, and
you see how vastly different the colours are, the colour of the flames
being dependent on the various forms of solid matter that I have
introduced into them.
[B] I have not forgotten Frankland's experiments on this subject,
but the lectures did not admit of dealing with exceptional cases.
Thus I have shown you that the heat of our flame is due to the clashing
of the two gases, and the light of the flame to the solid matter in the
flame, and the kind of light to the kind of solid matter.
Well, there is another point to which I desire to refer. Light is the
paint which colours bodies. You know that ordinary white light is made
up of a series of beautiful colours (the spectrum), which I show you
here. If I take all these spectrum or rainbow colours which are painted
on this glass I can, as you see, recompose them into white light by
rotating the disc with sufficient rapidity that they may get mixed
together on the little screen at the back of your eye. White light then
is a mixture of a number of colours.
Just ask yourselves this question. Why is this piece of ribbon white?
The white light falls upon it. White light is made up of all those
colours you saw just now upon the screen. The light is reflected from
this ribbon exactly as it fell upon the ribbon. The whole of those
colours come off together, and that ribbon is white because the whole of
the colours of the spectrum are reflected at the same moment. Why is
that ribbon green? The white light falls upon the ribbon--the violet,
the indigo, the red, the blue, the orange, and the yellow, are absorbed
by the dye of the ribbon, and you do not see them. The ribbon, as it
were, drinks in all these colours, but it cannot drink in the green. And
reflecting the green of the spectrum, you see that ribbon green because
the ribbon is incapable of absorbing the green of the white light. Why
is this ribbon red? For the same reason. It can absorb the green which
the previous piece of ribbon could not absorb, but it cannot absorb the
red. The fact is, colour is not an inherent property of a body. If you
ask me why that ribbon is green, and why this ribbon is red, the real
answer is, that the red ribbon has absorbed every colour except the red,
and the green ribbon every colour except the green, not because they are
of themselves red and green but because they have the power of
reflecting those colours from their surfaces.
This then is the consummated work of our tinder-box. Our tinder-box set
fire to the match, and the match set fire to the candle, whilst the heat
and the light of the candle are the finished work of the candle that the
tinder-box lighted.
The clock warns me that I must bring to an end my story of a tinder-box.
To be sure, the tinder-box is a thing of the past, but I hope its story
has not been altogether without teaching. Let me assure you that the
failure, if failure there be, is not the fault of the story, but of the
story-teller. If some day, my young friends, you desire to be great
philosophers--and such desire is a high and holy ambition--be content in
the first instance to listen to the familiar stories told you by the
commonest of common things. There is nothing, depend upon it, too
little to learn from. In time you will rise to higher efforts of thought
and intellectual activity, but you will be primed for those efforts by
the grasp you have secured in your studies of every-day phenomena.
"Great things are made of little things,
And little things go lessening, till at last
Comes God behind them."