takes this ore, with its impurities, and mixes it (as he finds it
essential and best) with its own weight of sulphuret of lead—lead
combined with sulphur. Both the lead and the sulphur are wanted; for the
iron that is there present, as you see by the table, is one of the most
annoying substances in the treatment that you can imagine, because it is
not volatile; and while the iron remains adhering to the platinum, the
platinum will not flow readily. It cannot be sent away by a high
temperature—sent into the atmosphere so as to leave the platinum behind.
Well, then, a hundred parts of ore and a hundred parts of sulphuret of
lead, with about fifty parts of metallic lead, being all mingled together
in a crucible, the sulphur of the sulphuret takes the iron, the copper,
and some of the other metals and impurities, and combines with them to
form a slag; and as it goes on boiling and oxidising, it carries off the
iron, and so a great cleansing takes place.
Now, you ought to know that these metals, such as platinum, iridium, and
palladium, have a strong affinity for such metals as lead and tin, and
upon this a great deal depends. Very much depends upon the platinum
throwing out its impurities of iron and so forth, by being taken up with
the lead present in it. That you may have a notion of the great power that
platinum has of combining with other metals, I will refer you to a little
of the chemist’s experience—his bad experience. He knows very well that
if he takes a piece of platinum-foil, and heats a piece of lead upon it,
or if he takes a piece of platinum-foil, such as we have here, and heats
things upon it that have lead in them, his platinum is destroyed. I have
here a piece of platinum, and if I apply the heat of the spirit-lamp to
it, in consequence of the presence of this little piece of lead which I
will place on it, I shall make a hole in the metal. The heat of the lamp
itself would do no harm to the platinum, nor would other chemical means;
but because there is a little lead present, and there is an affinity
between the two substances, the bodies fuse together at once. You see the
hole I have made. It is large enough to put your finger in, though the
platinum itself was, as you saw, almost infusible, except by the voltaic
battery. For the purpose of shewing this fact in a more striking manner, I
have taken pieces of platinum-foil, tinfoil, and lead-foil, and rolled
them together; and if I apply the blowpipe to them, you will have, in
fact, a repetition on a larger scale of the experiment you saw just now
when the lead and platinum came together, and one spoiled the other. When
the metals are laid one upon the other, and folded together and heat
applied, you will not only see that the platinum runs to waste, but that
at the time when the platinum and lead are combined there is ignition
produced—there is a power of sustaining combustion. I have taken a large
piece, that you may see the phenomenon on a large scale. You saw the
ignition and the explosion which followed, of which we have here the
results—the consequence of the chemical affinity between the platinum and
the metals combined with it, which is the thing upon which Deville founds
his first result.
When he has melted these substances and stirred them well up, and so
obtained a complete mixture, he throws in air upon the surface to burn off
all the sulphur from the remaining sulphuret of lead; and at last he gets
an ingot of lead with platinum—much lead, comparatively, and little
platinum. He gets that in the crucible with a lot of scoriæ and other
things, which he treats afterwards. It is that platiniferous lead which we
have to deal with in our future process. Now, let me tell you what he does
with it. His first object is to get rid of the lead. He has thrown out all
the iron, and a number of other things, and he has got this kind of
compound indicated in the table. He may get it as high as 78 per cent. of
platinum, and 22 of lead; or 5, or 10, or 15 of platinum, and 95, or 90,
or 85 of lead (which he calls weak platinum), and he then places it in the
kind of vessel that you see before you. Suppose we had the mixture here;
we should have to make it hot, and then throw in air upon the surface. The
combustible metal—that is, the lead—and the part that will oxidise, are
thoroughly oxidised; the litharge would flow out in a fused state into a
vessel placed to receive it, and the platinum remains behind.
[Illustration: Fig. 36.]
Here is the process which Deville adopts for the purpose of casting off
the lead, after he has got out the platinum from the ore. (Having made use
of your friend, you get rid of him as quickly as you can.) He gets his
heat by applying the combination of oxygen and hydrogen, or of carburetted
fuel, for the purpose of producing a fire. I have here a source of
coal-gas; there I have a source of hydrogen; and here I have a source of
oxygen. I have here also one of the blowpipes used by Deville in his
process for working platinum in the way I have spoken of. There are two
pipes, and one of them goes to the source of coal-gas, and the other to
the supply of oxygen.
[Illustration: Fig. 37.]
By uniting these we obtain a flame of such a heat as to melt platinum. You
will, perhaps, hardly imagine what the heat is, unless you have some proof
of it; but you will soon see that I have actually the power of melting
platinum. Here is a piece of platinum-foil running like wax under the
flame which I am bringing to bear against it. The question, however, is
whether we shall get heat enough to melt, not this small quantity, but
large masses—many pounds of the metal. Having obtained heat like this,
the next consideration is what vessel is he to employ which could retain
the platinum when so heated, or bear the effects of the flame? Such
vessels are happily well supplied at Paris, and are formed of a substance
which surrounds Paris; it is a kind of chalk (called, I believe, by
geologists, _calcaire grossière_), and it has the property of enduring an
extreme degree of heat. I am now going to get the highest heat that we can
obtain. First, I shew you the combustion of hydrogen by itself. I have not
a large supply, because the coal-gas is sufficient for most of our
purposes. If I put a piece of lime obtained from this chalk into the gas,
you see we get a pretty hot flame, which would burn one’s fingers a good
deal But now let me subject a piece of it to the joint action of oxygen
and hydrogen. I do this for the purpose of shewing you the value of lime
as a material for the furnaces and chambers that are to contain the
substances to be operated on, and that are consequently to sustain the
action of this extreme heat. Here we have the hydrogen and the oxygen,
which will give the most intense heat that can be obtained by chemical
action; and if I put a piece of lime into the flame, we get what is called
the lime-light. Now, with all the beauty and intensity of action which you
perceive, there is no sensible deterioration of the lime except by the
mechanical force of the current of gases rushing from the jet against the
lime, sweeping away such particles as are not strongly aggregated. “Vapour
of lime” some call it; and it may be so, but there is no other change of
the lime than that under the action of heat of this highly-exalted
chemical condition, though almost any other substance would melt at once.
Then, as to the way in which the heat is applied to the substance. It is
all very well for me to take a piece of antimony, and fuse it in the flame
of a blowpipe. But if I tried this piece in the ordinary lamp flame, I
should do nothing; if I tried a smaller piece, I should do little or
nothing; and if I tried a still smaller piece, I should do little or
nothing; yet I have a condition which will represent what Deville carries
to the highest possible extent, and which we all carry to the highest
extent, in the use of the blowpipe. Suppose I take this piece of antimony:
I shall not be able to melt it in that flame of the candle by merely
holding it there; yet, by taking pains, we can even melt a piece of
platinum there. This is a preparation which I made for the purpose of
proving the fusibility of platinum in a common candle. There is a piece of
wire, drawn by that ingenious process of Dr. Wollaston’s, not more than
the three-thousandth part of an inch in diameter. He put the wire into the
middle of a cylinder of silver, and drew both together until the whole
compound was exceedingly thin; and then he dissolved away the silver by
nitric acid. There was left in the centre a substance which I can scarcely
see with an eye-glass, but which I know is there, and which I can make
visible, as you see, by putting it into the candle, where the heat makes
it glow like a spark. I have again and again tried this experiment
up-stairs in my own room, and have easily fused this platinum-wire by a
common candle. You see we have, therefore, heat enough in the candle, as
in the voltaic battery, or in the highly-exalted combustion of the
blowpipe, but we do not supply a continuous source of heat. In the very
act of this becoming ignited, the heat radiates so fast that you cannot
accumulate enough to cause the fusion of the wire, except under the most
careful arrangement. Thus I cannot melt that piece of antimony by simply
putting it into the candle; but if I put it upon charcoal, and drive the
fiery current against it, there will be heat enough to melt it. The beauty
of the blowpipe is, that it sends hot air (making hot air by the
combustion of the flame) against the thing to be heated. I have only to
hold the antimony in the course of that current, and particle by particle
of the current impinges upon the antimony, and so we get it melted. You
now see it red-hot, and I have no doubt it will continue to burn if I
withdraw it from the flame and continue to force the air on it. Now, you
see it burning without any heat but that of its own combustion, which I am
keeping up by sending the air against it. It would go out in a moment if I
took away the current of air from it; but there it is burning, and the
more air I give it, by this or any other action, the better it is. So,
then, we have here not merely a mighty source of heat, but a means of
driving the heat forcibly against substances.
Let me shew you another experiment with a piece of iron. It will serve two
purposes—shewing you what the blowpipe does as a source of heat, and what
it does by sending that heat where it is wanted. I have taken iron in
contrast with silver or other metals, that you may see the difference of
action, and so be more interested in the experiment. Here is our fuel, the
coal-gas; and here our oxygen. Having thus my power of heat, I apply it to
the iron, which, as you see, soon gets red-hot. It is now flowing about
like a globule of melted mercury. But observe, I cannot raise any vapour:
it is now covered with a coat of melted oxide, and unless I have a great
power in my blowpipe, it is hardly possible to break through it. Now,
then, you see these beautiful sparks: you have not only a beautiful kind
of combustion, but you see the iron is being driven off, not producing
smoke, but burning in a fixed condition. How different this is from the
action of some other metals—that piece of antimony, for instance, which
we saw just now throwing off abundance of fumes. We can, of course, burn
away this iron by giving plenty of air to it; but with the bodies which
Deville wants to expose to this intense heat he has not that means: the
gas itself must have power enough to drive off the slag which forms on the
surface of the metal, and power to impinge upon the platinum so as to get
the full contact that he wants for the fusion to take place. We see here,
then, the means to which he resorts—oxygen, and either coal-gas or
water-gas[19], or pure hydrogen, for producing heat, and the blowpipe for
the purpose of impelling the heated current upon the metals.
I have two or three rough drawings here, representing the kind of furnaces
which he employs. They are larger, however, than the actual furnaces he
uses. Even the furnace in which he carries on that most serious operation
of fusing fifty pounds of platinum at once is not much more than half the
size of the drawing. It is made of a piece of lime below and a piece of
lime above. You see how beautifully lime sustains heat without altering in
shape; and you may have thought how beautifully it prevents the
dissipation of the heat by its very bad conducting powers.
[Illustration: Fig. 38]
While the front part of the lime which you saw here was so highly ignited,
I could at any moment touch the back of it without feeling any annoyance
from the heat So, by having a chamber of lime of this sort, he is able to
get a vessel to contain these metals with scarcely any loss of heat. He
puts the blowpipes through these apertures, and sends down these gases
upon the metals, which are gradually melted. He then puts in more metal
through a hole at the top. The results of the combustion issue out of the
aperture which you see represented. If there be strips of platinum, he
pushes them through the mouth out of which the heated current is coming,
and there they get red-hot and white-hot before they get into the bath of
platinum. So he is able to fuse a large body of platinum in this manner.
When the platinum is melted, he takes off the top and pours out from the
bottom piece, like a crucible, and makes his cast. This is the furnace by
which he fuses his forty pounds or fifty pounds of platinum at once. The
metal is raised to a heat that no eye can bear. There is no light and
shadow, no chiaro-oscuro there; all is the same intensity of glow. You
look in, and you cannot see where the metal or the lime is; it is all as
one. We have, therefore, a platform with a handle, which turns upon an
axis, that coincides with the gutter that is formed for the pouring of the
metal; and when all is known to be ready, by means of dark glasses, the
workmen take off the top piece and lift up the handle, and the mould being
then placed in a proper position, he knows that the issue of the metal
will be exactly in the line of the axis. No injury has ever happened from
the use of this plan. You know with what care it is necessary to carry
such a vessel of mercury as we have here, for fear of turning it over on
one side or the other; but if it be a vessel of melted platinum, the very
greatest care must be used, because the substance is twice as heavy: yet
no injury has been done to any of the workmen in this operation.
I have said that Deville depends upon intense heat for carrying off
vapour; and this brings me to the point of shewing how vapours are carried
off. Here is a basin of mercury, which boils easily, as you know, and
gives us the opportunity of observing the facts and principles which are
to guide us. I have here two poles of the battery, and if I bring them
into contact with the mercury, see what a development of vapour we have.
The mercury is flying off rapidly; and I might, if I pleased, put all the
company around me in a bath of mercury vapour. And so, if we take this
piece of lead and treat it in the same way, it will also give off vapour.
Observe the fumes that rise from it; and even if it was so far enclosed
from the air that you could not form any litharge, you would still have
those abundant fumes flying off. I may also take a piece of gold, and shew
you the same thing. I have here a piece of gold which I put upon a clean
surface of Paris limestone. Applying the heat of the blowpipe to it, you
see how the heat drives off the vapour; and if you notice at the end of
the Lecture, you will observe on the stone a purple patch of condensed
gold. Thus you see a proof of the volatilisation of gold. It is the same
with silver. You will not be startled if I sometimes use one agent and
sometimes another to illustrate a particular point. The volatility of gold
and silver is the same thing, whether it be effected by the voltaic
battery or by the blowpipe. [A lump of silver was placed in a charcoal
crucible between the poles of a voltaic battery.] Now, look at the fumes
of silver, and observe the peculiar and beautiful green colour which they
produce. We shall now shew you this same process of boiling the silver,
cast on a screen from the electric lamp which you have before you; and
while Dr. Tyndall is kindly getting the lamp ready for this purpose, let
me tell you that Deville proposes to throw out in this way all these
extraneous things that I have spoken of, except two—namely, iridium and
rhodium. It so happens, as he says, that iridium and rhodium do make the
metal more capable of resisting the attacks of acids than platinum itself.
Alloys are compounded up to 25 per cent. of rhodium and iridium, by which
the chemical inaction of the platinum is increased, and also its
malleability and other physical properties. [The image of the voltaic
discharge through vapour of silver was now thrown upon the screen.] What
you have now on the screen is an inverted image of what you saw when we
heated the silver before. The fine stream that you see around the silver
is the discharge of the electric force that takes place, giving you that
glorious green light which you see in the ray; and if Dr. Tyndall will
open the top of the lamp, you will see the quantity of fumes that will
come out of the aperture, shewing you at once the volatility of silver.
I have now finished this imperfect account. It is but an apology for not
having brought the process itself before you. I have done the best I could
under the circumstances; and I know your kindness well, for if I were not
aware that I might trust to it, I would not appear here so often as I have
done. The gradual loss of memory and of my other faculties is making
itself painfully evident to me, and requires, every time I appear before
you, the continued remembrance of your kindness to enable me to get
through my task. If I should happen to go on too long, or should fail in
doing what you might desire, remember it is yourselves who are chargeable,
by wishing me to remain. I have desired to retire, as I think every man
ought to do before his faculties become impaired; but I must confess that
the affection I have for this place, and for those who frequent this
place, is such, that I hardly know when the proper time has arrived.
NOTES.
[Footnote 1: Page 16. The Royal George sunk at Spithead on The 29th of
August, 1782. Colonel Pasley commenced operations for the removal of the
wreck by the explosion of gunpowder, in August, 1839. The candle which
Professor Faraday exhibited must therefore have been exposed to the action
of salt water for upwards of fifty-seven years.]
[Footnote 2: Page 17. The fat or tallow consists of a chemical combination
of fatty acids with glycerine. The lime unites with the palmitic, oleic,
and stearic acids, and separates the glycerine. After washing, the
insoluble lime soap is decomposed with hot dilute sulphuric acid. The
melted fatty acids thus rise as an oil to the surface, when they are
decanted. They are again washed and cast into thin plates, which, when
cold, are placed between layers of cocoa-nut matting, and submitted to
intense hydraulic pressure. In this way the soft oleic acid is squeezed
out, whilst the hard palmitic and stearic acids remain. These are further
purified by pressure at a higher temperature, and washing in warm dilute
sulphuric acid, when they are ready to be made into candles. These acids
are harder and whiter than the fats from which they were obtained, whilst
at the same time they are cleaner and more combustible.]
[Footnote 3: Page 19. A little borax or phosphorus salt is sometimes
added, in order to make the ash fusible.]
[Footnote 4: Page 27. Capillary attraction or repulsion is the cause which
determines the ascent or descent of a fluid in a capillary tube. If a
piece of thermometer tubing, open at each end, be plunged into water, the
latter will instantly rise in the tube considerably above its external
level. If, on the other hand, the tube be plunged into mercury, a
repulsion instead of attraction will be exhibited, and the level of the
mercury will be lower in the tube than it is outside.]
[Footnote 5: Page 29. The late Duke of Sussex was, we believe, the first
to shew that a prawn might be washed upon this principle. If the tail,
after pulling off the fan part, be placed in a tumbler of water, and the
head be allowed to hang over the outside, the water will be sucked up the
tail by capillary attraction, and will continue to run out through the
head until the water in the glass has sunk so low that the tail ceases to
dip into it.]
[Footnote 6: Page 37. The alcohol had chloride of copper dissolved in it:
this produces a beautiful green flame.]
[Footnote 7: Page 54. Lycopodium is a yellowish powder found in the fruit
of the club moss (_Lycopodium clavatum_). It is used in fireworks.]
[Footnote 8: Page 58. Bunsen has calculated that the temperature of the
oxyhydrogen blowpipe is 8061° Centigrade. Hydrogen burning in air has a
temperature of 3259° C., and coal-gas in air, 2350° C.]
[Footnote 9: Page 60. The following is the action of the sulphuric acid in
inflaming the mixture of sulphuret of antimony and chlorate of potassa. A
portion of the latter is decomposed by the sulphuric acid into oxide of
chlorine, bisulphate of potassa, and perchlorate of potassa. The oxide of
chlorine inflames the sulphuret of antimony, which is a combustible body,
and the whole mass instantly bursts into flame.]
[Footnote 10: Page 63. The “air-burner,” which is of such value in the
laboratory, owes its advantage to this principle. It consists of a
cylindrical metal chimney, covered at the top with a piece of rather
coarse iron-wire gauze. This is supported over an argand burner, in such a
manner that the gas may mix in the chimney with an amount of air
sufficient to burn the carbon and hydrogen simultaneously, so that there
may be no separation of carbon in the flame with consequent deposition of
soot. The flame, being unable to pass through the wire gauze, burns in a
steady, nearly invisible manner above.]
[Footnote 11: Page 74. Water is in its densest state at a temperature of
39.1° Fahrenheit]
[Footnote 12: Page 74. A mixture of salt and pounded ice reduces the
temperature from 32° F. to zero—the ice at the same time becoming fluid.]
[Footnote 13: Page 82. Potassium, the metallic basis of potash, was
discovered by Sir Humphrey Davy in 1807, who succeeded in separating it
from potash by means of a powerful voltaic battery. Its great affinity for
oxygen causes it to decompose water with evolution of hydrogen, which
takes fire with the heat produced.]
[Footnote 14: Page 98. Professor Faraday has calculated that there is as
much electricity required to decompose one grain of water as there is in a
very powerful flash of lightning.]
[Footnote 15: Page 101. A solution of acetate of lead submitted to the
action of the voltaic current, yields lead at the negative pole, and brown
peroxide of lead at the positive pole. A solution of nitrate of silver,
under the same circumstances, yields silver at the negative pole, and
peroxide of silver at the positive pole.]
[Footnote 16: Page 129. The gas which is thus employed as a test for the
presence of oxygen, is the binoxide of nitrogen, or nitrous oxide. It is a
colourless gas, which, when brought in contact with oxygen, unites with
it, forming hyponitric acid, the red gas referred to.]
[Footnote 17: Page 152. _Marble_ is a compound of carbonic acid and lime.
The muriatic acid being the stronger of the two, takes the place of the
carbonic acid, which escapes as a gas, the residue forming muriate of lime
or chloride of calcium.]
[Footnote 18: Page 186. _Lead pyrophorus_ is made by heating dry tartrate
of lead in a glass tube (closed at one end, and drawn out to a fine point
at the other) until no more vapours are evolved. The open end of the tube
is then to be sealed before the blowpipe. When the tube is broken and the
contents shaken out into the air, they burn with a red flash.]
[Footnote 19: Page 216. _Water-gas_ is formed by passing vapour of water
over red-hot charcoal or coke. It is a mixture of hydrogen and carbonic
oxide; each of which is an inflammable gas.]
Poster’s note: “combustion that makes!” was corrected from a misprint
“combusion that makes!” in the original.