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0:09
Yes when I was 10 years old I used to live in Shepherd's Bush in West London not that far from here And every Saturday afternoon I used to go to Shepherd's Bush market You see I was very keen to see what people were selling. What they were buying. But most importantly, what they were throwing away. Because very frequently among the rubbish I could find something useful For instance wooden boxes, from which I could make a cricket bat But, every Saturday afternoon There was a very strange man
0:38
He was completely bold and he was selling medicines He was selling medicines in the form of purple tablets Like this, and he told everyone That these tablets were very good for you Because they could cure you of any disease Whether you had a headache, tooth ache, muscle pain, neuralgia nostalgia, you name it, this medicine would cure it. So people were very fascinated to actually be able to buy this medicine
29:35
This was made about three hundred year ago. They rapidly realised that there are 2 main gases in the air, one of them which actually responsible for burning and that was called Oxygen. The other one they called Nitrogen.And they recognised that about one fifth of the air is Oxygen And then they started to do experiment to make fuels burn better, not by mixing with air but trying to mix them with Oxygen or get them to combine with Oxygen and I wanted to show you the result of one such experiment
30:13
which was a remarkable triumph of the science of Chemistry and it's occurred using the most mundane of substances you could possibly imagined and that is cotton wool. Cotton wool is a natural fibre. It has been known to human being for over five thousand year and we know that cotton was first grown and used on a large scale in India and also in Mexico . But it is in India where the great culture of cotton culture of cotton fabric, cotton weaving grew for thousand of years
30:49
well before it reach Europe. Now cotton, what is cotton chemically speaking? Well, it is a very delicate fabric , it is used in clothing of course as you know and for cosmetic application. and chemically speaking it's called cellulose . it's a natural polymer which occurs and which is very very comfortable to the touch and warm to the feel. and that is why it has a great place in our human history. Not to mention the fact that the cotton played a huge role in the industrial revolution
31:21
in this country during the 18 century when cotton weaving and spinning were developed in the . Now, if you watch carefully who on earth would think of burning. Well Chemist thought of burning everything and this is one of the things.They burnt cotton you see, they thought it may be wonderful. there it is , it is burning in air, a hugely boring experiment you see it is burning in air, because air contain only about 20 percent Oxygen you see and so this is what we have
31:49
incomplete combustion it is burning with this sort of slightly yellowy flame and there is tiny bit of ash left and entirely unspectacular almost like a piece of paper. Now What I wanted to to tell you next during the 19 century, during the middle of the 19 century, Chemists were experimenting with adding Oxygen to different fuels either mixing it or combining them chemically and the remarkable result was achieved by 2 scientists working independently with cotton, one of them was
32:27
Ascanio Sobrero who was an Italian. The other one was Christian Schönbein , who was a German organic chemist. Christian Schönbein was a good friend and colleague of the great Michael Faraday who delivered teacher's lectures in this precise spot here for over 30 years during the 19 century. So it was Christian Schönbein who actually was the first person who manage to add to cotton with some extra Oxygen. Now how did he achieve that? And also,
33:03
I told you in the beginning that we can frequently recognise a chemical change by a change of colour and yet you say look this cotton wool that I have got here, look the same and feels exactly the same as the cotton wool that I had a minute ago. And yet this has got the extra Oxygen as I will show you in a second. How therefore is this different? Dear children, the difference could be seen by weighing it. You see, if you weigh one of those. They weigh 1.7g but this weigh 2.8g.
33:37
An extra of 1.1g of Oxygen. Now that is huge amount of extra Oxygen added in. It actually does have some Nitrogen as well. You may say how did they add this? Well let's first of all see whether it works of course because there is no point in talking about the effect if we don't see how it was done if it doesn't work. So first of all, let's get ourselves something to talk about. So here then is our cotton wool which has had extra Oxygen added to it. And let"s see how
34:09
this burns and spot the difference. And as you see, there was an instantaneous combustion.The whole things burn very,very very quickly indeed and there was virtually nothing left at all you see. Now that of course, now that of course , dear children is the proof that the experiment was successful. Now when Schönbein achieve this experiment, he was absolutely delighted. And as I said Sobrero, they were working separately, one of them with cotton, the other with glycerin
34:41
and they were similar substances and they achieve this type of result. They thought this was absolutely amazing. For your information, the way this was made is using a very similar substance to what's used in gun powder. Gun powder contains sulphur, charcoal. and potassium nitrate, nitrates. This uses nitric acid and sulphuric acid. You mix them together and you soak the cotton wool for half an hour. And you end up with this. So the Oxygen
35:09
comes about indirectly. But then chemist are people who understood this processes and that's why they are able to bring about the effect. Now, what about I have actually made here is a high explosive. It is of course a very very dangerous substance. It is popular. Its name in the industry is called a gun cotton, nitro cellulose. It is hugely dangerous. It is used in engineering and military purposes. And I would like to demonstrate an explosion for you. Unfortunately, that would be far
35:39
too dangerous. So what I am going to do instead, I am going to demonstrate a propulsion and even 2 propulsions if we're lucky. And what is a propulsion? A propulsion is a directed explosion. A explosion is a rapidly release of gases everywhere, forcing something apart. A propulsion though is when you direct them in one direction. So I'm going to start off by showing you how to use this to make a simple mortar. It's rather similar to a canon. And I'm going to propel some ping pong ball,
36:15
hopefully to the ceiling of our dear Royal Institution Now what I have here is a special, It's a like canon. It's technically called the mortar and principle of it is that you burn the fuel at the bottom when the fuel which will be gun cotton burnt at the bottom. It releases a huge amount of hot gases and they will force the ping pong ball to fly, hopefully to the ceiling So that is the principle of this and what I wanted to tell you dear children is every time you watch one of those fireworks
36:51
displays where there are massive shells exploding in the sky and colours. They are all projected from these. Only they are made of thick cardboard and their shelves go up and they explode in the air. Today, we are not going to explode the shelves up there. We're simply going to fire ping pong balls. Now
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