The Best of the BA Festival

16 September 2007
Presented by Chris Smith, Kat Arney.

This week we're bringing you the very best bits from the BA Festival of Science in York. We discover a chewing gum that dissolves in the wash but still keeps your breath fresh, get some good news about cancer and find out why jogging may not be good for heart patient recovery if it's near a busy road. Also, Chris risks his health to find out about plague control in 17th Century York, and chocolate may be nicer than it is naughty, as Roger Corder explains how it could be good for your health. Out and about in the festival, Meera explores the psychology of commuter cooperation during the 2005 London bombings, tunes in to non-contact musical instruments and gets immersed in pure colour. Plus, we tackle your questions about good viruses, antioxidants and the Aurora Borealis. In Kitchen science Ben & Dave explore the physics behind a football and find out how to 'Bend it like Beckham.'

In this episode

Non-stick chewing gum

Chemists at the University of Bristol have come up with a formulation for chewing gum that could make gum-marked pavements a thing of the past. Terry Cosgrove and his team have developed a new polymer additive that, when added to the gum recipe, renders it water soluble. "Chewing gum contains a lot of hydrophobic - water hating - polymers, which prevent the material from breaking down in water," explains Cosgrove. This makes it difficult to wash off because the water-repellent quality of the gum prevents water from penetrating and dissolving the material. "But our formulation contains hydrophilic - water loving - groups, which, when added to the mixture, allow water molecules to interact with the gum and break it down." The result is a material indistinguishable from traditional gum but which disintegrates into fine flakes after prolonged water exposure; but hardcore chewers need not worry, it shouldn't melt in your mouth! Another spin-off is that the new polymer formulation, which is currently undergoing safety tests and is already licensed for use in human foods, also intensifies the flavour of the gum by helping the flavourants to interact with water in the mouth, and the tastebuds. "We're hoping to have this on the market by 2008," says Cosgrove.

Good news for a change…

Sometimes it can seem like the news is full of scare stories about cancer, but this week has seen two pieces of research that could be seen as good news.

First, six years of research have found that there's no link between mobile phones and brain cancer or brain function, at least in the short term. This adds to a number of other studies that have found no link between mobiles and cancer. Although we still don't know for sure if long-term use of mobiles could have an adverse effect on health, the new study does support the evidence that mobiles are safe.

And at the moment, scientists can't find any explanation as to how radio waves, produced by mobiles and masts, might actually cause cancer - if, indeed, they do at all. The energy of the radio waves is not sufficient to damage DNA. So even if the long-term studies do show an increase in risk, it will be down to a biological mechanism that we don't currently know about.

The other good news story is for women who take the contraceptive pill. Analysis of data from 46,000 women, collected over 36 years by the Royal College of General Practitioners, has shown that taking the pill for less that 8 years doesn't increase your long-term risk of cancer. In fact, the pill may actually cut your risk of certain forms of the disease, such as ovarian, womb and bowel cancer, by between 3 and 12%. That's around one less case of cancer per 2,200 women who have used the pill for a year. But if women took the pill for longer than 8 years, the scientists found a slightly increased risk of cervical cancer, and certain types of brain cancer. On average, the women in the study took the pill for just under 4 years.

It's important to note that this was a long-term study, looking at the risks of cancer many years after women have stopped taking the pill. But while women are actually taking the pill, their risk of breast cancer can be slightly higher. It's also important to remember that the pill is taken by younger women, whose risk of breast cancer is relatively low, so this increased risk is relatively small.

Cool it

US engineers have developed a system to keep houses cool without turning up the aircon.

Bill Miller and his team from Oak Ridge National Laboratory in Tennessee have come up with a roofing system that soaks up heat during the day and then re-radiates it out to space at night, keeping homes cool in the process.

There are four parts to the new system. The first three are concerned with reflecting heat off the roof surface using a combination of reflective tile surfaces, modified tiles that channel warm air out from within the roof cavity, and further reflective coatings beneath the rafters. These are fairly standard techniques, but the fourth line of defence is highly innovative. It consists of sheets of a material which melts at temperatures above 23 degrees Celsius. Melting a solid provokes a phase change, which consumes energy, so the material remains at the same temperature until it has all melted, which can take all day. In this way the material behaves as an energy sponge, preventing the heat from entering the living space below. Later, when the temperature falls at night, the material reverts to a solid again, releasing the energy it soaked up during the day and radiating it out into space.

During tests the material reduced attic temperatures on sunny days by more than 5 degrees Celsius. "We're able to intercept 90% of the heat energy that would otherwise penetrate into the living space through the attic floor. This could reduce the cooling bill for houses by up to 8%" says Miller.

Pollution pumps up heart risk

Although exercise is recommended for people who are rehabilitating from a heart attack, new research suggests that you'd probably want to try and steer clear of busy roads while you're doing it.  Scientists at the University of Edinburgh and Umea University in Sweden have measured the effects of diesel exhaust fumes on heart and blood vessel function in twenty men who'd previously had a heart attack.  Each man was put on an exercise bike, and given either clean or polluted air to breathe.  The team found that breathing in the fumes caused changes in the heart's electrical activity. This suggests that air pollution reduces the amount of oxygen that can get to the heart during exercise.  In addition, the researchers found that the body's ability to make a "guardian" protein known as t-PA, which can prevent blood clots from forming, was also reduced by more than a third following exposure to the fumes.

In fact, this explains why previous studies have found that patients with heart disease are more likely to be admitted to hospital on days in which air pollution levels are high. And it also reveals that as well as affecting our lungs, traffic fumes can have a major effect on our hearts too.  The research is particularly relevant for people in areas with lots of diesel-powered traffic.  Diesel engines produce 10 to 100 times more pollutant particles than petrol engines, and the number of diesel-powered automobiles is increasing in Europe and other parts of the world.  Perhaps in the future, we could see the introduction of devices that can filter out harmful particles from exhaust gases.  This might help to improve heart and lung health for us all.

But if you're a heart patient - or know someone who is - then it's probably worth steering clear of busy roads if you're out exercising.  The medical director of the British Heart Foundation, Professor Peter Weissberg, says that "Because of the overwhelming benefits of exercise on heart health, we would still encourage heart patients to exercise regularly, but preferably not when there is a lot of local traffic around.  Heart patients can look out for pollution levels on their local weather forecasts."

Only the lonely - genomically speaking

Scientists in America may have found out why loneliness is linked to health problems.

Writing in the journal Genome Biology, Steve Cole and his colleagues collected blood samples from 14 volunteers who had been matched for age, health status, weight, and medication use.

The only difference was that six of the subjects scored in the top 15% on a UCLA Loneliness Scale. The rest of the group were in the bottom 15% on the same scale.

The team analysed the blood samples to study patterns of gene expression from the 14 subjects and found 209 genes that were consistently differently expressed between the lonely and the contented subjects. Of those 209 genes, 78 were genes that were more active, and 131 were genes that were less active.

"White blood cell gene expression appears to be remodelled in chronically lonely individuals," said Cole.

This finding might explain why people who feel socially isolated tend to have a higher risk of certain diseases such as heart disease, high blood pressure, and susceptibility to infections and cancer compared with individuals with a stronger social network.

"These findings provide molecular targets for our efforts to block the adverse health effects of social isolation," says Cole. But the health-promoting effect of friends isn't just down to how many people you know.

"We found that what counts at the level of gene expression is not how many people you know, it's how many you feel really close to over time."

In the future Cole suggests that doctors might be able to use the team's genetic fingerprint of loneliness to monitor the effect of interventions intended to reduce the impact of a poor social network on health.

20:05 - York's Plagued Past

Chris travelled back in time with Sabine Clarke and James Sumner to find out how the people of 17th century York coped with the plague.

York's Plagued Past
with Sabine Clarke, Oxford University & James Sumner, Manchester University

Sabine -   We've been running an event here, and it's based on a real thing that happened in 1631 when there was an epidemic of plague in YorkChris -   So when you say an epidemic, how big?  How many people were involved?Sabine -   Well, it wasn't in fact one of the worst epidemics which ever affected York, but it was a time when some very draconian measures were introduced to control the plague.  And what the city council, the Aldermen, did at this period was institute a policy of boarding up the infected in their own homes.Chris -   So, crikey, what would have happened to those people then?Sabine -   Well in fact, guards would stand outside their houses and place boards across their doors and windows, and while they would pass the inhabitants of the house some food, they basically waited really until everyone was dead.  It was a way of stopping the plague from spreading.Chris -   But that was quite precient of them, really, to realise you could control a disease like that, wasn't it?Sabine -   Well certainly by 1631 York had had numerous epidemics of plague and over the years, of course, people had learned the best way to deal with these sorts of epidemics.Chris -   Which was to confine people?Sabine -   Yes, in fact other policies, such as building pest houses or hospitals would have been very expensive, boarding up people of course was effective and relatively cheap.Chris -   And in an average city like York in the 1600's, if plague came to town, how many people wouldn't come off so well?Sabine -   About a third of the population could be dead by the end of the plague period, so it could be an extremely serious disease.Chris -   Now James, you're dressed as a priest, so is it your job to bury these poor victims?James -   Amongst other things, yes.  I should explain; My name is Parson Grimsworth, I'm the parson of the parish of St Cuthberts in York, where there's been a severe outbreak of the plague.Chris -   And apart from burying people, what else would you have had to do as the parish parson?James -   I would have gone among them, ministering to them, and preying for the salvation of the city of York.  The plague, I believe, is punishment for the sins of the people of this city.  This is a very rich city full of very idle fellows who spend their time carousing and drinking in the taverns, and gambling down grape lanes...Chris -   And what about in the 17th Century?James -   In the 17th century it would have been pretty much the same.Chris -   So, stepping out of role for a minute, what do you normally do for a living?James -   I'm working on a project on the history of integrated software in Europe, so it's a bit of a step backwards for me.Chris -   Why do you do this?James -   It comes out of work with the British society for the history of science. All of us involved with this project work with the society as part of it's work in educational outreach.  It's useful to be able to develop these things which appeal to children much more immediately than the kind of static lecture presentation would.Chris -   And Sabine, has it been well received?Sabine -   Well absolutely, the children we saw this morning were superb, and thy were in role as part of our production; they were city Aldermen and they were able to ask a number of characters, like our parson and also a doctor some questions, and then at the end of our event they had to make the decision about how to control plague in York.

23:31 - A Chocolate Fix is Actually Good For You!

Being addicted to things is usually bad news but now there's evidence that getting hooked on chocolate might not be such a bad thing: Just so long as it's the dark variety. That...

A Chocolate Fix is Actually Good For You!
with Roger Corder, The Royal London Medical School & Peter Rogers, University of Bristol

Being addicted to things is usually bad news but now there's evidence that getting hooked on chocolate might not be such a bad thing: just so long as it's the dark variety. That said, is chocolate really addictive? To find out, Chris met up with anti-oxidant expert Roger Corder and addiction specialist Peter Rogers.

Chris - So Roger, what's the grounds for thinking (just like the chocolate manufacturers would love us to be thinking) that chocolate is actually good for us?

Roger -   Essentially, a number of population studies have shown a lower risk of heart disease in people who eat chocolate regularly and although this is a rather surprising finding a number of research groups over the past ten years or so have been looking at how this may be explained. What are the effects on blood vessel function that may be protective? We've done our own research in this area and shown that the potent effects; modifying the function of endothelial cells, which are the cells that line blood vessels. These effects could definitely be associated with reduced risk of heart disease; exactly the same molecules we're looking at when we're looking at red wine. There's so much similarity between the effects we're observing and the molecules that we're studying that there's a parallel between consumers of highly tannic red wine and those who like to eat dark chocolate, in terms of the effects observed.

Chris -   So it's got to be the dark chocolate?

Roger -   Absolutely. It's dark chocolate and no other choice. Cocoa as a drink may be an alternative but milk chocolate and white chocolate. These have virtually no flavinoids in; none of these protective polyphenols and therefore there's no scope for health benefits in consuming these products.

Chris - A lot of people criticise people who say, 'Drink red wine because it's good for you,'  because they say you couldn't possibly eat enough or drink enough to make a health-promoting effect.  Is that true with chocolate?  Can you eat enough to make a difference?

Roger -   I think we're at an early stage in defining what are the best chocolates to have. There's certainly some out there that have high cocoa levels with polyphenol levels you would say are consistent with the beneficial effects that are observed. So if you're talking about say 75%-85% chocolates: yes there are some. We need to have labelling of flavinoid levels on chocolate so that people can identify exactly what the best ones are. That's coming I'm sure.

Chris -   Talking to you Peter. A lot of people say, 'I am addicted to chocolate.' Now it sounds to me, from what Roger's saying, that it would be quite good to be addicted to chocolate if its going to make me live longer.

Peter -   Well, possibly. Although the other good news really about chocolate is it's not very addictive. In fact I'd say it's hardly addictive at all. We feel addicted sometimes but the way to sum it up is that we think of chocolate as 'nice but naughty.' We have conflicting attitudes about this food. We like its taste, its flavour, its texture. But it's also food we need to eat with restraint for a variety of reasons. We actually have in our heads that it's unhealthy food. Eating a lot of food is indulgent and that negative attitude is balanced against the nice part of it. Trying to resist a desire for nice things is difficult. Saying: 'I'm not going to eat chocolate because it will make me fat,' doesn't make the thought of chocolate go away. It makes it more intense if anything. We think about it more, we elaborate that thought and that then is a sort of feeling we then label as a craving. Craving them brings to mind the idea of addiction. So these things get linked in our heads together.

Chris -   Are there any chemicals in chocolate if you analyse it which could conceivably be addictive?

Peter -   Well this is one claim. What's special about it is it contains chemicals that have psychoactive effects/mood elevating effects and this is why we like it, crave it and perhaps even are addicted to it. But actually the evidence for this is pretty weak. Our own studies show that, where we have tested the idea directly. We don't find that cocoa solids (the key component containing these psychoactive chemicals) actually gives much of a mood lift beyond a caffeine buzz. We could probably get a stronger buzz from a cup of coffee. So it doesn't do any more than that so we don't think that plays a role at all.

Chris -   What about this idea that women say: 'I crave chocolate at certain stages of my menstrual cycle, either day 14 when I'm about to ovulate or just before I'm about to menstruate?'

Peter -   Well again we think that has more to do with our attitude to chocolate and how we use this food, rather than our biology. One thing here is if our biology puts us in a bad mood perhaps we feel it's legitimate to indulge in chocolate that will help cheer us up. We don't think there's anything special biologically about chocolate that gets rid of that bad mood, other than its just pleasurable to eat.

Chris -   So Roger, the best combo is then some red wine and a bar of chocolate?

Roger -   Absolutely. Although I prefer chocolate and coffee in the morning and a glass of wine in the evening.

Chris -   But isn't chocolate bad for you?

Roger -   There's lots of evidence to show that routine caffeine consumption decreases your risk of diabetes so to say it's bad is not really based on the evidence out there through population studies.

Chris -   So how much chocolate would I need to be eating on a daily basis in order to get the benefits?

Roger -   A very flavinoid, polyphenol-rich chocolate. About 30g would be a good choice and once you start exceeding that then you've got the problems of consuming excess calories. So we really need to have a good choice for consumers of high polyphenol chocolates (well labelled) that people will recognise.

Chris -   So there you have it. Chocolate is not addictive but it is very good for you as long as you eat the dark stuff: 30g. And combine it with a nice glass of red wine from the South of France (Roger tells me is the best place) and you should get set to live to 200!

How does the hand drier know when to switch on/off?

This type of hand drier contains a proximity sensor. If you look underneath the drier, near where the air outlet is, you can see a small glass section, almost like a mini window pane. If you were able to open this up and look inside, you would see an emitter which usually sends out an infra red pulse. Next to the emitter is an infra red receiver. The infra red source is too weak to bounce back from the floor to the receiver, but when you put your hand in the way the infra red beam bounces off your hands back into the receiver and it turns the hot air on. Others are designed with just a receiver to pick up the infra red radiation emitted by your hands.

Will bacteria become resistant to soap or bleach?

This is because antibiotics work in a very different way to bleach. Bleach will kill pretty much everything, but antibiotics work in a much more subtle way. Most antibiotics work by targeting specific proteins, such as ones in the cell wall, or those that help make other proteins. A population of bacteria will have variation in genes between one bug and the next and so some will survive. The bacteria that do survive are the ones with variations in their genes which let them cope against the drugs, and so these go on to divide and replace the ones killed by the antibiotics. Bleach is so utterly destructive that virtually nothing survives, so no 'resistance' genes get spread throughout the population.Most of the drugs we have are derived from a natural source, so bacteria have been locked in this 'arms race' for a very long time. Long before we started using these chemicals to fight off infection.

Are any viruses good for us?

This may be true, despite that fact that most people think of a virus as being something that makes them feel awful! A recent piece of research in Nature magazine, by US scientist Skip Virgin, suggests that being infected by one of the family of viruses known as Herpes viruses, such as herpes simplex, which causes cold sores, or Epstein-Barr virus, which causes glandular fever can help to boost the function of the immune system. When they infected mice with the rodent equivalents of either of these infections, the mice developed a much better immune response to other pathogens than mice which had never been infected. To prove this, they exposed the mice to the bacteria which causes plague and also listeria; they found these mice to be 100% protected against these bacteria compared with animals which had never been infected with a Herpes virus, which all died. When they studied these mice, they found signalling molecules called interferon gamma were being produced at a much higher level than normal, and this molecule is known to stimulate the immune system.

They think that, because we've been living with members of the herpes viruses for millions of years, the body has come to rely on infection to provide additional gene functions, or immune stimulating factors, which our body no longer has. This programmes a more powerful the immune system, and we get benefit. It's almost a symbiosis, we give the virus a home and it gives us a better immune system...

Why can the Aurora Borealis be different colours?

The Aurora Borealis occurs because the Earth has a magnetic field. The Sun is pumping out a million mile per hour maelstrom of charged particles and ions called the cosmic wind, and when this hits our magnetic field the two interact. This is because charged particles are deflected by a magnetic field. When this happens energy is released and this stimulates molecules in the atmosphere to get excited. The dominant molecules in our atmosphere are oxygen and nitrogen, and when excited, oxygen emits light which is mainly green. Sometimes, particles in the solar wind have much higher energy than average, and these can make the oxygen more excited, and you get other colours. In terms of why you only see it on certain nights, it depends to some extent on how much solar activity there is, on atmospheric conditions and whether or not it's cloudy! It only appears at the poles because the magnetic field lines are most concentrated near the north and south pole.

38:40 - Best of the BA Fest!

Meera went out to find the best bits of the festival, and wound up in crowd control!

Best of the BA Fest!
with John Drury, University of Sussex, & David Steven, Hypnotic Audio

Meera spoke to John Drury from the University of Sussex about his work on the psychology of crowds...

John -   I gathered data from 145 people, most of whom were actually caught up in the explosions. I was interested in whether their behaviour was selfish and competitive, or cooperative, because there's an assumption that people in emergencies tend to panic. Or if they don't panic, that they simply look after family members.

So first of all, I looked at their behaviour, and I found that selfish and competitive behaviour was extremely rare. On the other hand, mutual help was extremely common. My second question, was who were they with? As you know, most of the people on those trains were commuters. Therefore, most of the people helping each other were with strangers. People were helping strangers. My third question then, was to ask well what was the process behind this helping of strangers? They contrasted this with the experience you have everyday on the underground, where people feel atomised, and so it was this sense of unity, arising from the danger itself that brought people together that explains there helping behaviour.

Meera -   What were your overall conclusions from this?

John -   First of all, that the idea of panic is a myth. Secondly, that the authorities should make provisions for peoples willingness to provide help for each other (the public tend to be excluded from emergencies), and thirdly, the authorities, rather than treating people as irrational and overemotional, should try to give more information to the public on what's happening during emergencies.

Meera - You say communication should be improved. Why should communication be improved and how can it be improved?

John -   Well there is evidence, not just from my study but from other experimental studies, that when you give people more information about the nature of the danger, the location of the danger, people exit more effectively. The assumption is, that the more information you give people, the more they panic. Well that's not actually true. So you should have tannoy systems in place, that tell people the location and the nature of the danger so they can upon it.

Meera - Do you think we should all promote actually having contact with each other in normal circumstances?

John -   There is some lesson about everyday life here as well. Because, when you go on the tube, some of my people did draw a contrast between how people avoid each other's eyes, and I think the authorities and those in control of public spaces can do something about this by addressing people increasingly by their collective identity and in that way they can encourage more helping and cooperative and altruistic behaviour that we get in everyday life at the moment.

 - Music -

Meera -   So that, was music played purely by the use of sensors. I'm here with David who's going to explain to me how that's possible. Hello David!

David - Hello, yes I've been working with some children from the Applefield special needs school in York and the idea of the project is that it gives access to children who wouldn't normally have access to keyboards and so on for the reason of their disabilities and allows them to play music on the computer. The computer is a kind of software sampler and they control the playback with sensors. Now, these are things like infra-red distance sensors, the sort of thing that when you go to the supermarket the doors open automatically-there's something there that detects your movement. So we've got distance sensors, sensors you can tilt and move around and also pressure sensors.

Meera - There was something that resembled a cymbal but the children just kept there hand hovering over it, so that was a distance sensor?

David -   Yes that's a distance sensor. It sends out a little infra-red beam and then movement in front of it bounces back, the sensor detects the distance and then there's software inside the computer that translates that into control information for the sample playback. They can control the part of the sound that's playing back and also the pitch of the sound.

Meera -   Well, that was a very impressive result, so I hope the children are happy?

David -   There are some definite future stars among them I think....

Meera -   I've just arrived on Parliament Street and glaring me in the face is a huge, multi-coloured, multi-pathed inflated dome!So, I've just come inside the dome and it's really quite surreal. There's paths going in a quite a few directions and the colours are just really vibrant. All the paths are lined with bright blue and bright red. I'm here with Paul, who's going to explain how they get this effect. Hi Paul,

Paul -   Hello. You said it's very bright in here, in fact it is relatively dark in here compared to outside. The only light in here is from sunlight that comes through the plastic and is filtered into the structure, and red, green and blue are used specifically because they're the primary colours of light. So they're the colour we pick up in our eyes, all the other colours you see by the way are combinations of the three. While its relatively dark in here, you're more or less getting 100% of the red, blue and green from the sun separated from those three and tour eyes are, at the same time, dilated in order to receive more light because you're aware that it's actually fairly dark in here. So you're eyes are adjusted to get as much light as possible and you're still getting, independently, 100% of the three primaries which is what gives it that effect of being very bright and vibrant.

Meera -  I've just managed to bump into Helen. Hello Helen.

Helen -  Hello.

Meera -  This is all part of the BA Festival of science, did you know that was going on?

Helen -  I actually didn't know, no. I just came into town and found all these interesting things going on in town! And it's good for something to be different as well, because normally its fruit and veg ...flowers...food...and to see something to do with science is really interesting.

Meera - Well that's it from me, I hope you've enjoyed listening to the best of the fest!

44:59 - Cuckoo Confusion?

How does a cuckoo know it's a cuckoo? Most birds learn from their parents, so does foster parenting leave the cuckoo confused?

Cuckoo Confusion?

This question was answered by Dr Karen Spencer, a David Phillips Research Fellow at the University of GlasgowThe case of the cuckoo is a really interesting one, because originally it was thought that they were pretty much impervious to this imprinting mechanism [when a young bird learns to identify it's parent and act in a similar way, it is said to have imprinted on the parent] and they could ignore the visual, acoustic and social interactions with their foster parents, and they just innately knew, genetically knew who they were, and could basically ignore all environmental inputs. Recently it has been suggested that isn't quite the case, there's still a lot of questions to be answered certainly in this, but there is some evidence that in some species adult cuckoos visit the cuckoo chicks just before they're about to leave the nest. Now this means that they can then, obviously, imprint on this adult cuckoo but it still raises the question as to whether they've actually got a genetic filter to stop them imprinting earlier on, on their foster parents. Now if you take a cuckoo chick away from any other cuckoos and put it in a nest, and it grows up and doesn't actually get this later input from a cuckoo, it does seem to be slightly confused about who it is. It doesn't necessarily imprint totally on the foster parent, but it does have a few social problems when you introduce it to another cuckoo.

Do antioxidants survive stomach acid?

Yes they do. If we take, for example, vitamin C, which is actually ascorbic acid, which in theory shouldn't be broken down in an acid environment. We do rely on absorbing vitamins to remain healthy, so the body must have evolved mechanisms to absorb these without them being broken down.

What is fire, and why is it hot?

Fire is the burning products of vapours of a fuel. Whenever something burns it is first made into a vapour and mixed with oxygen, so when you have a fuel, such as paraffin or diesel, it works much better if it goes up a wick. This is because the heat can make vapour come from the wick to mix with oxygen in the air to cause a chemical reaction between the hydrocarbons in the fuel and the oxygen. When this happens, you get complete combustion. The flame that you see if just the hot gasses and soot particles, which are so hot that they glow. Gas burns blue because it burns clean, and so there are very few soot particles in the flame to glow yellow.

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