Peter Riddell asked:
Humphrey Davy invented the safety lamp in 1815 to prevent naked flame igniting the so-called fire-damp gas in mines. I believe that basically a gauze shield surrounded the flame. However air could enter or there would have been no flame for the lamp. My question therefore is why didn't the fire-damp gas also enter?
Peter's question really ignited the interest of Chris Smith...
Chris - Right. Well, it’s a really good observation because obviously, explosions in mines where there's lots of trapped gases because the coal when it got cooked into coal would’ve also produce gas and that gas is often in pockets down these mines and it can escape, it can build-up, and that’s why miners took canaries down coal mines. If it mixes with oxygen and a naked flame, whoosh! You can have an explosion and this frequently still happens today. And obviously, people didn’t have torches. They would take down a coal mine candles and things. so, what Humphrey Davy did was to say, “Well, how do we make these lamps safe?” in fact, you can do your own experiment to show how a Davy lamp works because if you light a candle and you take a gauze or your kitchen sieve will work actually…
Kat - Using a metal sieve, not a plastic sieve.
Chris - Don’t use a plastic one. That won't work. Well, it will work, but not the way we anticipate.
Kat - Very briefly.
Chris - But if you put that over the flame, what you'll see is that where the metal goes across the flame, the flame stops at the metal. And through the metal will come smoke and carbon particles. But it won't burn on the other side of the metal. The reason for this is that metal is a very good conductor of heat. It robs all of the hot gases which are mixing with oxygen and reacting of the energy which enables the chemical reaction to be sustained, which means that the gas is going through the sieve; add an insufficient temperature on the other side to continue the reaction and burn. And therefore, there is direct air contact with the flame and the fire damp will go into the lamp, but it won't be able to trigger an explosion outside the lamp.
Kat - So, it’s about stopping the fire going out rather than the gas coming in.