The gentle flicker of a candle's light - a symbol of hope and love.
A candle lit dinner is surely the epitome of romance. But a candle also reminds us of simpler time when a fire was our only source of warmth and a candle our source of light.
It wasn't that long ago that the convenience of modern electric lighting didn't exist and candles filled homes. Candle lit Christmas trees were found throughout Victorian England.
As inventions go, candles are incredibly simple.
They consist of little more than wax or a supply of oil and a wick. Indeed, they are so simple that they have been around for tens of thousands of years. The origin of the candle is lost in antiquity.
But underlying this simplicity is some fascinating science.
For example, why doesn't the wick burn up? If you light a piece of string or rope, it will obligingly smoulder into ash. Indeed, string or rope doesn't really burn very well at all. No flames - just a smouldering orange glow.
But despite being made of the same materials, a candle's wick doesn't smoulder or burn up quickly. Indeed, a wick seems to be the source of a bright glow flame.
The answer lies in what the wick is actually doing. The wick doesn't burn because the flame rarely touches it. The flame is actually separated from the wick.
A neat way to demonstrate this is with some metal gauze such as found in a screen door. If you insert the wick through the gauze, light it and let it warm up then you can actually lift the flame off of the candle a short distance and it will continue to burn on appear to be situated on the gauze.
The process behind this is straightforward. The flame melts the wax. The liquid wax is drawn up the wick, where it is further heated until it becomes a gas. The gas is released and it is this form of the hydrocarbon that actually burns.
And because a flame also requires oxygen, the combustion takes place over a larger volume than the immediate area around the wick - and well away from it.
This is why if you look at the heart of a candle's flame, it's not the same colour as the rest of the flame. It appears orange or brown when the rest of the flame is yellow. And it is surrounded by a nimbus of blue.
There is very little combustion occurring in the centre of a candle. There isn't much heat either at the centre of the flame - only gaseous hydrocarbons.
This is why you can use wetted fingers to snuff a candle. The wick is relatively cool. Not cold as it is still hot enough to melt wax but certainly not as hot as other parts of the flame.
The heat of a candle is a result of the combustion reaction of the hydrocarbons in the wax which, ironically, produces carbon dioxide and water. These two combustion products are commonly used in fire extinguishers - to put fires out. They are useful at putting out fires because they are spent fuel - fuel that has been burnt and can't burn any more.
The purity of the wax determines how quickly it will melt and at what temperature. Most candles are made with relatively impure wax which combines a number of different hydrocarbons and other compounds.
It is this impurity or variety of compounds that makes the candle drip as the wax melts too easy - faster than it can be burnt. Some of the compounds are more volatile and become gaseous at a lower temperature than others. It is the high temperature compounds that drip.
A higher purity wax - a wax with a narrow range of carbon compounds - will produce a "drip free" candle, which can be important for a candle light dinner.
The yellow colour of the flame results from the thermal excitation of carbon molecules. In burning up, heat is created as chemical bonds are broken and made. This heat causes some of the electrons in the resulting molecular bonds to shift to a higher energy level - to an "excited" state. In dropping back down to a lower energy level or "ground state", as the atoms move away from the heat and cool off, they release photons of light.
Other colours in the flame are from other compounds. For example, the blue colour originates with a gaseous dicarbon molecule which undergoes the same sort of excitation process. In addition, impurities in the wax - both accidental and deliberate - can produce other effects.
Of course, some people might think that knowing this about candles might just ruin the illusion or magic of candle light but it does provide conversation during those romantic dinners!
