Carbide within the caving or potholing community refers to Calcium Carbide which when mixed with water generates acetylene gas which is burnt to provide light.
CaC2 + 2 H2O ---> C2H2 + Ca(OH)2
Calcium carbide is purchased as fist sized lumps of a grey rock like material which is then crushed into large pebble sized lumps to provide more reactive surface area. The carbide is then placed in a dual chambered mechanism in which one of the chambers holds a quantity of water. A dripping mechanism allows the water to flow into the chamber and mix with the carbide solid. The acetylene gas which is created flows out of the carbide chamber to a nozzle where it is burnt.
Because the burning reaction requires 1.5 the amount of oxygen than the amount of acetylene produced...
2 C2 H2 + 3O2 ---> 4 CO2 + 2 H2O
a lot of free carbon hangs around for a bit being hot and giving off a nice white light until it forms the CO2 or just free C if not enough O2 is being provided at the burning point.
Carbide lamps have now started to fall out of favor with many caving communities due to the amount of smoke damage that stray flames can cause and also the mess that can be caused by dumping or spilling the Calcium Hydroxide waste. In many cave systems they are banned. Carbide lamps have been replaced by better batteries, halogen bulbs and LED systems which can last for the extended periods required in caving.
Carbide bombs made with PET bottles can be wicked fun but be careful as you will fuck yourself and your hearing up very rapidly by standing too close. The reaction does not reach an equilibrium and gas will continue to be produced until either the bottle bursts (small bang), spontaneous polymerisation followed by combustion occurs (BIG bang) or it runs out of water. The last case is bad as you now have to deal with a pressurized container of flammable gas.
Compressed Acetylene for welding use is packaged in cylinders meeting DOT specification 8 or 8AL. The cylinder contains a porous filler saturated with acetone. The acetylene stored in the cylinder is dissolved in the acetone to prevent the acetylene polymerising.
Needless to say making Carbide bombs with glass containers is way stupid and will definitely end up in tears and somebody will lose an eye (or worse). Strictly for those who want end up in hospital or jail.
When carrying spare Carbide underground make sure the container doesn't seal perfectly or the one of the above results will ensue
Some corrections provided by jasstrong
See also http://www.hoopersupply.com/msds/acetylene.htm