Clean 25 cm of magnesium ribbon and cut into 1 cm pieces Weigh a crucible plus lid put the pieces of magnesium ribbon in the crucible and weigh again Put the crucible on a pipe clay triangle supported on a tripod Heat very gently then strongly Hold the crucible lid in a pair of tongs close to the crucible The magnesium ribbon darkens just before it begins to melt At the first sign of burning place the lid on the crucible and remove the Bunsen burner Every 4 seconds raise the lid to allow more air to enter but do not allow any white magnesium oxide smoke to escape When the magnesium stops burning on raising the lid remove the lid Heat the crucible again strongly but still hold the crucible lid in a pair of tongs close to the crucible in case the magnesium starts to burn again Leave the crucible to cool When cool weigh the crucible plus lid plus contents Calculate the increase in mass of the magnesium