The flame and the light it givesPlatinum loop and salts in watch glassesBlack epoxy benchFlame tests. Clean the platinum loop by dipping it in concentrated hydrochloric acid and holding it in the non-luminous flame until it gives no colour. Make a paste of the salt with a few drops of the acid on a watch glass, dip the loop in it and hold it just above the inner blue cone. Each metal colours the flame its own way: lithium crimson red, sodium a persistent golden yellow, potassium a faint lilac, calcium brick red, strontium crimson, barium apple green, copper blue-green with a blue centre. Look again through blue cobalt glass: it absorbs sodium's yellow, so potassium shows crimson; strontium looks purple, barium bluish green, calcium green. The yellow luminous flame of a closed air hole is cooler, sooty and hides the colours. Inside the flame, heat lifts an electron in each metal atom up an energy level, and when it falls back it gives out light of one energy: that light is the colour.