When the vapour of an alkali salt is present in a Bunsen flame the vapour becomes luminous and also increases the electrical conductivity of the flame. The fraction of the salt molecules which are luminous is probably very small, and unless the concentration of the salt vapour is very small the fraction of the salt molecules which are charged electrically, or are ionised, is also very small. Any particular salt molecule enters the flame at the bottom and moves up the flame with the velocity of the flame gases which is of the order of 200 cm. per second. The molecule is therefore in the flame for only a small fraction of a second, but this time interval is probably long enough for a state of equilibrium to be established between the several states in which the molecules can exist. For example, in the case of sodium chloride there is probably an equilibrium between NaCl, NaOH, Na 2 O, Na, H 2 O, HCL, and possibly other bodies such as CO, CO 2 , and Na 2 CO 3 .