IONIZATION AND CHEMICAL ACTION 303 Connected with the chemical action and is probably directly caused by it. However, the slow oxidation of phosphorus is exceptional when compared with most chemical reactions at low temperatures, inasmuch as It Is accompanied by the emis- sion of light. It seems probable that both the lonization and the emission of light are direct and simultaneous consequences of the chemical reaction, but the possibility that the lonization is an Indirect photoelectric effect due to the action of the light emitted does not seem to be altogether excluded. Another case of ionization, apparently caused by chemical action, In which phosphorus takes part has been observed by the writer.1 At about 600° C. platinum reacts energetically with phosphorus vapour. During the occurrence of the re- action the platinum emits positive but not negative ions. After platinum has been left cold in contact with phosphorus vapour, a vigorous emission of positive ions takes place when the metal Is heated subsequently. This decays at constant temperature like the positive emission from new wires. Over- heating the wire was found to reduce the emission at the pre- vious temperature temporarily. There was some recovery at constant temperature from the reduction due to overheating which was subsequently followed by the general decay at constant temperature already referred to. These phenomena suggest that the emission involves two distinct processes whose rates are altered to different extents when the tempera- ture is changed. Increasing the temperature appears to re- duce the quantity of the substance which immediately gives rise to the emission of the ions without destroying the parent substance to an equal extent. Similar changes due to sudden disturbances of the temperature have been found to character- ize the emission of ions from heated salts (see p. 266). In some cases, though not invariably, the effect shown by salts Is in the opposite sense to that just referred to. Thus when sodium phosphate or sodium sulphate was overheated, the emission at the original lower temperature was found tem- porarily to be Increased. i 0. W. Richardson, « Phil. Mag.," (6), Vol. IX, p. 407 (*9°5).