ADDRESS. 47 



air is a phosphorescent body. All the alkaline earth sulphides which 

 phosphoresce brilliantly at the ordinary temperature lose this property 

 when cooled, to be revived on heating ; but such bodies in the first 

 instance may be stimulated through the absorption of light at the lowest 

 temperatures. Radio-active bodies, on the other hand, like radium, which 

 are naturally self-luminous, maintain this luminosity unimpaired at the 

 very lowest temperatures, and are still capable of inducing phospho- 

 rescence in bodies like the platino-cyanides. Some crystals become for a 

 time self-luminous when cooled in liquid air or hydrogen, owing to the 

 induced electric stimulation causing discharges between the crystal mole- 

 cules. This phenomenon is very pronounced with nitrate of uranium and 

 some platino-cyanides. 



In conjunction with Professor Fleming a long series of experiments 

 was made on the electric and magnetic properties of bodies at low tempera- 

 tures. The subjects that have been under investigation may be classi- 

 fied as follows : The Thermo -Electric Powers of Pure Metals ; The 

 Magnetic Properties of Iron and Steel ; Dielectric Constants ; The 

 Magnetic and Electric Constants of Liquid "Oxygen ; Magnetic Sus- 

 ceptibility. 



The investigations have shown that electric conductivity in pure 

 metals varies almost inversely as the absolute temperature down to minus 

 200 degrees, but that this law is greatly affected by the presence of the 

 most minute amount of impurity. Hence the results amount to a proof 

 that electric resistance in pure metals is closely dependent upon the mole- 

 cular or atomic motion which gives rise to temperature, and that the 

 process by which the energy constituting what is called an electric current 

 is dissipated essentially depends upon non-homogeneity of structure and 

 upon the absolute temperature of the material. It might be inferred that 

 at the zero of absolute temperature resistance would vanish altogether, 

 and all pure metals become perfect conductors of electricity. This con- 

 clusion, however, has been rendered very doubtful by subsequent obser- 

 vations made at still lower temperatures, which appear to point to an 

 tiltimate finite resistance. Thus the temperature at which copper was 

 assumed to have no resistance was minus 223 degrees, but that metal has 

 been cooled to minus 253 degrees without getting i-id of all resistance. 

 The reduction in resistance of some of the metals at the boiling-point of 

 bydrogen is very remarkable. Thus copper has only 1 per cent., gold and 

 platinum 3 per cent., and silver 4 per cent, of the resistance they possessed 

 at zero C, but iron still retains 12 per cent, of its initial resistance. 

 In the case of alloys and impure metals, cold brings about a much smaller 

 decrease in resistivity, and in the case of carbon and insulators like gutta- 

 percha, glass, ebonite, ikc, their resistivity steadily increases. The enor- 

 mous increase in resistance of bismuth when transvej-selv magnetised and 

 cooled was also discovered in the course of these experiments. The study 

 of dielectric constants at low temperatures has resulted in the discovery 

 of some interesting facts. A fundamental deduction from Maxwell's 



