HISTORY OF COLD AND THE ABSOLUTE ZERO. 237 



matter. Lord Kc^lviu's view is that the forces that hold together the 

 particles of bodies iiiay be accounted for without assuniino- any other 

 agency than gravitation or any other law than the Newtonian. An 

 opposite view is that the phenomena of the aggregation of molecules 

 depend upon the molecular vil)ration as a plwsical cause. Hence, at 

 the zero of al)solute temperature, this vibrating energy being in com- 

 plete a))eyanc(\ the phenpmena of cohesion should cease to exist, and 

 matter generally ])e reduced to an incoherent heap of cosmic dust. 

 This second view receives no support from experiment. 



The photogra|)hic action of light is diminished at tlu^ temperature 

 of liquid air to about i^o per cent of its ordinary etliciency, and at the 

 still lower temperature ot" licjuid hydrogen only about 10 per cent of 

 the original sensitivity remains. At the temperature of li((uid air or 

 liijuid hydrogen a large range of organic bodies and many inorganic 

 ones accpiire under exposure to violet light the property of phosphor- 

 escence. Such l)odies glow faintly so long as they are kept cold, but 

 become exceedingly l)rilliant during the period when the temperature 

 is rising. Even solid air is a phosphoresc-ent body. All the alkaline 

 earth sulphides which phosphoresce brilliantly at th(^ ordinary temper- 

 ature los<' this property when cooled, to be revived on h(niting; but 

 such bodies in the first instance may be stimulated through the absorp- 

 tion of light at the lowest temperatures. Radio-acti\'e l)odies, on the 

 other hiuid, like radium, which are naturally self-luminous, maintain 

 this luminosity unimpaired at the very lowest temperatures, and are 

 still capable of inducing phosphorescence 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 molecules. This phenomenon 

 is very pronounced Avith 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 Ixxlies at low 

 tempin-atures. The subjects that haxe been under investigation may 

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

 the Magnetic Properties of Iron and Steel; Dielectric Constants; the 

 Magnetic and Electric Constants of Licpiid Oxygen; Magnetic Sus- 

 ceptil)ility. 



The investigations have shown that electric conductivity in pure 

 metals varies almost inversely as the absolute temperatun^ down to 

 minus 200"'^, but that this law is greatly ali'ected b}-^ the pi-esence of the 

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

 proof that electric resistance in pure metals is (dosely dependent upon 

 the molecular or atomic motion which gives rise to temperatur(% and 

 that the process by which the energy constituting what is calked an 

 electric current is dissipated essentially depends upon non homogeneity 

 of structure and 'Ui)on the absolute temperature of the material. ' It 

 might be inferred that at the zero of absolute temperature resistance 



