(528 



SCIENCE. 



[N. S. Vol. XVI. No. 407. 



curvature of the tail depends, as Brediehin 

 has shown, on the rate at which the par- 

 ticles are driven, which in turn depends 

 on the size and specific gravity of the par- 

 ticles, and these will vary with the density 

 of the vapor from which they are formed 

 and the frequency of the negative ions 

 which collect them. In any case Arrhenius' 

 theory is a most suggestive one, not only 

 with reference to auroras and comets, and 

 the solar corona and chromosphere, but also 

 as to the constitution of the photosphere 

 itself. 



ViVEIOUS LOW-TEMPEEATURE RESEAECHES. 



We may now summarize some of the re- 

 sults which have already been attained by 

 low-temperature studies. In the first 

 place, the great majority of chemical inter- 

 actions are entirely suspended, but an ele- 

 ment of such exceptional powers of com- 

 bination as flourine is still active at the 

 temperature of liquid air. "Whether solid 

 fluorine and liquid hydrogen would inter- 

 act no one can at present say. Bodies nat- 

 urally become denser, but even a highly 

 expansive substance like ice does not appear 

 to reach the density of water at the lowest 

 temperature. This is confirmatory of the 

 view that the particles of matter under 

 such conditions are not packed in the clos- 

 est possible way. The force of cohesion is 

 greatly increased at low temperatures, as 

 is shown by the additional stress required 

 to rupture metallic wires. This fact is of 

 interest in connection with two conflicting 

 theories of matter. Lord Kelvin's view is 

 that the forces that hold together the par- 

 ticles of bodies may be accounted for with- 

 out assuming any other agency than gravi- 

 tation or any other law than the Newton- 

 ian. An opposite view is that the phenom- 

 ena of the aggregation of molecules de- 

 pend upon the molecular vibration as a 

 physical cause. Hence, at the zero of abso- 

 lute temperature, this vibrating energy 



being in complete abeyance, the phenomena 

 of cohesion should cease to exist, and mat- 

 ter generally be reduced to an incoherent 

 heap of cosmic dust. This second view 

 receives no support from experiment. 



The photographic action of light is di- 

 minished at the temperature of liquid air 

 to about twenty per cent, of its ordinary 

 efficiency, and at the still lower tempera- 

 ture of liquid hydrogen only about ten 

 per cent, of the original sensitivity re- 

 mains. At the temperature of liquid air 

 or liquid hydrogen a large range of organic 

 bodies and many inorganic ones acquire 

 under exposure to violet light the property 

 of phosphorescence. Such bodies glow 

 faintly so long as they are kept cold, but 

 become exceedingly brilliant during the 

 period when the temperature is rising. 

 Even solid 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 stimu- 

 lated through the absorption of light at 

 the lowest temperatures. Radio-active 

 bodies, on the other hand, like radium, 

 which are naturally self-luminous, main- 

 tain this luminosity unimpaired at the very 

 lowest temperatures, and are still capable 

 of inducing phosphorescence in bodies like 

 the platino-cyanides. Some crystals be- 

 come for a time self-luminous when cooled 

 in liquid air or hydrogen, owing to the in- 

 duced electric stimulation causing dis- 

 charges between the crystal molecules. 

 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 temperatures. The subjects 

 that have been under investigation may be 



