46 REPORT — 1902. 



of these objects absorb the sun's rays, and as they approach the sun 

 become heated on the side turned towards him until the volatile substances 

 frozen in or upon them are evaporated and diffused in the gaseous state in 

 surrounding space, where they get cooled to the temperature of lique- 

 faction and aggregated in drops about the negative ions. The larger of 

 these drops gravitate towards the sun and form clouds of the coma about 

 the head, while the smaller are driven by the incidence of the sun's light 

 upon them away from the sun and form the tail. The curvature of the 

 tail depends, as Bredichin has shown, on the rate at which the particles 

 are driven, which in turn depends on the size and specific gravity of the 

 particles, and these will vary with the density of the vapour 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 chromo- 

 sphere, but also as to the constitution of the photosphere itself. 



Various Low-Temperature Researches. 



We may now summarise some of the results which have already been 

 attained by low-temperature studies. In the first place, the great majority 

 of chemical interactions are entirely suspended, but an element of such 

 exceptional powers of combination as fluorine is still active at the tempe- 

 rature of liquid air. Whether solid fluorine and liquid hydrogen would 

 interact no one can at present say. Bodies naturally become denser, but 

 «ven 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 closest 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 particles of bodies may be accounted for without assuming 

 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 vibration as a physical cause. Hence, at the 

 zero of absolute temperature, this vibrating energy being in complete 

 abeyance, the phenomena of cohesion should cease to exist, and matter 

 generally be reduced to an incoherent heap of cosmic dust. This second 

 view receives no support from experiment. 



The photographic action of light is diminished at the temperature of 

 liquid air to about 20 per cent, of its ordinary efficiency, and at the still 

 lower temperature of liquid hydrogen only about 10 per cent, of the 

 original sensitivity remains. 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 

 xilliant during the period when the temperature is rising. Even solid 



