at the Royal Institution, 1 900-1 907. 388 



by liquid air* have furnished results of extraordinary interest and im- 

 portance. Professors Dewar and Fleming were led by their observa- 

 tions to the conclusion that, in all probability, at temperatures not 

 far from the absolute zero, in the case of substances generally (other 

 than metallic conductors), the value of the dielectric constant would 

 be only two to three times that of a vacuum. Taking into account 

 the very considerable difference in the results obtained on using 

 specially purified distilled water instead of ordinary distilled water,t 

 however, it is clear that even a minute proportion of impurity may 

 affect the results ; moreover the effect of reducing the temperature to 

 the degree possible with liquefied hydrogen has to be ascertained. If 

 induction through dielectrics such as water be at least mainly due to 

 the propagation of an " electrolytic impulse," at temperatures at which 

 all " fluidity " is destroyed and the molecules are firmly locked in 

 their positions, no such action should be possible : the specific inductive 

 capacity should fall at least to that of a vacuum. 



It was clear to Faraday that a vacuum might have inductive capa- 

 city — that a charge might be communicated across a space devoid of 

 conducting, contiguous particles; but he awaited proof before deciding 

 and we cannot do otherwise than follow his example ; probably we 

 shall do well to suspend our judgment also in the case of dielectrics. 



It is matter for congratulation that the laboratory in which such 

 transcendent problems were first stated should be that in which the 

 closest approach is being made to means of solving them. 



Chemical Interactions under Reduced Pressure. — Sir James Dewar's 

 observations on the interaction of sulphur and mercury and those on 

 phosphorescence also deserve special notice from the point of view 

 now under consideration. 



If sulphur be placed at the one end and mercury at the other of 

 a ri -shaped tube (Fig. 9) and the tube be exhausted and sealed up, 

 keeping the two ends in liquid air during the time required to reach 

 a high vacuum either by means of the pump or by cooled charcoal, 

 after several hours at the ordinary temperature the surface of the 

 mercury appears tarnished, owing to the formation of a film of the 

 sulphide. As the vapour pressure of mercury exceeds that of sul- 

 phur considerably, it was to be expected that the formation of 

 sulphide would have taken place on the sulphur side ; and, in fact, if 

 the ri tube be constricted at the bend (Fig. 10), sulphide is deposited 

 at the bend. The molecule of sulphur is known to be complex even 

 at temperatures considerably above its boiling-point but it is entirely 

 resolved into simple diatoniic molecules (S2) at high temperatures. 

 Sir James Dewar's experiments would indicate that such simple 



* Roy. Boc. Proc, vols. Ixi. aud Ixii. 

 t Ibid., 1897, Ixi. 319. 



