Mercury Vapour at tlie 2536 Absorption Line. 439 



It is clear from this table that the refractivity of the 

 vapour, or rather that part of it due to the absorption line in 

 question, does not increase in proportion to the vapour 

 density. It appears therefore very doubtful whether this 

 absorption line is due to the normal mercury molecule. We 

 may call it the dispersing molecule, which may either be an 

 atomic complex, or a molecule in a state of ionization, and it 

 seems probable that these exist in small numbers in compa- 

 rison with the normal molecules. At all events the percentage 

 of molecules in this condition is less as the vapour pressure 

 increases, for, as the table shows, a sevenfold increase in 

 pressure only triples the action on the wave velocity for 

 Line No. 1, and scarcely more than doubles it for Line 

 No. 2. 



I feel a little disappointed with the results as a whole, as I 

 had hoped to secure records over a much wider temperature 

 range, dealing with shifts amounting to many fringe widths, 

 but this appeared to be impossible with the arrangement o£ 

 apparatus adopted. It is a great pity that we do not have 

 some method intermediate in point of sensitiveness between 

 the interference methods and the prism. It is doubtful 

 whether reliable results could be obtained with a prism of 

 mercury vapour, since we should have to use one with a base 

 of 20 cm. to obtain a retardation of half a wave-length across 

 the entire wave-front at a temperature of 58°. I have, how- 

 ever, obtained fairly good photographs of the selective 

 dispersion at the 2536 line by employing a long tube of steel 

 similar to the ones used with sodium, but as we are dealing 

 in this case with non-homogeneous vapour^ the method is 

 wholly unsuitable for quantitative measurements. The 

 results given in this paper, while not as satisfactory as I had 

 hoped for, appear to me to be of some interest in that they 

 have pretty clearly established the fact that not all of the 

 molecules are concerned in the dispersion, and consequently 

 in the absorption. Some time during the coming winter I 

 plan to investigate the absorption of the vapour quantitatively 

 at different pressures and densities. The results obtained in 

 the study of the dispersion would lead us to expect that with 

 a given amount of mercury vapour we should have more 

 powerful absorption as we reduced the pressure, which is 

 precisely the opposite of what is usually observed. The very 

 homogeneous light from the resonance lamp, described in my 

 paper on the selective scattering &c. of resonating gas 

 molecules (Phil. Mag. 1912, vol. xxiii.) will greatly facilitate 

 this studv. 



