Vapour in the Visible and Ultra-violet Regions. ae 
density at £12° was six times as great as at 385°, and we should 
consequently look for a fringe-shift at the higher temperature 
six times as great as at the lower. This, however, was by nx 
means the case, the fringe-shift being only about twice as 
great. This is, on the whole, exactly what we should expect 
under the conditions of the experiment, for we must remember 
that our column of sodium vapour terminates in a vacuum, 
if the expression is allowable. If the sodium were heated in 
a tube provided with end plates which were not acted upon 
by the vapour, so that the whole could be brought to a con- 
stant temperature, it is probable that agreement with the 
results obtained by Jewett would be obtained. As it is, we 
have a cold condenser at each end of the column, and only 
obtain vapour of any considerable density through the 
circumstance that the rate of diffusion is comparatively 
slow. 
I hope in the near future to determine the actual density 
under the conditions of the experiment. 
The density at a given temperature was found to depend, 
to a large extent, upon the previous treatment of the tube. 
lf the tube was heated for the first time to a temperature of 
say 350 degrees, a shift of say 10 helium fringes would be 
observed. If the tube was then raised to a red heat and 
allowed to cool, on reheating it to the same temperature (350°) 
a shift of perhaps 15 fringes would be found. It appeared 
as if the density ata given temperature was much greater if a 
sodium ‘“ dew” had been formed over the entire inner surface 
of the tube, by the condensation of the dense vapour given 
off at the high temperature; in other words, the density at a 
given temperature was proportional to the surface of molten 
sodium which was exposed. | 
The determinations were made in glass tubes for the reason 
that in steel tubes the transition from the heated to the cool 
portion of the tube was not sufficiently sudden to ensure 
uniform density in the cylinder of vapour. The Jena-glass 
tubes could, however, be raised to a full red heat by means 
of the coil of iron wire without showing any trace of sodium 
vapour in the portions protruding from the helix. 
In order to obtain consistent results, it was found necessary 
to heat the tube very gradually in order to allow the thermo- 
element time to take the temperature of the vapour. One 
observer watched the scale of the galvanometer, the other 
counted the fringes as the tube was heated or cooled. The 
helium fringes disappeared after a shift of something over a 
hundred fringe-widths had occurred, but by dropping a plate 
of plane parallel glass into the air-path of the interferometer 
