﻿and Absorption by Resonating Gas Molecules. 703 

 The values found are given in the following table. 



Air pressure 

 in mm. 



Intensity of primary 

 resonance radiation. 



Absorbed energy. 



•01 



300 







•45 



230 



70 



110 



200 



100 



2 20 



170 



130 



6-20 



100 



200 



9 -no 



70 



230 



1 4-20 



50 



250 



1800 



40 



260 



82 00 



12 



288 



If we plot these values, taking intensities as ordinates and 

 air pressures as abscissae, we obtain a curve practically iden- 

 tical with the curve obtained with iodine vapour, which shows 

 that the effect of the air upon the intensity of the emitted 

 radiation is about the same in the two cases. In the third 

 column I have given the amounts of the energy absorbed in 

 each case. These values are merely the differences between 

 the amounts of the emitted energies and the energy emitted 

 when the vapour is in a high vacuum (300), and are calculated 

 on the assumption that the total energy diverted from the 

 primary beam is the same in the two cases, i. e., that the 

 presence of the air does not influence the amount of energy 

 removed from the beam by the resonating gas molecules. 



That this is in reality the case was shown by the following 

 experiment. A double cell, PL X. fig. 4 b, was made by 

 soldering a cross partition along a diameter of a short section 

 of large brass tube, the ends of which were closed with quartz 

 windows. The length of the tube was 17 mm., and the 

 diameter 30 mm., and tw T o small brass tubes permitted 

 either compartment to be exhausted to any desired pressure. 

 In measuring the energy diverted from the primary beam by 

 the vapour, we must be certain that we use light which is in 

 exact synchronism with the resonating molecules. The light 

 must be far more homogeneous than the ray isolated by the 

 quartz spectrograph from the light of the mercury arc. I used 

 therefore what I shall hereafter refer to as the resonance lamp, 

 a small quartz bulb, closed at the bottom with a flat plate of 

 polished fused quartz, which was fused on in the same manner 

 as the end plates of the long tube previously described. 

 This bulb contained a drop of mercury, and was highly ex- 

 hausted and sealed. The light from the quartz spectrograph 



