﻿Spectrum of Iodine Vapour. 331 



time for their development. The heating of a gas by absorp- 

 tion of radiant energy must in some way result from a 

 transformation of the energy stored in the molecule during 

 absorption (which we may regard as potential) into kinetic- 

 energy at the moment of collision. That is, the velocity of 

 the rebound is increased by actions or motions going on 

 within the molecule. If we consider that the molecule is 

 capable of storing up energy to a certain point after which 

 further absorption results in a re-emission of radiant energy 

 (fluorescence), and if we further assume that at the moment 

 of collision the stored energy is liberated, or spent in increas- 

 ing the molecular velocity, it is clear, if the time-interval 

 between collisions is sufficiently small, that the internal energy 

 of the molecule cannot be increased up to the point at which 

 fluorescence manifests itself. At atmospheric pressure, how- 

 ever, the average time-interval between two collisions is 

 sufficient for the passage of about 130,000 light-waves. It 

 may appear at first sight improbable that fluorescence would 

 be excited at all if the passage of such an enormous number 

 of waves failed to awaken any response. The capacity of a 

 molecule for storing energy may be, and probably i<. very 

 great ; and it is by no means unlikely that a molecule can 

 absorb the energy of many thousand lieht-waves before being 

 brought to the unstable condition at which a re-emission of 

 radiant energy commences. If molecular impacts are re- 

 sponsible for the suppression of the fluorescence, it would 

 appear probable that a light gas, such as hydrogen, would be 

 less detrimental. Bulbs were accordingly filled with 

 hydrogen, and the fluorescence of the iodine vapour examined 

 as the pressure was gradually reduced. Faint traces of 

 fluorescence appeared at a pressure of 30 cms., much higher 

 than was the case with air : but at lower pressures there did 

 not seem to be very much difference, the intensity of the 

 fluorescence in air and hydrogen at 2 cms. pressure being 

 about the same. It was found, moreover, that if a bulb con- 

 taining just enough air to prevent the fluorescence were 

 warmed, the fluorescence at once appeared. This circum- 

 stance may perhaps be taken as evidence against the theory 

 of molecular collisions, and we may adopt as an alternative 

 what may be termed a solution theory. I have been of the 

 opinion for many years that we may speak with perfect cor- 

 rectness of a solution of a gas in a gas. Experiments made 

 in 189G on the absorption spectrum of iodine and bromine 

 mixed with the vapours of bisulphide of carbon, showed that 

 a portion of the iodine was in a state of solution, giving an 

 absorption spectrum devoid of lines or bands, while a portion 



