SPECTROPHOTOGRAPHY OF CHEMICAL REACTIONS. 55 



that the mobility of the negative ions in the pure gases was much greater 

 than when traces of oxygen were present. In other words, the electrons 

 existed largely in a free condition, and as soon as oxygen was introduced, the 

 electrons and negative ions were supposed to condense on the oxygen mole- 

 cules. Small traces of the less electro-negative gases are found to act in the 

 same way. Similarly, Franck and Wood believe that the more electro-nega- 

 tive elements have a greater power to destroy fluorescence, and this agrees 

 with the relative destructive power as given by the series hydrogen, air, car- 

 bon dioxide, and ether vapor, the latter having the greatest effect. The con- 

 clusion reached is that the gases that interfere with the motions of the free 

 electrons are also the ones that modify the motion of the bound electrons 

 to the greatest extent. In this connection the influence of the presence of 

 helium, argon, nitrogen, oxygen, and chlorine on the fluorescence of mercury 

 and iodine has been studied. 



According to Lorentz's hypothesis that the damping results from col- 

 lisions, the damping factor would be a function of the molecular weight. On 

 the other hand, the destructive effect of helium on the fluorescence of iodine 

 is much less than that of hydrogen, the effects due to hydrogen and argon 

 being about the same, although the molecular weight of the latter is forty 

 times as great as the former. The gas having the greatest effect is chlorine, 

 and next to it ether. It therefore seems that the power of a gas to combine 

 with electrons is a very important factor in determining the role which that 

 gas will play in the suppression of fluorescence. 



The fact that the maximum intensity of fluorescence in a pure gas occurs 

 at different pressures for different gases may also be explained on the above 

 hypothesis. To obtain visible fluorescence there must be a sufficient number 

 of molecules present to give a certain intensity of the emitted light, but this 

 number of molecules must not be so great as to disturb each other. In a 

 strongly electro-negative gas the vibration of electrons in one molecule will 

 be influenced by the presence of other molecules in the neighborhood, so that 

 for a gas like bromine the fluorescence would be expected to take place at much 

 lower pressures than is the case for a much less electro-negative gas like 

 iodine ; while in the case of mercury the pressure would be very much greater. 

 This has been found to be the case. 



In treating the following cases of chemical reactions, the theory of aggre- 

 gates, as fully developed in the general remarks at the end of this monograph, 

 will be assumed. It may be possible to obtain some knowledge of these aggre- 

 gates by other methods. In the study of a, /3, and 7 rays it is found that, in 

 general, the absorption of these radiations depends on the atomic structure 

 of the matter they traverse, rather than on the molecular structure. There 

 may possibly be some exceptions to this rule. For instance, uranium and 

 thorium salts emit a rays, the a rays probably coming from the uranium and 

 thorium atoms themselves, before they break down into the next radio-active 

 product. If one had a very thin layer of a uranium or thorium salt, it would 

 be expected that the range of most of the a particles would be less if the ura- 

 nium or thorium existed as an aggregate or in the simple molecular condi- 

 tion. It might, therefore, be found that the range of the a particles from 

 uranyl nitrate would be related to the wave-length of the uranyl or uranous 



