SUMMARY AND GENERAL DISCUSSION OF RESULTS. 99 



bands become more diffuse, the edges becoming hazier. When there is a 

 mixture of salts in the same solvent, the bands may become much weaker 

 with rise in temperature. This is the case in a mixture of neodymium and 

 calcium chlorides in water. In a similar manner the absorption of a salt in 

 two solvents probably decreases in intensity with rise in temperature. An 

 example of this kind is that of urarous bromide in 40 per cent water and 60 

 per cent alcohol. At ordinary temperatures the bands are of about equal 

 intensity. At 80 the water-bands have practically disappeared, without the 

 alcohol bands having widened to any great extent. 



(3) In general the center of intensity of single bands changes but little. 

 Whenever there is any change of wave-length, the shift is invariably towards 

 the red.. It seems that this shift is greater the wider the band, so that it is 

 difficult to say in most cases whether the shift is real or only apparent. In 

 the case of solutions of pure neodymium and erbium salts the shift is, in general, 

 too small to be observed. 



(4) A study of gaseous aggregates such as N20 4 + 2N0 2 indicates that 

 raising the temperature or lowering the pressure increases the relative number 

 of the simpler molecules. Many vapors like those of the fatty acids show 

 molecular clustering at low temperatures. In a similar maimer it would be 

 expected that aggregates would gradually break down at the higher tempera- 

 tures. In considering specific examples, it would be expected that acid uranyl 

 sulphate aggregates in breaking down would result in a shift of the uranyl 

 bands, the shift being towards the violet. On the other hand, if the nitric 

 acid uranyl nitrate aggregates are broken down, it would be expected that 

 the uranyl bands would be shifted towards the red. According to this view 

 the shift of the uranyl bands of uranyl nitrate in nitric acid and of uranyl 

 sulphate in sulphuric acid, with rise in temperature, should be quite different 

 if the only effect of rise in temperature is a breaking up of the aggregates. 



In advancing an hypothesis of this kind it is assumed that it is only the 

 molecules in an aggregate that are effective in changing the frequency of 

 vibration of the absorbing systems of the light centers. This means that the 

 kinetic energy of the aggregate corresponds to that of a molecule at the same 

 temperature in the solution, the individual molecules in an aggregate all 

 moving together. Whether there is a constant interchange of these molecules 

 and the molecules of the solution, spectroscopic evidence does not as yet show. 

 Neither can it be said with certainty that molecules outside the aggregates 

 do not affect the frequencies of vibration of the absorbing system within the 

 aggregate. Assuming that this is not the case, then it follows of necessity 

 that with rise in temperature the acid aggregates are not broken up, because 

 the uranyl bands of acid solutions are then shifted to the red. This shift 

 seems to be about as great for uranyl sulphate in sulphuric acid as it is for 

 uranyl nitrate in nitric acid. In a similar manner, acid solutions of neodym- 

 ium salts do not have their absorption spectra changed so as to resemble more 

 closely that of the neutral salt, with rise in temperature, as one would expect 

 if the acid neodymium aggregates were broken down. 



(5) When foreign salts like calcium chloride are added to solutions of 

 neodymium chloride, it is probable that aggregates containing the two salts 

 are formed. In the case of aqueous solutions of these salts it is found that 



