SFECTROSCOPIC EVIDENCE FOR THE HYDRATE THEORY. 235 



place. Iii fact, when we say that pure water is transparent to light of one 

 given period (for example, the D 3 line of helium), we do not mean that the 

 absorption is absolutely zero, with mathematical exactness, but we do mean 

 that it is perhaps indefinitely less than we can detect experimentally. Doubt- 

 less, if a column of absolutely pure water of sufficient length could be ob- 

 tained, it would be found that the region of absorption of the Schumann 

 waves would widen out simultaneously with the bands in the infra-red, until 

 the entire visible spectrum would be dimmed by general absorption. The 

 next question is how should we expect the hydrates to affect the width 

 of a given absorption band, or a limit of a region of absorption. The maxi- 

 mum of absorption of a band would correspond to that vibration of the 

 resonator that was the least damped, and which, therefore, had the greatest 

 amplitude; whereas the penumbra at the boundary of the band would corre- 

 spond to those vibrations of the resonator which were damped to a great 

 degree and which, therefore, had very small amplitude. It has been tacitly 

 assumed that the incident radiation was continuous, and practically com- 

 prised all of the periods of the resonating system. If, now, the hydrate 

 associated with a given resonator increased in mass by the addition of water, 

 it would, of course, damp the vibrations more and more, and thus, by 

 decreasing the amplitudes of the penumbral vibrations, cause the band to 

 become narrower and narrower. That absorption still existed in the region 

 from which it had apparently disappeared, could be shown at once by simply 

 increasing the length of absorbing column until the penumbra had widened 

 out to its original value, everything else being kept constant. Conversely, 

 when the extent of hydration decreases, the regions of absorption would 

 widen out and extend over a greater range of periods and wave-lengths. 

 Assuming that the argument advanced above is valid, then it does not seem 

 possible to escape the conclusion that hydrates exist, since the theory accounts 

 so perfectly for the facts. We shall deduce the evidence for this agreement 

 in some detail from the spectrograms and other sources of information. 



Plate 2 shows that the region of absorption in the ultra-violet widens out 

 as the concentration of the cobalt chloride increases. The same fact is shown 

 by the absorption band in the green by plates 2 and 3 (a). As the concen- 

 tration increases, the relative amount of water at the disposal of one cobalt 

 system decreases, and hence, in accordance with the preceding theory, the 

 band must become wider. 



The same phenomenon in the ultra-violet is shown by plate 5 for the solu- 

 tions of copper chloride in water. Plate 3 (6) also brings out the facts for 

 the band in the red. 



It is obvious that in interpreting the spectrograms, the difference between 

 the observed widening of the absorption bands, and the widening that 

 would have theoretically been produced if a change in concentration had 



