of Chemical Compounds. 139 



and that the pale yellow colour usually ascribed to them is 

 due to the presence of a trace of free bromine or other impurity. 



Of the remaining ten exceptions, six (viz. Au 2 0, WC1 4 , 

 WI 2 , UC1 5 , UC1 4 , and A11I3) are green instead of being brown 

 or black, and even these will be in accordance with the rule 

 if we accept the modified theoretical explanation given under 

 (b), {vide infra). There remain, therefore, only four excep- 

 tions (viz. r)iCl 3 , V 2 5 , Cr0 3 , and CdO) which do not admit 

 of any satisfactory explanation ; and even of these CdO is not 

 altogether decisive either one way or another, for though it is 

 brown when in the amorphous state whereas HgO is orange- 

 yellow, yet CdO and HgO are both red when crystalline. 



Theoretical Explanation. — For the following theoretical 

 explanation of the phenomena mentioned above, I am chiefly 

 indebted to my friend and colleague Mr. J. W. Capstick, B.Sc. 



1. Influence of Atomic Weight. — (a) In a solid the molecules 

 vibrate as wholes about certain mean positions. Other thino-s 

 being the same (e.g. temperature, &c.) the period of vibration 

 will be less as the mass of the molecule is less. If the time of 

 vibration of the molecule is so small as to coincide with some 

 vibration outside the violet end of the spectrum, no visible 

 vibration will be absorbed and the colour of the substance 

 will be white. This state of things must hold until (from 

 chemical substitutions &c.) the mass of the molecule becomes 

 so great as to bring the period of vibration up to the violet 

 end, when the violet rays will begin to be absorbed and the 

 salt will appear of the complementary colour of violet, i. e. 

 greenish yellow. If the mass of the molecule be made still 

 greater its period of vibration further increases, and the blue 

 rays begin to be absorbed, and the light left unabsorbed com- 

 bines to give yellow. Next the green rays are absorbed, and 

 the result is orange. Then the yellow is cut out in addition, 

 leaving only red ; and, finally, the red also is cut out, and the 

 substance becomes black. 



(b) From the colours observed it would appear to be an 

 almost general rule that when the period of vibration becomes 

 great enough to cut out a given colour, it also cuts out all 

 those with smaller wave-length, e.g. when yellow is cut out 

 the blue and green are also cut out, otherwise the colours 

 would go from orange to reddish-purple, and then, when the 

 red is cut out (if the blue, green, and yellow were not also cut 

 out), the colour would be greenish instead of the usual black. 

 The green colour of Au 2 0, WC1 4 , WI 2 , UC1 5 , UC1 4 , and Aul 3 

 may possibly be accounted for in this way. 



2. Influence of Temperature. — In the case of a solid the 

 molecules vibrate about certain mean positions, and a rise of 



