﻿414 Sir W. Ramsay and Dr. Spencer on Chemical and 



cells were gold, copper, and cadmium ; in every case a change 

 of potential was observed when one of the electrodes was 

 exposed to the light. The changes were small, but quite 

 distinct, and of the order of magnitude of ten millivolts, 

 although they were somewhat variable. In these experiments 

 sunlight was found to be quite as active as the mercury lamp. 

 Additional confirmation was afforded by the fact that if a 

 cell with one of its electrodes uncovered was left exposed to 

 light for some days, a deposit appeared and accumulated upon 

 the uncovered electrode, whilst no change occurred on the 

 other. In the case of copper there was a green deposit, with 

 gold the deposit was orange. The change on the exposed 

 metal can be explained on the disintegration hypothesis : the 

 light falling on to the electrode causes it to lose electrons ; 

 i. e. ions of the metal are formed which combine, as rapidly 

 as they are formed, with the hydroxyl ions of the water 

 forming hydroxides. All chemical actions influenced by 

 light come under this category, for the light will obviously 

 increase ionization, and this fact is well known to be a con- 

 trolling factor in the velocity of reactions. 



6. Reference has already been made to the tiring of metals 

 when exposed to ultraviolet light. It was found that when 

 a plate was measured several times, without re-cleaning, that 

 each successive measurement was slower than the preceding- 

 one. In some cases, more especially those with a rapid rate 

 of discharge, the most rapid value could be regained by re- 

 polishing the plate ; whilst in others, <?. g. palladium and 

 platinum, the quickest value could not be regained until after 

 several days had elapsed. To investigate this point a cleaned 

 plate was clamped on the electroscope, and its rate of dis- 

 charge was measured immediately, and at intervals over a 

 period of twenty-four hours. Such series of measurements 

 were made for plates of magnesium, zinc, tin, and aluminium; 

 the rates of discharge were then plotted against the times of 

 exposure. The curves obtained showed a number of breaks 

 which was different for each metal, but with the exception of 

 aluminium, the numbers corresponded with the number of 

 valencies of the metals. When an element combines with 

 another, it expels a number of electrons representing its 

 valency ; these electrons are more lightly held in the atom 

 than the other electrons which constitute its mass. This 

 would mean that the lightly bound electrons are expelled by 

 the ultraviolet light in definite stages, represented by the 

 breaks in the curves. Aluminium shows at least five or six 

 breaks which may be explained by a still further splitting off 

 of electrons after those, corresponding to the valencies, have 

 been expelled. 



When a metal, e.g. Mg, has been subjected to one series of 



