﻿Ultra-violet Light on Chlorine. 750 



Long ago Lenard showed that the action of ultra-violet 

 light on gases produced three kinds of effects * : — 



1. The gas may conduct electricity owing to the pro- 



duction of positive and negative ions. 



2. Condensation nuclei may be formed, devoid of electric 



charge f . 



3. Chemical molecular change may take place (30 2 to 



20,). 



All these effects are due to the action of the periodic 

 electric forces on the molecules of the gas. 



Nuclei. — The condensation nuclei persist longer than the 

 ions, from which they can be separated by means of a 

 powerful electric field ; they are unaffected by cooling to 

 — 70° C, but destroyed at 180° C; their diameter appears to 

 be some 25 times that of the molecule of the gas, but varies 

 with the duration of illumination, increasing with the time ; 

 their formation was at first thought to be analogous to the 

 production of ozone, but is more probably due to secondary 

 chemical reactions (e. g., NH 3 oxidized to NH 4 N0 2 , &c). 

 When the gases, not specially purified by cooling, are 

 exposed to short wave-lengths (below ISO fifi), there is no 

 diminution in the number of condensation nuclei (as judged 

 by means of a steam jet) whether the ions are removed by a 

 powerful electric field or not. Hence the nuclei are more 

 effective than ions in bringing about the condensation of 

 supersaturated steam. 



When, however, the gases are highly purified, most 

 effectively by freezing out impurities by means of liquid air, 

 no nuclei are formed, proving that they are due to minute 

 traces of vapours present in the air ; the effect of the merest 

 traces of water vapour, ammonia, or vapour from rubber 

 tubing is most striking. 



Ions. — The production of ions of molecular dimensions is 

 explained by supposing the loss of a negative electron by one 

 molecule and the gain of it by another. Larger ions could 

 arise by agglomeration, but this process does not go far, or 

 by the attachment of a small ion to a larger particle (nucleus) 

 suspended in the gas. 



With gases not highly purified, the longer and more 

 powerful the illumination the larger the ions, but with gases 

 purified by cooling to — 78° C. only ions of molecular size 

 are formed ; hence, it appears that large ions are due to the 

 acquisition of an electron by a nucleus. 



* Lenard & Wolf, Ann. d. Vhys. u. Chem. vol. iii.F (1889) ; Ann. d. 

 Phys. vols. i. & iii. (1000). 



t See also C. T. R. Wilson, Phil. Trans, vol. cxcii. (1899). 



