MOTIONS OF ELECTRONS 187 



which are too slow to give red light "infra-red" and 

 those which are too fast to give the violet light which 

 lies at the other end of spectrum " ultra-violet ' 

 The radiations of the ultra-violet range have only 

 begun to be investigated, but we shall find several 

 known facts of considerable interest. 



For example, the ultra-violet radiations reaching us 

 from the sun are the cause of those chemical transfor- 

 mations whereby the leaves of plants exposed to sun- 

 light turn green, while the growth beneath the ground 

 remains white. Ultra-violet radiations if not suf- 

 ficiently reduced in intensity by the air through which 

 they travel may also produce severe burns, particu- 

 larly in the inner eye of a human being, as has been 

 recognized by workers with electric arcs or oxyacety- 

 lene welding outfits. For this reason, where street 

 railway rails are being welded electrically, goggles are 

 worn by the workers and signs are displayed advising 

 the passer-by not to look at the flame. 



For our immediate purpose, however, the importance 

 of ultra-violet light is its ionizing effect. Ions are pro- 

 duced in gases exposed to light, rich in ultra-violet 

 radiations, as is easily verified by their increased ability 

 to conduct electricity. Thus if the ah- between the 

 charged leaves of a gold-leaf electroscope is exposed to 

 ultra-violet radiations the leaves quickly collapse. 

 The explanation lies in the formation in the ah- of 

 positive and negative ions. Whether the leaves were 

 charged positively or negatively, there are thus made 

 available carriers of the opposite kind, which may 

 move to the leaves and neutralize their charges, allow- 

 ing them to collapse. 



