1] RADIO-ACTIVE SUBSTANCES 5 
waves. At the same time Le Bon? showed that, by the action of 
sunlight on certain bodies, a radiation was given out, invisible to 
the eye, but active with regard to a photographic plate. These 
results have been the subject of much discussion ; but there seems 
to be little doubt that the effects are due to short ultra-violet light 
waves, capable of passing through certain substances opaque to 
ordinary light. These effects, while interesting in themselves, are 
of quite a distinct character from those shown by the radio- 
active bodies which will now be considered. 
3. Uranium. The first important discovery in the subject of 
radio-activity was made in February, 1896, by M. Henri Becquerel?, 
who found that a uranium salt, the double sulphate of uranium 
and potassium, emitted some rays which gave an impression on a 
photographic plate enveloped in black paper. These rays were 
also able to pass through thin plates of metals and other substances 
opaque to light. The impressions on the plate could not have 
been due to vapours given off by the substances, since the same 
effect was produced whether the salt was placed directly on the 
black paper or on a thin plate of glass lying upon it. 
Becquerel found later that all the compounds of uranium as 
well as the metal itself possessed the same property, and, although 
the amount of action varied slightly for the different compounds, 
the effects in all cases were comparable. It was at first natural to 
suppose that the emission of these rays was in some way connected 
with the power of phosphorescence, but later observations showed 
that there was no connection whatever between them. The uranic 
salts are phosphorescent, while the uranous salts are not. The uranic 
salts, when exposed to ultra-violet light in the phosphoroscope, 
give a phosphorescent light lasting about ‘01 seconds. When the 
salts are dissolved in water, the duration is still less. The amount 
of action on the photographic plate does not depend on the par- 
ticular compound of uranium employed, but only on the amount of 
uranium present in the compound. The non-phosphorescent are 
equally active with the phosphorescent compounds. The amount 
of radiation given out is unaltered if the active body is kept 
1 
2 
C. R. 122, pp. 188, 233, 386, 462. 1896. 
2 C. R, 122, pp. 420, 501, 559, 689, 762, 1086. 1896. 
