190 J. MORISON : ADDRESS — 



plate and mantle in a dark cloth, and depositing the whole in 

 a dark place for eight days. When taken out the plate will be 

 found to have a photograph of the mantle imprinted upon it. 

 The mantle, indeed, has photographed itself by means of its own 

 thoriimi rays. 



According to Hahn's investigations, thorium, which in itself is 

 rayless, is continually undergoing change into a substance which 

 he calls radio-thorium. Radio -thorium emits a rays and is 

 changed into thorium X, which in its turn, with the emission of 

 an a particle, is transformed into the thorium emanation. This 

 emanation is very like that of radium, but its life is much shorter, 

 being half transformed in 54 seconds. It has been shown to be 

 a heavy inert gas of high atomic weight. Like the radium 

 emanation, with the emission of an a particle, it is changed into 

 an active deposit. This active deposit is a mixture of two sub- 

 stances, thorium A and thorium B. The emanation is first 

 converted into thorium A, which is a rayless product, and is soon 

 changed into thorium B ; then thorium B breaks up, violently 

 emitting a, ft, and 7 rays, and is transformed into a substance 

 which is apparently stable. 



Actinium is a very rare element which was discovered by 

 Debierne in pitch-blende residues. It seems to be nearly equal 

 to radium in activity. It has not yet been separated in a state 

 sufficiently isolated to allow of the determination of its atomic 

 weight or spectrum. According to Hahn, actinium, in itself 

 rayless, is continually giving rise to a substance which he calls 

 radio-actinium, which with the emission of an a particle is 

 changed into actinium X ; then actinium X, emitting another 

 a particle, is converted into an emanation like that of thorium, 

 but still more short-lived, being half transformed in 3 or 4 

 seconds. The active deposit, like that of thorium, is a mixtui'e 

 of two substances, actinium A and actinium B. The emanation, 

 with the emission of an a particle, becomes actinixun A, a rayless 

 body ; actinium A is soon changed into actinium B ; and actinium 

 B, emitting a, ft, and 7 rays, is quickly transformed into a sub- 

 stance which appears to be stable. 



Traces of radium or radio-active bodies are found in most 

 rocks, in the soil, in rain and snow, in the atmosphere, 

 indeed very generally over the earth's surface. Eadium is 

 found in more considerable quantity in the deposits from certain 

 thermal springs such as those of Bath and Buxton. The waters 

 of these springs not only contain small quantities of radimn or 

 its products, but also traces of helium. 



