24. RADIO-ACTIVE SUBSTANCES 

earths, and is precipitated with them. The method of preparation 
of this material is the same as that employed for the separation 
of the rare earths. This substance is similar in radio-active be- 
haviour to thorium, but intensely active in comparison. From 
the method of separation, thorium itself cannot be present except 
in minute quantity. 
The substance gives out easily absorbed and penetrating rays 
and also an emanation. On account of the intensity of the 
emanation which it emits, Giesel has termed this active material 
the “emanating substance.” 
If a piece of paper is placed in a small closed vessel contaiming 
the active material, in a short time the paper itself becomes power- 
fully active. This is especially the case if 1t 1s moistened with water. 
The emanation lights up a zinc sulphide screen. An electric field 
has a marked action on the luminosity of the screen. The action 
is discussed in more detail in section 186. 
Giesel found that the activity of the material seemed to increase 
slightly during the six months’ interval after separation. In this 
respect it is similar to radium compounds, of which the activity 
increases for a time after separation. 
Both the method of preparation and also the radiating properties 
of this “emanating substance” indicate that 1t 1s the same as the 
actimum of Debierne. Neither of these active substances has 
been studied in the same detail as uranium, thorium, or radium, 
and further comparative data on the nature of the radiations and 
emanations are necessary before any definite conclusion can be 
reached. The distinctive character of the radiations and ema- 
nations is of far more value in establishing the dissimilarity of 
two active bodies than differences in their chemical behaviour. 
This is especially the case where the active substance is present 
only in small quantity im inactive material. 
22. The similarity of the chemical properties of actinium and 
thorium has led to the suggestion at different times that the 
activity of thorium is not due to thorium itself but to the presence 
of a slight trace of actinium. In view of the difference in the rate 
of decay of activity of the emanations of thorium and actinium, 
this position is not tenable. If the activity of thorium were due 
