256 EXCITED RADIO-ACTIVITY [CH. 
with sand or emery paper. The amount of active matter deposited 
is extremely small, for no difference of weight has been detected 
in a platinum wire when made extremely active. On examining 
the wire under a microscope, no trace of foreign matter is observed. 
It follows from these results that the matter which causes excited 
activity is many thousand times more active, weight for weight, 
than radium itself. 
It is convenient to have a definite name for this radio-active 
matter, for the term “excited activity” only refers to the radiation 
from the active matter and not to the matter itself Since the 
matter which produces the phenomena of excited radio-activity is 
derived from the emanation of thorium and of radium, the name 
emanation X will be given to it. This is chosen from analogy to the 
active products Ur X and Th X which are continuously produced 
from uranium and thorium respectively. The emanation X from 
thorium is different in chemical and other properties from the 
emanation X from radium. For example, each type of matter has 
a distinctive rate of decay of activity, as well as some differences 
in solubility by acids. 
On the view developed in section 127, the emanation X is the 
residue left behind from each atom of the emanation of thorium 
or of radium after one or more @ particles have been expelled. The 
emanation X is an unstable substance, and its atoms again break 
up, giving rise to “excited activity,” ae. to the radiation from 
“emanation X.” 
The emanation X is quite distinct in chemical and physical . 
properties from the emanation which produces it. For example, 
emanation X behaves as a solid, which is deposited on the surface 
of bodies, while the emanation exists in the gaseous state. The 
emanation is insoluble in hydrochloric or sulphuric acids, while 
emanation X is readily soluble in both. 
169. Decay of the excited activity produced by thorium. 
The excited activity produced in a body after a long exposure to 
the emanations of thorium, decays in an exponential law with the 
time, falling to half value in about 11 hours. The following table 
shows the rate of decay of the excited activity produced on a brass 
rod, 
