Ix] EXCITED RADIO-ACTIVITY 2 
strongly active and its activity decayed at a normal rate. Von 
Lerch found that the amount of activity obtained by placing 
different metals in active solutions for equal times varied greatly 
with the metal. For example, he found that if a zinc plate and 
an amalgamated zine plate, which show equal potential differences 
with regard to hydrochloric acid, were dipped for equal times in 
two solutions of equal activity, the zinc plate was seven times as 
active as the other. The activity was almost removed from the 
solution in a few minutes by dipping a zinc plate into it. Some 
metals became active when dipped into an active solution while 
others did not. Platinum, palladium, and silver remained inactive, 
while copper, tin, lead, nickel, iron, zinc, cadmium, magnesium, 
and aluminium became active. These results strongly confirm the 
view that excited activity is due to a deposit of active matter 
which has distinctive chemical behaviour. 
G. B. Pegram? has made a detailed study of the active deposits 
obtained by electrolysis of pure and commercial thorium salts. 
The commercial thorium nitrate obtained from P. de Haen gave, 
when electrolysed, a deposit of lead peroxide on the anode. This 
deposit was radio-active, and its activity decayed at the normal 
rate of the excited activity due to thorium. From solutions of 
pure thorium nitrate, no visible deposit was obtained on the anode, 
but it was, however, found to be radio-active. The activity 
decayed rapidly, falling to half value in about one hour. Some 
experiments were also made on the effect of adding metallic salts 
to thorium solutions and then electrolysing them. Anode and 
cathode deposits of the oxides or metals obtained in this way were 
found to be radio-active, but the activity fell to half value in a few 
minutes. The gases produced by electrolysis were radio-active, 
but this was due to the presence of the thorium emanation. The 
results of Pegram and von Lerch would seem to indicate that, 
besides those already known, other radio-active products with 
a distinctive rate of decay are produced during the changes 
occurring in thorium. 
180. Effect of temperature. The activity of a platinum 
wire which has been exposed in the presence of the thorium 
1 Phys. Review, p. 424, Dec. 1903. 
