A450 Mr. Garrett and Dr. Willows: Chemical 
Eaperiments with Zinc Lodide. 
Sodium iodide and zinc iodide are two compounds that 
may be formed when the above mixture is heated. 
No increased current could be observed when the former 
was heated to 360°; the latter gave large currents above 320° 
when ( was negatively charged. If these currents are mul- 
tiplied by a constant, they are represented by the same curve 
as that showing the leak from a mixture of zinc-salt-iodine. 
The crosses in fig. 7 show the leak from zine iodide alone, 
the other points from the mixture of zinc-salt-iodine. The 
positive currents, i. e. C charged positively, are the same in 
the two cases. The rate of decay with time when kept at a 
constant temperature agrees pretty closely with that given 
by the mixture (see fig. 8). We scarcely think that the 
maximum shown here is an experimental error, due to the 
temperature not having become steady. Neglecting this 
part, the remainder is an exponential curve. 
Suppose a gram of zinc iodide is giving, at a steady tem- 
perature, a current I. Leta certain fraction A become inactive 
during each minute due to chemical dissociation. Then 
dl . 
a eee Al 
and 
f=[e4 
where I, is the maximum value of the current. 
The time, t, for the leak to fall to half its maximum value is 
log 2 
eiven by t=: - This is equal to 42 min. for the mixture 
yinc-salt-iodine and 40 min. for zine iodide. It thus 
becomes apparent that the conduction observed by Beattie 
when salt, sprinkled with iodine, is heated on a zine plate 
is due to the formation of zine iodide at the lower tem- 
peratures, and a subsequent breaking up as the temperature 
rises. The sodium chloride appears to have a catalytic 
action during the formation, as the direct heating of zinc and 
iodine together causes no leak. 
It is to be noted that the expression [=aT™ does not give 
a definite temperature at which the conduction begins ; if, 
therefore, it accurately represents the current, a more sensitive 
apparatus would detect this at a lower temperature than in 
our experiments. 
