25 
portion of the a-rays gets already absorbed in the emitting substance, 
because they penetrate into solid matter but a few hundredths of a 
m.m.; consequently the active substance is heated above the tempe: 
rature of the surrounding air and, of course, remains warmer, because 
the radiation process proceeds conunuously. a-Particles which do 
not possess a sufficient velocity to ionise gases, or to render the 
zinc sulphide luminous, may still have a considerable kinetie energy ; 
and when they are absorbed, their energy being converted into heat, 
they might raise the salt from which they originate to a higher 
temperature. This argument has also been applied by GRRINACHER ') 
during an investigation on the radioactivity of several ordinary 
substances; he, however, did not study the salts which are now of 
particular interest to us. 
I have investigated this question by placing in a large galvanised 
iron basin, on pieces of cork, two silvered vacuum flasks of about 
1'/, litre capacity. The basin was placed in another and the 
space between was filled with ice; the whole was placed in a 
wooden box isolated by means of slag-wool. A third bath serves as 
a cover, which was also filled with ice and covered with blankets. 
In this manner, the flasks are entirely surrounded by ice, and it 
may be assumed that the surrounding air possesses a constant tem- 
perature. Every two days, the accumulated water is drawn off and 
fresh ice is added. The flasks are filled with about 2 kilogrammes 
of potassium or sodium chloride respectively, and closed with a 
solid plug of cotton-wool, upon which is poured a layer of paraffin. 
Through this seal penetrates a very thin-walled glass tube which 
reaches to the centre of the bulb and contains one of the junctions 
of a thermo-couple copper-constantan. The eccnstantan wire con- 
nects directly the junctions, the copper wires are carried away 
through an opening in the box and connected to the galvanometer 
which is suspended according to Jutivs and read off by means of a 
mirror and a telescope. 
If now potassium chloride emits a-rays, it may be expected to 
reach a higher temperature than sodium chloride and, owing to a 
thermoelectric force the galvanometer will deviate; by gauging with 
a definite difference in temperature it may be found with how many 
degrees corresponds a deviation of, say, 1 mm.; this proved to be 
0.003°. As soon as the circuit is closed a deviation of the galvano- 
meter is observed, but without further discussion we may not con- 
clude to a difference in temperature between the two salts. There 
are, necessarily, always some places of contact between different 
') Ann. der Phys. [4] 24, 79 (1907). 
