1440 
units, and after an hour 18.2 units. We can thus assert that in this 
experiment, which was evidently conducted under very favourable 
circumstances the cooling had not caused a change in the main 
current of as much as 1 in 5000. 
We made another experiment at the temperature of liquid oxygen. 
The current measured had a strength of 1.8 units. Measurements 
made during an hour at the temperature of liquid oxygen gave a 
value of 2.6 units for the current measured, which corresponds to 
a decrease of 0.7 in 1000 in the main current. 
It would have been desirable to have made a greater number of 
experiments and to continue these during a greater length of time; 
nevertheless it would appear to be justifiabie even now to state, 
that cooling of radium down to the temperature of liquid hydrogen (about 
20°.3 absolute) during a period of not more than 1*/, hours does 
not cause a change in the gamma radiation of 1 in 1000 and pro- 
bably not even of 1 in 5000. 
It is thus probable paying due regard to the degree of accuracy 
attained, that this decrease of temperature has no immediate or 
quickly discernable influence upon the emanation or the active 
deposits of short period (radium A, B and C). But in these expe- 
riments there was no opportunity for detecting an eventual effect 
upon the radium itself, or a slowly developing effect upon its evolu- 
tion products. 
Experiments with polonium. 
A few preliminary experiments on the influence of low tempera- 
tures upon the radiation of polonium have been made in Paris. 
The experiment which was made only with liquid air, gives rise 
to some difficulties. A plate on which was some deposit of polo- 
nium was placed at the bottom of a long glass tube, which could 
be immersed in liquid air. This plate radiated through a thin alu- 
minium plate that closed the tube, into an airtight ionisation’ cham- 
ber, where the polonium rays were absorbed by the air. The polo- 
nium tube was as far as possible exhausted; and the vacuum was 
further improved by iminersing a side tube containing a little 
charcoal in liquid air. The radiation was measured at ordinary tem- 
perature, and later, when the bottom of the tube was immersed in 
liquid air. In these experiments changes of current of inconstant 
amount were observed when cooling was applied. These changes 
were smaller in proportion as the vacuum was made more com- 
plete and kept more constant. It is thus highly probable that they 
