< 160 ) 
“phosphors” under the same conditions and immediately after each 
other. 
Between the two plates p,,/?, lies the reservoir 8 , of the helium 
thermometer which indicates the temperature of the “phosphors”; 
it is an elliptical cylinder of german-silver. It is attached to the 
steel capillary 8 X which, enclosed in g x , passes through the tube /, 
and by this means is connected with the glass capillary d t which 
allows readings for accurate determinations of temperature between 
— 260° C. and —180° C. and with the reservoir 8 V on which 
temperatures higher than —180° C. can be read by approximation. 
The purpose served by the valves 8 6 and 8„ needs no further 
explanation. 
To the bottom of the hydrogen glass b is brought a german-silver 
warming coil whose leads are insulated and made airtight in the 
cap h. This coil serves to vaporise the hydrogen ; when this has 
vaporised the temperature of the space b slowly rises, and by this 
means the simultaneous change of the phosphorescence and of the 
temperature can be followed. 
In order to work at the temperature of solid hydrogen, the 
hydrogen gas is pumped away by the vacuum-pump along </, and 
the pressure is lowered until a crystal layer is formed on the liquid 
hydrogen; after the crystal layer has grown to a certain thickness 
it is kept in that state by regulating the rate at which the gas is 
pumped away. 
As the source of light for exciting phosphorescence an iron-arc 
was used with or without the Wood ultra-violet filter (arranged 
as in “1904” p. 244) which had been very suitable in former 
experiments, and also a carbon arc with the same filter and con¬ 
centrating lenses. Ultra-red light which often proved serviceable, was 
obtained from the same arc lamp by using as light filter a layer of 
water and a layer of a solution of iodine in carbon bisulphide. We 
found no noticeable absorption of this light in the layers of liquid 
air and liquid hydrogen which it had to traverse. In order to put 
this to the proof we so manipulated the height of the surface of 
the liquid hydrogen that the rays falling upon the “phosphor 
passed only through glass and gas, and the “phosphor” came just 
above the surface of the liquid and no more, so that although in 
the vapour space, it remained just as strongly cooled; in these 
Circumstances we observed not the slightest change, 
The substances embraced by our experiments were the “phosphors 
Ca Cu Ui, Ca Mn Na (yellowshade), Ca Ni FI, CaBiNa, Sr Zn FI, 
Sr Mn Na, Sr Bi Na, Ba Cu Li, Ba Bi K. They were all prepared so 
