172 Dewar — The Nadir of Temperature, etc. 



be seen that by the use of liquid or solid hydrogen as a cooling 

 agent we ought to be able to liquefy a body having a critical 

 point of about 6° to S° absolute and boiling point of about 4° 

 or 5° absolute. Then, if liquid helium could be produced 

 with the probable boiling point of 5° absolute, this substance 

 would not enable us to reach the zero of temperature ; another 

 gas must be found that is as much more volatile than helium 

 as it is than hydrogen in order to reach within 1° of the zero 

 of temperature. If, the helium group comprises a substance 

 having the atomic weight 2, or half that of helium, such a 

 gas would bring us nearer the desired goal. In the meantime 

 the production of liquid helium is a difficult and expensive 

 enough problem to long occupy the scientific world. 



A number of miscellaneous observations have been made in 

 the course of this inquiry, among which the following may be 

 mentioned. Thus the great increase of phosphorescence in the 

 case of organic bodies cooled to the boiling point of hydrogen 

 under light stimulation is very marked, when compared with 

 the same effects brought about by the use of liquid air. A 

 body like sulphide of zinc cooled to 21° absolute and exposed 

 to light shows brilliant phosphorescence on the temperature 

 being allowed to rise. Bodies like radium that exhibit self- 

 luminosity in the dark, cooled in liquid hydrogen maintain 

 their luminosity unimpaired. Photographic action is still 

 active although it is reduced to about half the intensity it bears 

 at the temperature of liquid air. Some crystals when placed 

 in liquid hydrogen become for a time self-luminous, on account 

 of the high electric stimulation brought about by the cooling 

 causing actual electric discharges between the crystal mole- 

 cules. This is very marked with some platino-cyanides and 

 nitrate of uranium. Even eooling such crystals to the tem- 

 perature of liquid air is sufficient- to develop marked electrical 

 and luminous effects. Considering that both liquid hydrogen 

 and air are highly insulating liquids, tiie fact of electric dis- 

 charges taking place under such conditions proves that the 

 electrical potential generated by the cooling must be very high. 

 When the cooled crystal is taken out of either liquid and 

 allowed to increase in temperature, the luminosity and electric 

 discharges take place again during the return to the normal 

 temperature. A crystal of nitrate of uranium gets so highly 

 charged electrically that, although its density is 2*8 and that of 

 liquid air about 1, it refuses to sink, sticking to the side of the 

 vacuum vessel and requiring a marked pull on a silk thread, 

 to which it is attached, to displace it. Such a crystal rapidly 

 removes cloudiness from liquid air by attracting suspended 

 particles to its surface. The study of pyro-electricity at low 

 temperatures will solve some very important problems. 



