262 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1935 



from the surroundings. At 0.5°, however, we have a vapor pres- 

 sure 10"^, which is no longer negligible. In this case the substance 

 would warm up much more quickly, and so we have the paradox 

 that it is much easier to keep a temperature below, let us say, 0.3°, 

 than one above this temperature.^ 



Table 1.- — Vapor pressures of helium , 



T p ( mtn ) 



1. . 1. 5X10-" 



0. 7 3. 2X10-' 



0. 5 2. 5X10-" 



0. 3 7 XIO-'" 



0. 2 3 X10-" 



0. 1 3 XlO-'' 



0. 05 4 XlO-*" 



0. 03 6 XlO-'"' 



At these very low temperatures one can get any isolation one likes ; 

 for instance, at 0.03° we have a vapor pressure of 10-^°-. The sur- 

 roundings of the substance are at a temperature of about 1°, where 

 the radiation is certainly negligible, and one can make the suspen- 

 sion so that very little heat is conducted to the substance. Thus we 

 have really no difficulty in keeping temperatures as long as we like, 

 even working with very small amounts of substances. 



You saw that using different paramagnetic salts, we reached dif- 

 ferent final temperatures, which means that all substances are not 

 equally good for this method. Of the many substances we investi- 

 gated, iron ammonium alum was found the most suitable, and with 

 it we have got down to about 0.04°, using a field of 14,000 gauss. 

 Do Haas reached 0.015° with this procedure, using potassium chro- 

 mium alum, and having the huge magnet of the Leiden laboratory 

 at his disposal. 



I may remind you that the temperature of a material body in the 

 interstellar space cannot fall below 2° or 3° K., as it always has 

 to be in equilibrium with the stellar radiation. So you see that in 

 this case we can realize in the laboratory a lower temperature than 

 we can find in nature, and we can surpass the conditions found in 

 nature in still another way. We will look once again at the table 

 of the vapor pressures of helium, the most volatile gas existing. In 

 the interstellar space there is a vacuum of about 10"^^ cm Hg. You 

 see that we have already reached this pressure in a space surrounded 

 by a body at a temperature of about 0.15°, even when it is filled 

 with the most volatile gas. At 0.03° the pressure would be so small 

 that in the whole Galaxy we would not find one single atom in 

 equilibrium with it. So, in the directions of low density and low 



^ Of course, one could establish a vacuum by means of pumps, but at low temperatures 

 the helium is absorbed in big amounts on the walls, and it would take a very long time to 

 obtain a sufficiently high vacuum. 



