ABSOLUTE ZERO — SIMON 259 



low temperature; it is always necessary to compare it with the 

 phenomenon in which we are interested. For instance, room tem- 

 perature is a very low temperature if we look for the evaporation of 

 diamond, because its heat of evaporation is very high (i. e., only at 

 high temperatures is the thermal agitation big enough to push a 

 carbon atom out of the crystal). But room temperature is a very 

 high temperature if we look for the evaporation of hydrogen, as 

 its heat of evaporation is very small. So the question is, are there 

 any phenomena connected with very small energy changes; that 

 means, phenomena which will still happen at very low temperatures ? 



If the atoms were only points possessing attractive or repulsive 

 forces, then certainly nothing much of interest would happen within 

 the new region. The thermal agitation would become smaller, but 

 this would not give rise to any new phenomena. However, we know 

 that although in the kinetic theory it was for a long time sufficient 

 to treat the atoms as points with attractive and repulsive forces, yet 

 this is certainly not a complete picture. We know that the atoms 

 are built up from nuclei and electrons. In general one is accus- 

 tomed to find the effects of this complexity of the atoms only in gases 

 at high temperatures, as most of them are connected with big 

 changes of energy. It is true that at normal temperatures the effects 

 originating in the complexity of the structure of the atom are not 

 very striking in a solid body, but certainly some do exist. For in- 

 stance, we spoke just now of the magnetic properties of some salts. 

 If the atoms were only points with forces, they could not show any 

 magnetic properties. These are due to the motion or the spin of the 

 electrons, and here we have one effect of the structure of the atom. 

 We have seen already that this effect becomes more and more strik- 

 ing as the temperature is lowered. We have also considered another 

 phenomenon that would not have been possible if the atoms had 

 only been points. A system of points could not show metallic con- 

 ductivity. That is due to electrons split off the atoms within the 

 metal, and we have seen that with these electrons, something happens 

 only at very low temperatures, namely, supraconductivity. Here 

 some change takes place connected with an energy difference of such 

 an order of magnitude that it becomes equal to the thermal agita- 

 tion only at very low temperatures; and I may mention that at 

 present it is not known what exactly is happening in the metal. 



It would be very important to see whether at still lower tempera- 

 tures all metals become supraconductive ; that is, whether it is a 

 general property of all metals. 



So we see that it is of interest to extend our temperature range to 

 lower temperatures, and we will find later on that there are still more 

 phenomena that can be expected to take place below 1°. 



36923—36 18 



