228 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1950 



during the past 25 years, especially by Prof. William F. Giauque, have 

 shown, however, that the applications of the third law of thermody- 

 namics to the calculation of entropy values for crystalline substances 

 by measurements of heat capacity made at low temperatures are often 

 not reliable in practice, unless there is available some structural in- 

 formation about the residual entropy of the crystals at the lowest 

 temperatures at which measurements are made. Thus some simple 

 substances, such as hydrogen, carbon monoxide, nitrous oxide, and 

 nitrogen dioxide, have residual entropies of significant amount, caused 

 by such structural features as a randomness of orientation of molecules 

 in the crystal lattice. It may be said, with justice, that the experiments 

 have not yet been carried out to sufficiently low temperatures, or that 

 sufficient time has not been allowed for the crystals to achieve a state 

 of true thermodynamic equilibrium ; nevertheless, the practical prob- 

 lem still exists — the reliable application of the third law of ther- 

 modynamics requires a penetrating understanding of the structure 

 of the crystalline substance under investigation. 



The recent decades have seen an extraordinary development of the 

 art of cryogenics, the production of low temperature. The pioneer 

 work of Dewar was extended by Kamerlingh Onnes, whose feat of 

 reaching a temperature as low as 0.71° K. seemed for many years to be 

 incapable of significant betterment. Then, in 1924, Giauque suggested 

 and later put into practice the astounding new method of cooling 

 by demagnetization, with which he and other investigators have been 

 able to reach temperatures as low as about 0.001° K. 



Although the production of low temperatures might well be con- 

 sidered to be a part of the science of physics, the fact that this final 

 great achievement of reaching the temperature of 0.001° K. was 

 made by a professor of chemistry, using a method invented by himself, 

 justifies mention of it in this discussion. The work done by Professor 

 Giauque illustrates the fact that the border line between chemistry 

 and physics is a difficult one to define, as is also the border line between 

 chemistry and biology. The logarithmic dependence of certain ther- 

 modynamic quantities on temperature is, of course, responsible for the 

 great difficulty found in decreasing the temperature by successive 

 factors of 10, and leads to the theorem of the impossibility of reaching 

 the absolute zero itself. It has recently been pointed out to me by 

 Prof. Franz Simon at Oxford, however, that it is not true that there 

 is an interesting portion of nature to which access is denied to man, 

 namely, the portion of nature that deals with the properties of matter 

 at temperatures lower than those that can ever be achieved in the 

 laboratory. Professor Simon points out that the only low-temperature 

 range that is inaccessible to man is that in which no interesting phe- 

 nomena occur, because if any phenomena were to occur, they them- 



