CHEMICAL ACHIEVEMENT — PAULING 227 



electronic and atomic architecture of molecules and crystals almost 

 as confidently as architects can talk about the structural elements of 

 skyscrapers and bridges. By the methods of spectroscopy, X-ray 

 diffraction, and electron diffraction accurate interatomic distances 

 have been determined for thousands of substances. The magnitudes 

 of the forces operating between the atoms have also been determined 

 experimentally for very many molecules and crystals. Further in- 

 formation about the nature of substances has been obtained by the 

 application of many different techniques of modern physics — the 

 study of the diamagnetic, paramagnetic, and ferromagnetic prop- 

 erties of the substances, their electrical properties, and the spectros- 

 copy not only of the visible, infrared. X-ray, and ultraviolet regions, 

 but even, in recent years, of the microwave and long-wave radio 

 regions of the spectrum. The structural knowledge obtained in this 

 way about molecules permits the calculation of thermodynamic prop- 

 erties for many substances. 



A significant start has already been made on the task of formu- 

 lating a complete system of chemical thermodynamics of pure sub- 

 stances. This task involves the determination for each substance at 

 one temperature of its enthalpy, relative to the elements that compose 

 it. It is further necessary to determine the entropy of the substance 

 at one temperature, which can be done by any one of three methods : 

 the measurement of a chemical equilibrium involving the substance 

 and other substances of known thermodynamic properties, the meas- 

 urement of the heat capacity down to very low temperatures and the 

 application of the third law of thermodynamics, or the calculation of 

 the entropy from structural data obtained by spectroscopic and 

 diffraction methods. Knowledge of the heat capacity of the sub- 

 stance over a wide range of temperatures, obtained either by direct 

 experiment or by calculation from known structural properties, then 

 permits the extension of the tables of thermodynamic properties over 

 this temperature range. It may well be expected that at some time 

 in the distant future there will be available extensive tables of the 

 enthalpy, entropy, and free energy of thousands of substances over 

 wide ranges of conditions. There would then still remain, however, 

 the problem of the thermodynamic properties of solutions, for which 

 no such simple and inclusive set of data could be formulated. 



It is interesting to note that, in a practical sense, the third law of 

 thermodynamics differs from the first and second laws, in that it 

 cannot be applied completely independently of structural considera- 

 tions. In general, thermodynamic deductions are expected to be in- 

 dependent of any structural considerations, and to be reliable, pro- 

 vided only that true thermodynamic equilibrium has been approxi- 

 mated or achieved in the experiment. Investigations carried out 



