SOME CONTEMPORARY ADVANCES IN PHYSICS— XI 491 



tion per se, apart from all knowledge or assumption concerning the 

 processes which effect it. There is a thermodynamic method of deter- 

 mining the percentage of dissociated molecules in a molecular gas as a 

 function of the temperature and the pressure of the gas, which can be 

 used if we know the amount of energy required to dissociate a single 

 molecule, the specific heats of the undissociated and the dissociated 

 gas, and the chemical constants of the undissociated and the disso- 

 ciated gas. An analogy may be establish^ between dissociation and 

 ionization: the ionizing-energy of a monatomic gas corresponds to the 

 heat of dissociation of (say) a diatomic gas; the electrons and the ions 

 resulting from the ionizations may be taken as the particles of two dis- 

 tinct gases mingled with one another and with the gas composed of the 

 neutral atoms; the chemical constant of the ion-gas is taken as equal 

 to the chemical constant of the original gas, and the chemical constant 

 of the electron-gas is identified with that which a gas composed of 

 neutral atoms, each possessing the same mass as an electron, would 

 possess. Utilizing this analogy, a formula may be deduced for the per- 

 centage of ionized atoms present in a monatomic gas in thermal equilib- 

 rium at any temperature and pressure. 



Without developing the formula, it may be taken as a rather obvious 

 inference that the higher the temperature of the gas at a given pres- 

 sure, or the lower the pressure at a given temperature, the greater the 

 percentage of ionization will be; and of two gases maintained at the 

 same temperature and pressure, the gas having the smaller ionizing- 

 energy will be the more ionized. 



Measurements of the degree of ionization in a iiame of known tem- 

 perature, into which a known amount of caesium was introduced, have 

 yielded values in good agreement with the percentage calculated from 

 the thermodynamic formula; and measurements upon the conductivi- 

 ties of the vapors of the alkali metals have shown that they stand in the 

 order of the ionizing energies reversed, although in other respects the 

 agreement with the theory is not good. 29. The tests and the value of 

 the theory, however, appear chiefly in the realm of astrophysics. The 

 hotter the region of a star in which the lines observed in its spectrum 

 have their source, the more the lines of ionized atoms predominate 

 among these. In many cases it happens that lines of ionized atoms are 

 the only ones characteristic of a given element to be found at all. The 

 assertion once commonly made, that certain elements are absent from 

 the sun or other stars, is invalidated by the fact that under the actual 

 conditions of temperature and pressure prevailing in these bodies, 



"B. T. Barnes, Phys. Rev. (2) 23, pp. 178-188 (1924); M. N. Saha, Phil. Mag. 

 46, pp. 534-543 (1923). 



