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CHEMISTRY: NO YES AND WILSON Proc. N. A. S. 
ionization of all elements, and may well suffice to convert the neutral 
calcium atoms into calcium ions, which produce the H and K lines ; and 
also to convert completely the neutral sodium atoms, to which the D 
lines are due, into sodium ions, which do not yield strong lines in the 
visible spectrum. A second interesting application of Saha's equations 
has been made by Russell,^ who was led to the discovery of rubidium in 
sun spots by the considerations that the absence of its lines in the general 
solar spectrum is probably due to the complete conversion by the high 
temperature there prevailing of the neutral rubidium atoms (which pro- 
duce the characteristic lines of the element) into rubidium ions (Rb+), and 
that at the lower temperature prevailing in sun 'spots this conversion 
might well be only partial. 
The thermodynamic expressions may first be presented which are ap- 
plicable to a reaction of the type under consideration, by which the neutral 
atoms of a gaseous elementary substance M are converted into positive 
ions and electrons in accordance with the chemical equation M = M+ + 
B~. At any definite temperature the equilibrium-constant K of such a 
reaction is expressed in terms of the partial pressures, pM, pM+y Pe-j oi 
the three substances, regarded as perfect gases, by the following equations : 
Pmi1^=K (1) =K. (2) 
pM I - X 
Unlike the first equation, which is general, the second equation, in which 
X represents the fraction ionized and p the sum of the pressures pu and 
pM +, is valid only when pM + = Pe- that is, only when electrons do not 
originate from any other source, as from the presence of another ionizing 
element or from thermionic causes. 
The second law of thermodynamics leads to a simple expression for the 
change of this ionization constant K with the temperature, in the case where 
the heat-content-increase AH attending the reaction can be expressed as a 
linear function of the absolute temperature T, thus by the formula AH = 
AHq + TACp, where AH^ is an empirical constant and ACp is the in- 
crease in the heat-capacity at constant pressure that results from the oc- 
currence of the reaction. The second law leads, namely, to the following 
equations, in which R is the gas-constant (1.985 calories per degree), and 
I is the integration-constant resulting from the integration of equation (3) . 
dlnK =^^^^-^tI^dT. (3) 
InK = - 4- ^Mn r + — . (4) 
RT R R 
