LE. A. Mill— Notes on Argon and Helium. 365 
force of heat (if such an expression be permissible), renders 
all combination impossible ; which is the physical condition in 
which argon, at ordinary temperatures, is by this theory 
assumed to be. 
Chemical affinity, then (the tendency for instance of the 
molecules H’ and Cl’ to form 2HCl), is not a simple force, but 
rather the resultant of at least three other atomic, not molecu- 
lar, forces, viz: the attraction between the atoms H and Cl, 
which is opposed by the attractions between H and H and Cl 
and Cl. In general 
where F'= the chemical affinity of chlorine for hydrogen 
J = the attraction between the atoms H and Cl 
e= 66 ce (19 Cl 6¢ Cl 
A= 66 66 6< H « H 
The theory assumes 7, c and A to decrease with a rise, and 
increase with a fall in temperature, but 7 more slowly than c 
h 
and A. When f = i =0. If temperature be reduced 7 
: h 
increases more slowly than — hence the forces of molecular 
ageregation ¢ and f/ are stronger the lower the temperature, 
while F, the affinity for other molecules, has a negative value. 
Per contra, if temperature be increased, 7 decreases more 
+t fy 1 
slowly than =<, and when 5 + a= 0, f will have a positive 
value, and F' will have its maximum value, that is F=/; the 
case corresponding to the point where heat has dissociated the 
molecules into nascent atoms. Or rather the maximum value 
of I’ will occur somewhere between the dissociation tempera- 
tures of Cl and H. When both molecules are dissociated, 
since =+ * 0, increase of temperature reduces IF and where 
fF’ = 0, which case, for mercury vapor, is reached at 800°, we 
reach the upper limit of chemical combination ; that is, the 
kinetic energy of the molecule is too great to allow of its 
combining with other molecules. 
The following might at first seem an objection to this theory, 
viz: that considering the molecule HCl, if temperature is 
reduced 7, the force binding the atoms H and Cl into a mole- 
h : 
_ cule finally becomes less than “= hence reduction of tempera- 
ture ought to produce decomposition of HCl, whereas nothing 
of the sort occurs. | 
The answer is this, before HCl can be dissociated into H 
and Cl so that H? and Cl’ can be formed, we must first over- 
come the force of attraction between the molecules of HCl, 
