﻿Molecular Thermodynamics. 241 



we get roughly the following upper limits (of total concen- 

 tration C) for the applicability of the four successive stages 

 of approximation : 



Approximation I 



M. 



10 



II 2M. 



Ill 5M. 



IV 8M. 



(38) 



— that is, considering aqueous solution, and assuming m to be 

 about 40. The limits would be considerably different, par- 

 ticularly in the case of III. and IV., if m were given a very 

 different value, as can readily be seen. 



The practical criterion of a perfect solute in a complex 

 solvent is now that its behaviour should be expressed by that 

 one of the above succesive approximations appropriate to the 

 total-solute concentration of the solution, with the "general " 

 terms omitted. 



If the assumption of perfect behaviour in the case of a 

 particular solute be made, an experimental determination of 

 the quantities ra , 0, rj, etc., can be made and concordance in 

 several such determinations made upon different solutes 

 would tend to justify the assumption that perfect behaviour 

 persisted up to the concentration at which concordance was 

 found. 



According to the concentration reached (with concordance) 

 some of the quantities m , 0, 77, etc. would then be known 

 with some approximation (closest in m , next in #, and so on). 



Then, on the assumption of "perfect" behaviour on the 

 part of the solvent-molecular species — that is, a sufficient 

 approximation thereto, — these quantities would suffice to 

 discover something about the constitution of the solvent. 



Thus two solvent-molecular species would be completely 

 determined by a knowledge of m alone (that is, the propor- 

 tions of the two kinds present would be determined), — and 

 in this case the question of "perfect" behaviour of the two 

 species would not enter. m and 6 would, suffice to deter- 

 mine completely three species, or would provide a check if 

 only two species were present, and so on. 



The problem, however, as a practical problem is compli- 

 cated by considerations which are the subject of a succeeding 

 paper, viz. the question of solvation and partial solvation of 

 solutes. 



Finally, it is proposed to consider one point with regard 

 Phil. Mag, S. 6. Vol. 44. No. 259. July 1922. R 



