SOLID SOLUTIONS. 129 



varying but little with the actual composition of the mixtures 

 taken. The general theory of solutions asserts that when a 

 substance(here /3-naphthol) is divided between two immiscible 

 solvents (here water and naphthalene, or naphtholnaph- 

 thalene) it will be distributed in a constant ratio between 

 the two solvents, no matter what amount of it be taken, 

 provided only the molecular weight of the substance is the 

 same in both solvents. In the case investigated this does 

 not hold — the ratio of the concentrations in the two solvents 

 is not constant ; and the molecular weight of /3- 

 naphthol dissolved in water is therefore different from the 

 molecular weight of /3-naphthol " dissolved " in naphthalene. 

 The theory further asserts that when, as in the present 

 instance, the concentration in one of the solvents is pro- 

 portional to the square root of the concentration in the 

 other solvent, the molecule in the second solvent must be 

 twice as great as the molecule in the first. We know that 

 /3-naphthol dissolved in water has the normal molecular 

 weight corresponding to the formula C IO H s O ; in naphthalene 

 solution it has consequently the molecular weight corre- 

 sponding to the formula (C IO H 8 0) 2 . 



The theory of solutions likewise enables us to calculate 

 the molecular weight of the naphthalene in the above 

 experiments from the diminution of the solubility of the 

 /3-naphthol in water as it dissolves more and more naph- 

 thalene. In the case before us the question is slightly com- 

 plicated by the existence of naphtholnaphthalene molecules, 

 but Kiister was able to arrive at the result that naphthalene 

 must have double the molecular weight in the state of solid 

 solution that it has in the state of vapour, viz., (C IO H 8 ) 2 . 



Another well-investigated case of solid solutions is that 

 offered by the absorption of hydrogen by palladium. 

 T roost and Hautefeuille, in order to obtain information as to 

 the state in which the hydrogen existed within the metal, 

 made an extensive series of observations of the pressure of 

 hydrogen in equilibrium with palladium containing different 

 amounts of hydrogen. They found that with compositions 

 of the solid up to one atom of hydrogen to two atoms of 

 palladium the pressure of hydrogen remained constant 



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