8o MOLECULAR WEIGHT AND POLYMER1SM 



which behave like two liquid layers l . Thus e. g. a similar 

 behaviour as regards pressure would be found if unsatu- 

 rated ether vapour were compressed in contact with water. 

 At first the pressure will rise, with increasing absorption 

 of ether by the water ; then a second layer will be formed, 

 very rich in ether, i. e. moist ether, and then the pressure 

 will remain constant whilst the new layer increases in 

 bulk at the cost of the old. When the latter has vanished 

 the pressure again rises. The analogy with two liquid 

 layers has been actually found in solid solutions, in 

 isomorphous mixtures as a matter of fact 2 . Beryllium 

 sulphate and selenate, e.g., do not mix isomorphically in 

 all proportions, but between S : Se = 7-33 : i and S : Se 

 = 4:1 there is a gap. So that from a solution containing, 

 say, S : Se = 5 : i two isomorphous mixtures will crystallize 

 out, one tetragonal (7-33 : i), the other rhombic (4 : i). 



In following out further the behaviour of palladium 

 hydride with regard to the molecular character of the 

 hydrogen, only the parts of the series of observations 

 before and after the region of constant pressure are 

 available, and especially the former, because it refers to 

 small concentrations of hydrogen, and so satisfies the con- 

 dition implied in the theory of solution. 



According to what precedes (p. 28), if the pressure and 

 the concentration of hydrogen in palladium are proportional, 

 the formula H 2 is probable for the dissolved gas. The 

 following results are obtained, with the aid of the density 

 of palladium hydride :-- 



15-8 



166 

 16-6 



1 According to recent observations by v. Jiiptner on steel, there are 

 indications of a similar phenomenon involving solutions of carbon in iron. 

 ' 2 See also Part I, p. 55. 



