Tension and Complex Molecules. 395 



The points m and n, which correspond to the concentra- 

 tions of the two saturated layers, are situated on one side of 

 the eutectic point and must be regarded as solutions in the 

 same dissolvant. I£ two solutions in the same dissolvant 

 have the same freezing-point (resp. boiling-point) they 

 must contain an equal number of molecules per unit volume. 



Such is only possible if the molecules of B form a com- 

 pound with some of the molecules in the solutions, and 

 further addition of B would not increase the number 

 of molecules present in the solution and all properties 

 depending on the number of molecules (and not their 

 dimensions) would remain invariable. The Phase Rule 

 specifies some conditions under which monovariant systems 

 can be formed *, e. g. in a system water-salt, the addition of 

 salt to its solution alters the properties thereof, until the 

 solution is saturated. Adding more salt has only one effect, 

 it only increases the quantity of solid salt with which the 

 solution is in equilibrium. The properties of the solution 

 are monovariant [i. e. they only depend upon the tempera- 

 ture) until the solid phase and liquid are coexisting. In 

 this case the system remains monovariant while it is 

 heterogeneous. 



But if the molecules of the added substance, instead of 

 forming a precipitate, adhere to the molecules in solution 

 forming complex molecules, then the same monovariance 

 mav be attained in a quite homogeneous system. 



Such cases of monovariance (i. e. when some properties 

 remain invariable with a change of concentration at a given 

 temperature) are actually known for some pairs of liquids 

 not far from the critical point of the separation into two 

 liquid layers f- Such properties may be the vapour pres- 

 sure, the surface tension, freezing-point, &c. 



All the above considerations are equally applicable to the 

 case represented in fig. 5. In this case the points m and n 

 are situated between the two eutectic points, and in this 

 region the ice formed at the freezing-points is not one of the 

 substances A or B, but a compound of A q B r . 



The solutions m and n are therefore also solutions in the 

 same dissolvant whatever it may be with regard to its 

 chemical nature, and must also contain equal numbers of 

 molecules per unit of volume. 



The above must also be true for pairs with very small 



* Systems for which some" properties depend upon the temperature 

 only "being independent of the concentration. 

 f G.N. ^ntonoffj loc. cit. 



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