S70 



REPORT — 1901. 



are not the only necessary constituents. The simultaneous existence of 

 the two hydrates of Magnesium sulphate, as already pointed out, is only 

 possible when the solution contains, in addition, a certain proportion of 

 Magnesium chloride — viz., 73 molecules, the determination of which is the 



outcome of tentative trials. 



Turning to solutions saturated in presence of three equilibi'ators, a 

 difficulty arises in expressing the composition of the solution, as chemical 

 analysis only gives a measure of the amount of the various radicles pre- 

 sent, and aifords no information whatever as to the nature of the salts 

 present and their relative amounts — i.e., apart from hypothesis nothing is 

 known as to the state in which salts exist in solution. As in practice the 

 solution is saturated in presence of three known salts, its constitution is 

 most rationally represented by expressing the analytical results as much as 

 possible in terms of these. However, bearing in mind the equation for a 

 reciprocal salt pair, and the fact that tlie constitution of a solution is 

 expressed in molecular proportions, a little consideration shows that it is 

 of minor importance how the composition of the solution is expressed, 

 the important fact being that, when saturated in presence of three known 

 substances, it has a definite chemical composition. The table printed 

 above shows the composition of the sixteen saturated solutions which 

 can be made by using one or more of the equilibrators derivable from die 

 reciprocal salt pair. Geometrically, there are many possible ways of 

 graphically representing such a set of results — that chosen by van't Hoff 

 practically involves plotting the four salts on axes at right angles to each 

 other in such a manner that reciprocal salts are measured in opposite direc- 

 tions on the same axis. In such a diagram (fig. 3) the solutions saturated 

 with a single salt are represented by points on the axes, all other saturated 

 solutions giving points between the axes. Thus, the points A, b, c, and 

 D fall on the four axes, -whilst a point E representing the solution saturated 



