i 4 o SCIENCE PROGRESS 



mental proof for the existence in solution of double salts, 

 an existence which moreover one would be led to expect 

 theoretically. The methods used in these proofs of course all 

 depend on the comparative measurement of physical properties, 

 the values found for the solution of the double salt XY 

 being compared with those calculated for a mixture of the 

 constituent simple salts from the experimental values obtained 

 for X and Y separately. 



The following are examples of such comparative measure- 

 ments : 



(i) Electrical Conductivity. — Measurements have been carried 

 out by many investigators, who found that in moderately 

 strong solutions, N and N/2, the double salts are but little 

 dissociated. Thus the differences between the conductivities of 

 2KCI . ZnCl 2 and of 2KCI -f ZnCl 2 (calculated) amount to nearly 

 36 per cent., which greatly exceeds the value of the difference 

 which would be due to the influence on one another of salts 

 containing a common ion. 1 



(ii) Lowering of the Freezing-point} — For 2KCy + HgCy 2 the 

 calculated molecular lowering is by 477° greater than that 

 observed for K 2 Hg(Cy) 4 , corresponding to a disappearance of 

 about two and a half molecules (or ions) in the formation of one 

 molecule of the double salt. 



Calculated from 2KCy + HgCy 2 . 7*57 + 1 "93 = c/50 

 Found for K 2 HgCy 4 =477 



Difference . . 473 



(iii) Rotation of the Plane of Polarisation. — Measurements 

 have shown that, whilst for solutions containing mixtures of salts 

 known not to yield double salts the effect measured is additive, 

 solutions containing substances such as 2KI and Hgl 2 show 

 marked deviation from the additive values. 2 



(iv) Colour. — The yellow chloride of iron and the colourless 

 chloride of potassium form at 22 C. and above a double salt of 

 the composition FeCl 3 . 2KCI . H 2 0, which in the solid state is 

 red. A concentrated solution of the mixture of the two simple 



1 Jones, H. C. (and others), "Contributions to our knowledge of the Aqueous 

 Solutions of Double Salts," Amer. Chem.J. 22, 1899, p. 5 ; 25, 1901, p. 349. 



2 Schonrock, " Elektromagnetische Drehung der Polarisationsebene in 

 Fliissigkeiten und Salzlosungen," Zs. physik. Chem. 11, 1893, p. 776. 



