140 SCIENCE. PROGKESs 
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 
2KCl1. ZnCl, and of 2KCl + ZnCl, (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.! 
(11) Lowering of the Freezing-point!—For 2KCy + HgCy, the 
calculated molecular lowering is by 4'77° greater than that 
observed for K,Hg(Cy),, 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 + po : 7°57 + 1°93 = 9°50 
Found for K,HgCy, . : : 5 ee Sl 
Difference . : 4°73 
(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 HglI, show 
marked deviation from the additive values.? 
(iv) Colour—tThe yellow chloride of iron and the colourless 
chloride of potassium form at 22°C. and above a double salt of 
the composition FeCl,.2KCl.H.O, which in the solid state is 
red. A concentrated solution of the mixture of the two simple 
Jones, H. C. (and others), “ Contributions to our knowledge of the Aqueous 
Solutions of Double Salts,” Amer. Chem. J. 22, 1899, p. 5 3 25, 1901, p. 349. 
* Schénrock, “ Elektromagnetische Drehung der Polarisationsebene in 
Flussigkeiten und Salzlésungen,” Zs. physik. Chem. 11, 1893, p. 776. 
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