( 375 ) 
For salts with low melting points the 1s* boiling point will 
generally be situated at higher temperatures (Tl NO; does not con- 
form to this rule) because the solubility at lower temperatures is 
generally greater than with salts having a high melting point. In 
such a case the distance from £ to C and therefore also from Z to 
F becomes smaller. This will as a ruie be coupled with the fact 
that the maximum pressure at B is lower. Of the examples cited, 
Ag NO; has been more closely investigated. 
Drs. Conen and Sarrs, in a preliminary experiment, found the 
following vapour pressures of the saturated solution. 
t P t Pp 
133° 760 mM. B70". KO 10 mM. 
135° 800 » 185° 900 » 
i; 150° 9605s Jak 760 » 
16075 1000: <a 
From this we derive a maximum at 167° and 1015 m.m. The 
lower the maximum point of the curve, the greater is the chance 
that the points # and C will lie at widely different temperatures 
because the part of the curve BC must now bend round more 
suddenly in order to end vertically in C. The greatest diffe- 
rence between the second boiling point and the melting point has 
been found in the case of Ag NOs. If we take salts with a still 
lower melting point, the maximum pressure may then fall below 
1 atm. and the line DEFG will, therefore, lie entirely in the region 
of the unsaturated solutions above the curve. 
Such salts will therefore, completely liquefy, at lower tempera- 
tures, in aqueous vapour of 1 atm. pressure and this solution will 
never deposit solid salt when the temperature is raised. 
Such a case occurs for instance with NH, NO; (m. p. 164°) and 
of course always with salts melting below 100° as was shown in 
the case of ammonium acetate (m. p. 89°). 
In the case of such salts the two boiling points of the saturated 
solutions can only appear at a lower pressure or they may appear 
at 1 atm. if a more volatile solvent is taken which boils below the 
melting point and in which the solubility of the salt at lower tem- 
peratures is not too large. Dr. Sirs succesfully used alcohol in the 
case of NH, NOs. The alcohol boiled at 78°.4, the first boiling point 
of the saturated solution was situated at 83°, the second at 161°, 
therefore 3° below the melting point. 
Consequently there can be no doubt about the generality of the 
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