Z 
-YAPOUR PRESSURES AND REFRACTIVE INDICES OF SALT SOLUTIONS. 287 
Bee) 
vapour pressure of water by the value <P ; Where p = pressure of 
vapour of pure water, p' = pressure of water vapour from salt solution 
containing 7 molecules per 100 H,O, and this, as was to be expected, is 
in all cases a diminishing quantity with rise of temperature—showing 
that, in a constantly saturated solution, a salt exercises a less restraining 
effect on the water the higher the temperature. 
2. Vapour Pressure of Water from Non-saturated Salt Solutions. 
The experiments on this subject are not yet complete, but are suffi- 
ciently advanced to justify certain conclusions regarding the behaviour 
of salts under varying conditions of temperature and concentration. 
The method employed was the same as that in the previous section, 
with this difference, that a dilute, not a saturated, solution of the salt 
was employed, and successive portions of water were distilled off and 
weighed. In this way the concentration at different pressures and at a 
definite temperature was readily determined. 
Four salts have, as yet, been examined, NaCl, KCl, NaNO;, and 
KNO;. The temperature chosen was 70°, though some experiments 
were made at 90°, 
Two of the above salts have been examined in solutions of constant 
strength at temperatures of 70°, 75°, 80°, 85°, and 90°, 
The general results are as follows :— 
(a) When temperature is constant and m varying, then 27? 
n 
increases with increase of n in the case of NaCl; is constant, or nearly 
so, with KCl, and diminishes more or less rapidly with NaNO, and 
KNO;. These results are fully confirmed by Tammann’s results, obtained 
by the Barometric method (Wiedem. Ann. 24), a close agreement being 
found between the two sets of figures. 
(8) When the concentration 8 constant but temperature varying, 
then the value of ee or 1— ae is a diminishing one with NaCl and 
a slowly increasing one in the case of the other three salts. This also is 
confirmed by Tammann’s results, and general agreement is to be found 
with the experiments of Legrand (1835), conducted in an entirely 
different way. 
Tt is believed that there is an intimate connection between this 
behaviour of the salts and their solubility, but the discussion of this 
question is postponed till the results are more numerous and complete. 
V. Expansion of Salt Solutions. 
The dilatation of solutions containing definite numbers of molecules, 
1, 3, 5, or 2, 4, 6, &c., of NaCl, KCl, NaNO,, and KNO,, have been 
determined by means of specially constructed dilatometers, and a special 
constant temperature bath, by means of which a tube 700mm. long can 
be kept for any length of time at a definite temperature, the tempera- 
ture of the one end differing from that of the other not more than 0%1. 
Thus all necessity for correction of the results for the exposed portion of 
the stem of the dilatometer is avoided. 
As in the previous section, the experiments are not yet complete, but 
have fally established the following conclusions :— 
