ON VAPOUR PRESSURES AND REFHACTIYE INDICES OF SALT SOLUTIONS. 205 



The results confirm in all respects those obtained in the previous pre- 

 liminary experiments. They are as follows : — 



(a) When temperature is constant and concentration (n) varying, 



then ^H^ increases rapidly with NaCl, more slowly with KCl; diminishes 

 n 



slowly with NaNOs, and very rapidly with KNOg, the order being 



NaCl, KCl, NaNOs, KlNOa. The figures show a clear agreement with 



those of Tammann (Wiedemann 'Ann.' xxiv), obtained by the barometric 



method. This is entirely at variance with Wiillner's statement (Pogg. 



' Ann.' ex.) that ^ — ^ is constant for all salts ; a statement not borne 



P 

 out by his figures, discordant as they are. 



(/3) When n is constant and temperature varying, then the value of 



P~P ^^ i,e., the restraining efiect of each salt molecule, is a diminishing 



np 

 quantity in the case of NaCl, practically constant with KCl, slowly in- 

 creasing with NaNOs, and rapidly increasing with KNO3, the order 

 being the same but reversed. This also is confirmed by Tammann, and 

 agrees with the results of Legrand ('Ann. Chim. et Phys.' 1835). 



(•y) When, too, the temperature and concentration increase, the salts 

 form the same series : decrease of restraining efiect with NaCl, less so 

 with KCl, no change with NaNOs, and a marked increase with KNO3. 



Connected with the above are : — 



(^) The order is the same when the solubility as a function of 

 the temperature is considered. NaCl has its solubility only slightly 

 affected by rise of temperature, KCl more so, NaNOs still more, and 

 KNO3 greatly so. 



(e) The value of ^ — ^, where n^l, is very nearly the same for all 



four salts at the same temperatures. 

 (^) The heat of solution for — 

 NaCl =-1-180 



NaN03= -5-200 



KCl =-4-400 

 KN03= -8-500 



Again the same series. 



The behaviour of these four salts can be satisfactorily explained on 

 the lines of the theory of solution laid down in a paper on the nature of 

 solution ('Phil. Mag.' 1883); but for the details reference must be 

 made to the memoir. 



II. Refractive Indices of Salt Solutions. 



The work of nearly all previous experimenters on this branch of 

 the subject of solution is unavailable for any systematic examination of 

 the point, inasmuch as but few salts have been examined, and only a 

 few solutions of each ; while even in these cases the results require re- 

 calculation, as the solutions examined were of percentage composition, 

 and the conversion of these into terms of even molecules of salt per 100 

 H2O is a laborious process, requiring a large amount of interpolation, 

 for which the data are generally insuQicient. 



Recently, however, a paper by Ostwald has come into our hands 

 (' Volum. u. Optisch-Chem. Studien,' Dorpat, 1878), which contains the 

 necessary data for a partial examination of the subject. Ostwald's ex- 



