376 Dr. W. W. J. Nicol on the Mutual 



The salts hitherto examined are not very numerous, but are 

 typical examples of two important classes, the chlorides and 

 nitrates. Indeed, it was found that only a limited number of 

 salts were available for this form of experiment, inasmuch as 

 all hydrated salts are unsuited, for the following reason. If a 

 hydrated salt be added to a solvent solution of a definite 

 strength, the quantity of water present is increased by the 

 amount present in the quantity of the hydrated salt that is 

 dissolved. What part this water plays is still a moot point. 

 Does it remain attached to the salt, or does it become part of 

 the solvent ? Until this question is settled, experiments with 

 such salts only complicate a problem already sufficiently dif- 

 ficult. Nor can this complication be avoided by dehydrating 

 the salt before adding it to the solvent solution ; no water 

 is indeed added to the solution, but the other objection men- 

 tioned above still remains ; and we are met with a still 

 greater difficulty in addition — the excess of dehydrated salt 

 that is perforce added removes an unknown quantity of 

 water from the solution, and the composition of the solvent 

 solution is altered. Such an alteration is not of course fatal, 

 but necessitates separate determinations of the amount of the 

 salts present, involving an amount of labour which it would 

 be foolish to undertake until it is seen from experiments with 

 anhydrous salts that the method of investigation is likely to 

 yield valuable results. 



The salts examined were the chlorides of potassium and 

 sodium and the nitrates of the same two metals. These salts 

 present special advantages for this and all other solution expe- 

 riments. They can be easily obtained pure. They crystallize 

 from solution in the anhydrous state, and are not known to 

 form hydrates except at low temperatures. The physical 

 properties of their solutions have been more fully studied than 

 those of any other salts. Their solubility presents no abnor- 

 malities on rise of temperature. They differ greatly in amount 

 of their solubility at the ordinary temperature, and still more 

 markedly in the effect of temperature on the amount dissolved. 

 Above all, they are all sufficiently soluble in water at 20° C. 

 to render any small experimental error of little consequence. 

 These four salts were arranged, for the purposes of expe- 

 riment, in two series of four pairs, the two series being 

 complimentary the one to the other. 



(1) KC1 in NaCl. (2) NaCl in KC1. 



(3) KC1 in KN0 3 . (4) KNO3 in KC1. 



(5) NaN0 3 in NaCl. (6) NaCl in NaN0 3 . 



(7) NaN0 3 in KN0 3 . (8) KM) 3 in NaN0 3 . 



