393 



nearer the fusing point of either of the constituents. Thus TILDEN and 

 SHENSTONE (1) have shown that, taking different salts soluble in water, the 

 rate of increase of solubility with temperature above 100 C. stands 

 related to the melting point the lower the melting point of the salt 

 the more rapidly will its solubility increase with temperature. They also 

 show that benzoic acid which melts at 120 C. is miscible with water 

 in any proportion at a few degrees above this temperature. Such a 

 mixture may still be regarded as a solution, but it is no more correct 

 to say that the benzoic acid is dissolved in the water than it is to say 

 that the water is dissolved in the benzoic acid. Dr. GUTHRIE^ in one of his 

 most important papers on cryohydrates and eutectic mixtures also calls 

 attention to the fact that water and certain fused salts are miscible in all 

 proportions, and that an extremely small quantity of water has the effect 

 of lowering the fusing point of the salt. Thus 1*14 p.c. of water lowered 

 the fusing point of nitre from 320 C. to 300 C., i.e., no less than 20 C. 



The distinction between "dissolver" and "dissolved" also becomes 

 somewhat confused if we consider the phenomena accompanying the 

 solidification of solutions. Take the case of nitre and water. Dr. GUTHRIE 

 has shown that a solution containing 1T2 p.c. of nitre will solidify as a 

 whole at a temperature of 3 C. and he gives the name cryohydrate (3) 

 to the mixture containing this percentage of nitre. On cooling a solution 

 containing more than 11 '2 p.c. of nitre the salt crystallizes out until the 

 temperature of 3 C. is reached, when the mother liquor will be found 

 to contain 11'2 p.c. of nitre and to solidify at 3 C. On cooling a 

 solution containing less than 11 '2 p.c. ice crystallizes out, and when the 

 temperature of 3 C. is reached the mother liquor will be found to 

 contain the same percentage of salt as before. In short, take any aqueous 

 solution of nitre and cool it to 3 C. : the mother liquor at that 

 temperature will have a constant composition and will solidify at that 

 temperature provided heat can escape. We see, therefore, that between the 

 solidifying point of ice and the solidifying point of the cryohydrate the 

 order of crystallization of the two constituents does not depend upon their 

 fusibilities but on the relative proportions of the two constituents. 

 Which constituent is to be regarded as the solvent medium between C. 

 and 3 C. ? Shall we say that the nitre is dissolved in the water ? or the 

 ice in the nitre ? or would it be preferable to regard both salt and nitre, 

 when either are present in excess of the proportion existing in the cryo- 

 hydrate, as dissolved in the cryohydrate ? The best plan appears to be 

 to look upon solution as a liquid mixture of two or more compounds 

 without attempting to determine in every case which compound should 

 be regarded as the solvent medium. 



A solution is said to be saturated with any particular compound when 



(1) Phil. Trans., Vol. CLXXV. (1884), p. 36. 



(2) Phil. Mag., Vol. XVIII., 5th Se. (1884), p. 114. 



(3) Each salt soluble in water is capable of forming with water a crjohydrate. Thus 

 the cryohydrate of common salt contains 23 - 6 p.c. of salt and solidifies at - 5 C. See papers by 

 DE, GUTHEIE : Phil. Mag., 4th Series, Vol. XLIX. ; oth Series, Vols. I., VI., XVII. and XVIIT. 



