560 Intelligence and Miscellaneous Articles. 



ON THE FREEZING OF SALINE SOLUTIONS. BY DR. RUDORFF. 



The author showed in a previous communication that pure ice is 

 formed by the freezing of saline solutions, and that some salts affect 

 the freezing-point of the water in which they are dissolved as an- 

 hydrous salts, others as salts containing* water. M. Dufour tried to 

 prove, in a paper which appeared simultaneously, that the ice 

 which separates from a saline solution contains salt which separates 

 in the solid form at the same time as the ice. The well-known fact, 

 that the proportion of salt contained in ice so formed is always 

 smaller than that contained in the original saline solution, and that 

 it is smaller the slower has been the formation of the ice, is explained 

 by M. Dufour by supposing that the remaining solution redissolves 

 the salt which separates out with the ice. The following experi- 

 ments aiford, however, an additional proof that the salt contained in 

 ice produced in this way is due to a portion of the solution adhering 

 to it or being enclosed by it. 



It is well known that a solution of the magnificent dichroic 

 double salt, the platinocyanide of magnesium, is perfectly colour- 

 less. If such a solution is allowed to freeze, the resulting ice is 

 equally free from colour. At last, when the quantity of water 

 separated in the form of ice is so great that the remainder no longer 

 suffices to retain the whole quantity of salt in solution, the ice 

 begins to get coloured red and green from the separation of crystals 

 of salt. 



The following experiment proves in a still more striking manner 

 that salt does not separate out from a saline solution in a solid form 

 at the same time as ice. As stated in the author's former paper, 

 all saline solutions may be cooled below their freezing-point without 

 the formation of ice taking place, such over-cooled solutions being 

 thus analogous to supersaturated solutions of salts. A bit of ice 

 causes the formation of ice in an over-cooled solution, just as a crystal 

 of the dissolved salt causes the salt to separate from the supersatu- 

 rated solution. 



Now, with due care, it is possible to cool a supersaturated solu- 

 tion of sulphate of soda below its freezing-point ; and in such a 

 supersaturated and over-cooled solution, a bit of ice causes the 

 separation of ice only and not of salt, while a crystal of sulphate of 

 soda causes the salt alone and no ice to separate out, as may be 

 easily seen by the ice floating at the top of the solution, whereas 

 the sulphate of soda quickly sinks to the bottom. If both ice and 

 salt are caused to solidify simultaneously, they separate from each 

 other in this way. If the smallest trace of solid salt were to separate 

 on throwing a morsel of ice into the solution, it would occasion the 

 crystallization of the whole quantity of salt with which the solution 

 is supersaturated. 



The author has determined the influence of a great number of 

 salts on the freezing-point of the water in which they are dissolved. 

 For this purpose he operated in the same way as in his former ex- 

 periments • that is, he cooled the solutions examined a few tenths of 



