196 



MR J. Y. BUCHANAN ON THE 



fluctuations in the results of the single observations of each series which indicates a 

 condition of unrest in the solution, which is most accentuated in the fifth series of 

 observations, during which the specific gravity fell from 1 "445719 to 1 "444886. The 

 most remarkable feature of these changes is that they occurred without being ac- 

 companied by any change in the temperature of the solution. Confirmation of the 

 occurrence of this remarkable condition of unrest was furnished by a repetition of the 

 experiment, made with a solution containing 7 "196 gram-molecules CaClg per 1000 

 grams HgO, recorded in Table lie, and in it similar fluctuations of density, although 

 not quite so pronounced, occurred as the forerunner of crystallisation. When the 

 temperature of the solution had been observed after completion of the third series, the 

 side of the cylinder was accidentally rubbed by the thermometer and crystallisation took 

 place; but the behaviour of this solution and that of the 7"225 CaClz solution, in the 

 case of the first three series, exhibit similar features of unrest. 



§ 114. It is interesting to inquire into the nature of the changes of displacement 

 which have occurred in the transition of the solution from a condition of supersaturation 

 to that of a mixture of saturated solution and crystals at the same temperature, 19 "5° C. 

 They are clearly shown in the accompanying table. 



We commence with 806"61 grams of a solution having a specific gravity 1-444886, 

 the displacement of this weight of solution being therefore .558 "25 grams, and this re- 

 solves itself into 596"3 grams of mother-liquor with a specific gravity of 1 "423500, giving 

 a displacement of 418'89 grams, and 210"3 grams of CaCl26H20 crystals with a specific 

 gravity of 1*654 (Landolt's Tables), giving a displacement of 127*15 grams, from which 

 we see that the displacement of the mixture of crystals and mother-liquor is 12*21 grams, 

 or 2 "2 per cent less than that of the original volume of supersaturated solution. 



The following table shows this clearly : — 





Weight in 

 Grams. 



Displacement in 

 Grams of Water. 



Volume in c.c. 

 at 19-5° C. 



Percentage 

 Displacement. 



Original solution 



Mother-liquor . 

 CaClgeHgO 



Total 



Diflference . 



806-6 



558-25 



558-75 



10000 



596-3 

 210-3 



418-89 

 127-15 



419-48 

 127-36 



75-03 

 22-83 



806-6 



546-04 



546-84 



97-86 



... 



12-21 



11-91 



2-14 



§ 11 5. When we compare the behaviour of the chlorides of magnesium and calcium 

 in supersaturated solution, it seems strange that the salt which has the greater amount 

 of heat to lose by crystallising should be the more difiicult to bring to crystallisation. 

 The difficulty, however, lies only in the starting of crystallisation ; there is none in its 

 continuation. To start crystallisation in any solution, no matter how supersaturated 



