178 MR J. Y. BUCHANAN ON THE 



Section XIV. — The Specific Gravity and the Displacement of Solutions 

 OF THE Chlorides of Beryllium, Magnesium, and Calcium. 



§ 98. For the purpose of determiniDg these constants, hydrometric observations 

 were made on solutions of the three salts the concentrations of which ranged from 1/2 to 

 1/1024 gram -molecule per thousand grams of water, the experiments being made with 

 the closed hydrometers Nos. 3 and 17. Experiments were also made on strong solutions 

 of calcium chloride and magnesium chloride, using the open hydrometers A and B ; 

 the concentrations of these solutions varied from 1 gram-molecule per thousand grams 

 of water to the highest attainable degree of supersaturation. It was when the experi- 

 ments on a supersaturated solution of calcium chloride were in progress that the 

 observations were made which revealed the remarkable state of unrest in that solution 

 which preceded its partition into crystals and mother-liquor with liberation of heat. 

 The details of this experiment are given in Section XV., and from them it will be 

 seen that the range of supersaturation which can be explored hydrometrically when the 

 salt in solution is chloride of calcium is considerable. The solution of magnesium 

 chloride which is saturated at 19"5°C. contains 5*918 gram-molecules (564*123 grams) 

 of MgClg in 1000 grams of water. A supersaturated solution, containing 5*982 gram- 

 molecules of salt per thousand grams of water was cooled to 16'5° C, at which tempera- 

 ture the saturated solution contains 5*853 MgClg per thousand grams of water; yet, 

 with this small degree of supersaturation, the slightest disturbance, such as lifting the 

 beaker, induced crystallisation in the solution. This shows that the limits of super- 

 saturation are restricted, and that it would certainly be discharged by an attempt to 

 make hydrometric observations in the solution. 



This difference in the behaviour of the supersaturated solutions of these two salts is 

 interesting. On the one hand we have the calcium chloride, which produces a high 

 degree of supersaturation with great absorption of heat, and offers great resistance to 

 crystallisation ; while magnesium chloride can produce solutions attaining only to a 

 moderate degree of supersaturation with very moderate absorption of heat, and the salt 

 crystallises from such solutions on the slightest provocation. With a view to a com- 

 parison with the thermal behaviour of chloride of calcium, some observations were made 

 on the heat of solution of magnesium chloride in water, while experiments were being 

 made to determine the concentration of solutions saturated with the salt at different 

 temperatures. It was found that when the quantities of the crystallised salt 

 MgClgjBHgO and water used were such as to produce a solution the concentration of 

 which was about 2*0 gram-molecules of MgClg per thousand grams of water, the 

 dissolution of the salt was accompanied by an appreciable liberation of heat. When 

 the conditions of the experiment were such that a saturated solution was formed, some 

 of the crystals remaining undissolved, the dissolution of the salt was accompanied by 

 absorption of heat. When the saturated solution was produced by fractions, it was 

 observed that during the dissolution of the first fraction the thermometer indicated a 



