SPECIFIC GRAVITY AND DISPLACEMENT OF SOME SALINE SOLUTIONS. 185 



Section XV. — On a Remarkable State of Unrest in a Supersaturated 

 Solution of Calcium Chloride before Crystallising. 



§ 104. The primary purpose for which the open hydrometer was designed was to 

 investigate the specific gravity and the displacement of solutions having concentra- 

 tions in the neighbourhood of that of saturation. In § 90 we have seen the satisfactory 

 result of experiments made for this purpose with solutions of chloride of calcium 

 containing 6-3 grm.-mols. CaCla per 1000 grams of water. Experiments in the same 

 direction were made with solutions of chloride of calcium of still higher concentration. 

 A parent solution was made, which, on the basis of published data relative to the 

 solubility of the salt, should be supersaturated at 19 "5° C. With it, it was intended to 

 produce the solution saturated at this temperature, and to study its specific gravity 

 and that of solutions formed by diluting it with small quantities of water. 



As the solution showed no inclination to crystallise, although every opportunity was 

 offered to it to do so, it seemed to me to be an example of a supersaturated solution 

 peculiarly adapted to closer study. 



Its composition was determined, and it was found to contain 7 '225 grm.-mols. of 

 chloride of calcium dissolved in 1 000 grams of water. Its specific gravity was determined 

 with the hydrometer exactly as if it had been a non-saturated solution. Two hydro- 

 meters were used for this purpose. They are designated A and B respectively. That 

 designated A is the hydrometer whose constants have been set out in detail in 

 Section XL The hydrometers were made at different dates and on different specifica- 

 tions, though possessing the same general characteristics. 



In Table I. are given the constants of both these instruments when loaded so as to 

 float with small added weight, (a) in distilled water, and (h) in a supersaturated solution 

 of calcium chloride, respectively. The entries opposite " weight of glass " and " weight 

 of lead shot " in these tables are the approximate weights in air of these substances, 

 which are required for the estimation of the corresponding volumes. The entries 

 opposite "weight of the loaded hydrometer" are the exact weights, as in vacuo, of the 

 glass -f- lead forming part of each instrument. To each of these weights has to be 

 added that of the air contained in the instrument. The external volume of the 

 hydrometer is independent of the internal load which it carries. It is entered only in 

 (a), line 6. The " internal space occupied by air" is arrived at by subtracting the sum 

 of the volumes of glass and of lead from the external volume of each hydrometer 

 respectively. The mass of the air which fills this space depends not only on the 

 volume of that space, but also on the density of the air which forms the atmosphere of 

 the laboratory at the time of the experiment. 



trans. ROY. see. EDIN., VOL. XLIX., PART I. (NO. 1). 24 



