Super saturation of Salt- Solutions. 299 



pared from water and dehydrated salt without heating these 

 out of contact with the air ? and what effect has this heating ? 



There is no doubt that one, if not the sole, cause which 

 brings about crystallization of a supersaturated solution is the 

 presence of a crystal of the hydrated salt; and it is for this 

 reason that supersaturated solutions can exist only out of con- 

 tact with the air of the laboratory. This also is the reason 

 that the w T ater and anhydrous salt must be heated before they 

 are brought in contact ; for minute crystals of the solid hy- 

 drate may lie hid in the scratches in the neck of the bottle or 

 in the mass of dehydrated salt, and these must be heated above 

 33° before they are decomposed, and so lose their pow T er of 

 determining the union of the dehydrated salt and the water 

 with Avhich it is brought in contact. The fact that no hydra- 

 tion of the salt takes place before solution is conclusively 

 proved, not only by the formation of a supersaturated solution 

 which cannot exist in the presence of undissolved hydrate, 

 but also by the fact that the finely powdered dehydrated salt 

 dissolves at once ; or if excess be present, the portion remain- 

 ing undissolved retains its powdery form, no caking together 

 or change in appearance taking place. 



Finally, a very considerable rise of temperature attends the 

 solution of the salt quite equal to — so far as it is possible to 

 judge without quantitative determination of its amount — if not 

 actually greater than, that attending the solution of the dehy- 

 drated salt under ordinary conditions. Now r , as shown above, 

 there is no hydration, and the question remains — To w^hat is 

 this change of temperature due ? This cannot be fully answered 

 until our knowledge of the volume and temperature changes 

 attending the solution of the dehydrated salt is more complete 

 than at present. The probable explanation however is, I ven- 

 ture to suggest, to be found in the great contraction attending 

 the act of solution. According to the most reliable determi- 

 nations, the density of Na 2 S0 4 (solid) is 2-6618 at 20° C. 

 This gives the molecular volume 53*35. But, as shown in my 

 previous paper, the molecular volume of Na 2 S0 4 (dissolved) 

 in a solution containing 6*244 molecules of salt (that is, 49*26 

 per 100) is 32*79. A saturated solution, as shown above, 

 contains not more than 55*6 per 100, and the molecular volume 

 in such a solution cannot exceed 33*5. The contraction, then, 

 attending the solution of one gramme-molecule of Na 2 S0 4 is 

 53*4 — 33*5 = 19*9, or nearly 20 cubic centim., or about 40 per 

 cent. ; a quantity possibly sufficient to account for the thermal 

 change, exceeding, as it does, the volume-change on solution 

 of other anhydrous salts, which dissolve w T ith either a slight 

 rise or fall of temperature. For a further extension of this 

 argument reference must be made to the papers of Miiller- 



