394 



a change in temperature (generally a lowering) causes the separation of 

 that compound. It is said to be super-saturated when it is cooled below 

 the temperature at which the compound would separate if crystals of that 

 compound were present. When once crystals start to grow in a super- 

 saturated solution they do so rapidly and the temperature rises. A liquid 

 may be over-cooled, and in this condition crystallization, when once 

 started, proceeds rapidly and the temperature rises to the normal 

 consolidating point. 



Dr. SORBY has shown that pressure decreases the solubility of those 

 salts which expand on dissolving. Hence, in solutions of such salts, increase 

 of pressure will act in the same way as decrease of temperature. Moreover, 

 it follows, as a consequence of the same principle, that a substance may 

 crystallize from a solution under great pressure at a higher temperature 

 than it would crystallize from the same solution under a low pressure ; 

 and, further, that relief of pressure may be followed by a solution or 

 corrosion of pre-existing crystals. 



A homogeneous solution remains homogeneous so long as the 

 temperature remains uniform, but a disturbance in the equilibrium of 

 temperature brings about heterogeneity in the solution. The compound 

 or compounds with which the solution is nearly saturated tend to 

 accumulate in the colder parts. This important principle has been estab- 

 lished by SORET (1) and may therefore be referred to as SORET'S principle. 

 Vertical tubes containing aqueous solutions of various salts in various 

 proportions were placed under such conditions that their upper portions 

 were maintained at a temperature of 80 and their lower portions at 20. 

 The experiments were continued for about fifty days. In every case the 

 same feature was observed there was always a motion of the salt towards 

 the colder parts ; or, in other words, the passage of heat through the 

 solution was accompanied by a passage of salt in the same direction. 



Reference has already been made to Dr. GUTHRIE'S researches on 

 cryohydrates. It follows from these researches that each salt which is 

 soluble in water is capable of forming with water a mixture (or compound) 

 which possesses a constant solidifying point and a constant composition. 

 An important question arises as to the nature of these cryohydrates. Are 

 they definite chemical compounds or are they simply mixtures of definite 

 compounds which have a fixed melting point ? Some light may be thrown 

 on this point by a microscopic examination of the corresponding mixtures 

 of certain anhydrous salts. 



Nitre fuses at 320; nitrate of lead is infusible without decom- 

 position. Nitrate of lead is soluble in liquid nitre, just as salt is 

 soluble in water. If we mix nitrate of lead and nitre in the propor- 

 tion of 46*86 of the former to 53'14 of the latter, the solution may be 

 cooled to 207 C. without the separation of either of the constituents/ 2 ) 

 At this temperature the whole mass will consolidate, provided the 



(1) Ann. Chim Phy. Paris, 1881 (5) 22, p. 293. 



(2) GUTIIBIE. Phil. Ma^., 5th Series, Vol. XVII., p. 470. 



