1820.] Solubility of the Salts in Water. 3 



temperature of 25° (77° Fahr.) I placed at the same time m 

 the same place two flagons, one containing crystals of nitre, the 

 other crystals of sulphate of soda, into which I poured water of 

 the temperature of 8° (46-5° Fahr.) ; so that the liquid did not 

 cover the salt. After an interval of a fortni^iit, 1 evaporated 

 known quantities of each solution, and obtamed the following 

 results : 



Temperature of the cavern 11-67° (53° Fahr.) 



Saturated solution of nitre, 



By cooUng, 100 water contain 22*24 salt 



By simple contact 22*22 



Saturated solution of sulphate of soda, 



By cooling, 100 water contain 10-11 salt 



By simple contact lU* 14 



Other experiments made by the processes which I have 

 usually employed to form saturated solutions gave me results 

 whose differences, in general very small, were sometiuies on one 

 side and sometimes on the other. I admit then as a certain fact 

 that water for a determinate temperature comes to ti!e same 

 degree of saturation, either by allowmg the excess of salt which 

 it contains in solution to precipitate by cooling, or by imme- 

 diately dissolving the same salt, provided it remain for a sufficient 

 time in contact with it. This result might have been foreseen, 

 for the circumstances in both cases are rigorously the same. 

 I may likevvise remark, that the volume of crystals which fonn in 

 a solution, or which are put into it, has no sensible intiuence on 

 the term of saturation. This is the consequence of the nature 

 of chemical affinity which -iicts only at distances infinitely small. 



Yet Dr. Thomson found that water retains more oxide of 

 arsenic, when saturated by cooling, than when put in contact 

 with the oxide without any elevation of temperature ; but the 

 reason I am persuaded was, that he employed too little oxide of 

 arsenic relatively to the water, and that he did not prolong the 

 contact sufficiently. We perceive in fact, on a little reflection, 

 that saturation follows in its progress a decreasing geometrical 

 progression, and that the time necessary for completing it 

 depends upon the surface of contact of the solvent and the body 

 to be dissolved. 



It happens often that the solution of a salt which does not 

 crj'^stalh/e, and which, for that reason, we consider as saturated, 

 yields sahne molecules to the crystals of the same nature plunged 

 mto it ; and it has been concluded from this, that the crystals of 

 a salt impoverish a solution, and make it sink below its true point 

 of saturation. The fact is certain ; it is even very general ; but 

 I am of opinion that it has been ill explained. 



Saturation in a sahne solution of an invariable temperatiue is 



A 2 



