Aqueous Solutions of Acids and Alkalies. 451 



If, to the same quantity of salt, less water be taken, there 

 will be an expansion of the volume of the water ; but if more 

 water be taken, there will be a visible condensation. 



Messrs. Playfair and Joule place carbonate of soda among 

 the substances that dissolve in water without changing the vo- 

 lume of the water, and they give an experiment to prove its 

 possession of this property. (Phil. Magazine, xxviii. 506.) But 

 the experiment proves nothing beyond the fact mentioned 

 in the last paragraph. It shows a property characteristic of 

 a particular solution, but not a property manifested by the 

 salt in all its solutions. Instead of being a general character 

 it is only a speciality. 



I may perhaps be permitted to make here a passing remark 

 on the theory of the aqueous solution of hydrates advanced 

 by M. Gay-Lussac (Ann. Chim. Phys. t. lxx. p. 426) : — 



" As it is of interest to know whether a salt, susceptible of 

 forming a hydrate, dissolves in water hydrated or anhydrous, 

 I will cite a fact which appears to me sufficient to destroy the 

 uncertainty. It is, that whenever an anhydrous salt, or any 

 other body not having the property to form a hydrate, dis- 

 solves in water, there is always a production of cold, and that, 

 on the contrary, whenever the salt can form a hydrate, there 

 is a production of heat. When the hydrate is completed, 

 previous to the solution in w ? ater, the case is the same as when 

 the salt cannot form a hydrate. It is conceivable that it might 

 sometimes happen, that the heat produced by hydration was 

 less than the cold produced by the change of state, but I have 

 not yet met with an exception." 



The remark I wish to make on this passage is, that it is 

 fallacious in ascribing the production of heat and cold (mean- 

 ing thereby such as can be detected in the solutions by the 

 thermometer), each to a single reaction. There is, according 

 to my observations, always a double reaction, and it is for 

 want of noticing the double reaction that the " exceptions " 

 alluded to in the last sentence of the extract were never de- 

 tected. It is due to this eminent philosopher to offer suffi- 

 cient reasons for my objections to his opinions. 



When a test atom of anhydrous carbonate of soda is dis- 

 solved in water much heat is disengaged. 38*66 septems of 

 the salt combine with 160*7 septems of water, and produce 

 179*91 septems of hydrated carbonate of soda. Here there 

 is a loss of volume equal to 19*45 septems, and this condensa- 

 tion is the source of the heat produced. But there is another 

 reaction also to be taken into consideration : the 38*66 sep- 

 tems of dry salt, in combining with the 160*7 septems of 



