528 Prof. Marignac an Messrs. Playfair and Joule's Memoir 



these salts in the solid state, but also their volume in the state 

 of solution, that is to say, the increase in volume which the 

 water in which they are dissolved undergoes. Their me- 

 thod consists in introducing a determined weight of salt into 

 water contained in a glass bulb, capable of holding 1000 to 

 4000 grains of water, surmounted by a graduated stem, in 

 which they measure the increase in volume of the water after 

 the solution of the salt. The same apparatus served to mea- 

 sure the volume of the salts in the solid state, either by employ- 

 ing a saturated solution of the salts or a liquid in which they 

 were insoluble, such as the oil of turpentine. 



We shall examine successively the results relative to the 

 volumes occupied by the salts in a state of solution and in a 

 solid state. 



In 1840 Dalton observed that sugar and certain hydrated 

 salts, on solution in water, increased its volume by a quantity 

 precisely equal to the volume of water they themselves con- 

 tained. He generalised this observation, extending it to all the 

 hydrated salts, and he thence concluded that the anhydrous 

 salts did not increase the volume of the water in which, they 

 were dissolved. 



Messrs. Playfair and Joule confirmed this interesting re- 

 sult for a sufficiently large number of salts, but not for all*. 

 Those which obey this law are in general the salts containing 

 a large proportion of water of crystallization, as for in- 

 stance — 



The sulphates of the magnesian group with 5, 6 or 7 equiv. water 



The chlorides of calcium, strontium and magnesium with... G equiv. water 



The alumsf with 24 equiv. water 



The phosphates or arseniates of soda, neutral or basic, with 24 equiv. water 

 The carbonate, borate, sulphate, pyrophosphate of soda with 10 equiv. water 

 The sulphate of alumina with 18 equiv. water 



Lastly, cane-sugar, considering as water the 1 1 equiv. oxy- 

 gen and hydrogen which it contains. 



For all these compounds the increase in volume of the 

 water in which they are dissolved is precisely equal to the 

 volume of water they contain ; so that if these salts be used 

 in the anhydrous state, they dissolve in the water without 

 causing any change of volume. 



Several other salts, both anhydrous and hydrated, follow 

 different laws. Messrs. Playfair and Joule advance the fol- 



* This had been previously done to a certain extent by Mr. Holker ; see 

 his paper in vol. xxvii. p. 207 of this Journal. — Ed. Phil. Mag. 



t There is however an exception, for the ammoniacal alums possess a 

 volume equal to 25 equiv. water and not 24 like the potash alums, although 

 they contain like them only 24 equiv. water of crystallization. 



