530 Prof. Marignac on Messrs. Playfair and Joule's Memoir 



When it is seen that the atomic volume of sulphate of pot- 

 ash and magnesia may vary more than 10 between 0° and 

 27°} it will readily be conceived that it will never be a diffi- 

 cult task to make the volume of each compound an exact 

 multiple of 9; for this purpose it is merely requisite to make 

 the experiment at a suitable temperature. 



The relative proportions of water and of salt likewise cause 

 the volume occupied by the latter in solution to vary. Messrs. 

 Playfair and Joule are fully aware of this; they made experi- 

 ments on the very subject, and proved this influence by the 

 following results relative to the volume occupied by an equi- 

 valent of sugar in solution in different quantities of water: — 



Relative proportions of , P Volume of one equivalent 



1 \ . Temperature. c ■ , v 



sugar and water. * ot sugar in solution. 



1 : 120 15°-5 99 



1 : 10 11 105-89 



1:1 11 107'01 



3:1 11 108-06 



The difference between the first number and the following 

 would have been still greater if the experiments had been 

 made at the same temperature. 



What then are the relative proportions of salt and water 

 which should be employed in order that the results may be 

 capable of comparison ? We know not, and the authors leave 

 us in total ignorance of the subject ; we only find that in their 

 experiments they have not restricted themselves to uniform 

 conditions j thus they employed 



Salt. Water. Proportion. 



for the sulphate of alumina and potash 59 1000 1 : 17 



of iron and ammonia 30-06 1000 1: 33 



of oxide of chrome and potash ... 32 4100 1 : 129 

 of alumina and ammonia 20 4100 1 : 205 



Evidently, if under such circumstances they have found an 

 agreement between the volumes of these salts, it may be con- 

 cluded that had they operated under uniform conditions they 

 would not have found the least trace of one. In short, the 

 atomic volume of a salt in solution depends both on the tem- 

 perature and the relative proportions of water and salt, and 

 these circumstances cause the volume to vary within consider- 

 able limits. As long as these influences are neglected, or we 

 do not operate so that they act in all cases in the same manner, 

 the results obtained cannot be compared in any possible way 

 with one another. 



Let us now pass to the atomic volume of the salts in the 

 solid state. Messrs. Playfair and Joule have advanced for 

 these the following law : — The atomic volume of any salt 



