OF SEA- WATER. 



225 



solid mass is submitted to the action of alcohol, its operation, 

 as a solvent, may, on the same principle, cause the reverse 

 combinations to take place, of muriate of magnesia and sul- 

 phate of soda ; the quantity of this sulphate being, of course, 

 equivalent to the quantity of sulphate of magnesia ; and the 

 quantity of muriate of magnesia formed, being added to the 

 quantity of that salt which the sea-water contains as a primary 

 ingredient. Thus is explained the diversity of results obtain- 

 ed by the two modes of analysis ; and this diversity itself af- 

 fords an excellent illustration of the change of combination 

 which may be produced in mineral waters by analytic opera- 

 tions, and a very conclusive proof that the substances obtained 

 by the analysis are not always to be regarded as the original 

 ingredients, since here they are varied according to the mode 

 in which the analysis is performed *. 



Vol. VIII. P. I. F f Lavoisier 



* The small portion of sulphate of soda obtained with the sulphate of mag- 

 nesia, in the second analysis, may have been formed by the action of the alcohol, 

 which, though employed much less extensively than in the first, was still intro- 

 duced to a certain extent. Or it might originate from other circumstances in- 

 dependent of this ; for a similar result, I have been informed, sometimes oc- 

 curs in the large way, sulphate of soda being procured in boiling down the bit- 

 tern of sea- water to obtain its sulphate of magnesia, or in purifying this sul- 

 phate. The circumstances on which this depends, it may be difficult to assign 

 with perfect precision ; but it probably arises from the relative quantities of the 

 different salts, and their tendency to crystallization, as influenced by the state of 

 concentration, and the temperature. That both of these have a considerable 

 effect on the combinations established in a compound saline solution, has been 

 sufficiently shewn by the experiments of Berthollet and others. A striking 

 proof of it was derived from the very salts which are the subject of the present 

 observations, in a singular case of affinity, first observed by Sciieele, and af- 

 terwards confirmed by Gren : that of muriate of magnesia and sulphate of so- 

 da, which decompose each other in a concentrated solution at a high tempe- 

 rature, producing muriate of soda and sulphate of magnesia ; but, at tempe- 

 ratures below 32°, the reverse effect takes place, muriate of soda and sulphate 



of 



