2 4 An Ana lysis of Sea -water ; 



may, on the same principle, cause the reverse combinations to 

 take place, of muriate of magnesia and sulphate of soda ; the 

 quantity of this sulphate being, of course, equivalent to the quan- 

 tity 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 ex- 

 plained the diversity of results obtained by the two modes of 

 analysis ; and this diversity itself affords an excellent illustration 

 of the change of combination which may be produced in mineral 

 waters by analytic operations, and a very conclusive proof that 

 the substances obtained by the analysis are not always to be re- 

 garded as the original ingredients, since here they are varied ac- 

 cording to the mode in which the analysis is performed*. 



• The small portion of sulphate of soda obtained with the sulphate of 

 magnesia, in the second analysis, may have been formed by tlie action of 

 the alcohol, which, though employed much less extensively than in the first, 

 was still introduced to a certain extent. Or it might originate from other 

 circumstances independent of this : for a similar result, I have been informed, 

 sometimes occurs in the large way, sulphate of soda being procured in 

 boiling down the bittern of sea-water to obtain its sulphate of magnesia, or 

 in purifying this sulphate. 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 difterent salts, and their tendency to crystalli- 

 zation, 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 shown by the experi- 

 ments of BerthoUet and others. A striking proof of it was derived from 

 the very salts which are the subject of the present observations, in a singu- 

 lar case of affinity, first observed by Scheele, and afterwards confirmed by 

 Gren: that of muriate of magnesia and sulphate of soda, which decompose 

 each other in a concentrated solution at a high temperature, producing mu- 

 riate of soda and sulphate af magnesia ; but, at temperatures below 32% 

 the reverse effect takes place, muriate of soda and sulphate of magnesia 

 reacting, and being converted into sulphate of soda and muriate of mag- 

 nesia. This singidar case is evidently owing to the relation of the solubi- 

 lity of these salts to temperature. Muriate of soda has its solubility little 

 increased by heat, of course little diminished by cold ; sulphate of soda is 

 in this respect precisely tlie reverse : hence, at an elevated temperature, 

 muriate of soda is the less soluble salt ; and this determines its formation 

 and separation from a compound solution, containing its elements ; at a 

 low temperature, again, sulphate of soda is the less soluble salt; and thia 

 equally determines its formation, of course occasions the reverse decom- 

 positions. Now, according to the proportion of saline ingredients, and ac- 

 cording to the state of concentration, and the temperature favouring the 

 tendencj' of certain salts to crystallization more than others; it is easy to 

 conceive, that in a compound solution, different combinations may be esta- 

 blished, as these circumstances vary, and thus products may be obtained, 

 under certain conditions, which arc not obtained under others. Althoui>-h 

 sulpiiate of magnesia, therefore, is usually obtained by evaporation from 

 sea-wa^er, sulphate of soda, at some stages of the operation or under pecu- 

 liar circumstances, with regard either to relative quantity of the elements, 

 or to temperature, may likewise be formed. 



Lavoisier 



