200 Scientific Intelligence — Chemistry and the Arts. 



Coins. — Professor Hausmann has published in Karsten's Archiv 

 (vol. xviii. p. 505), a curious account of the chemical examination of 

 ancient coins, by Mr Briiel of Hanover. The most remarkable re- 

 sult of the investigation is the discovery of chloride and bromide of 

 silver in most of the coins analysed. The largest quantity of these 

 substances was met with in Greek coins, and in Saxon coins of the 

 thirteenth century, but they were also found in Roman coins. Briiel 

 expresses no opinion as to the origin of the chloride and bromide of 

 silver, but seems inclined to the probable belief, that the chlorine 

 and bromine were not originally contained in the coins, but that 

 they were derived from without. In order, however, to elucidate 

 the subject, it would be desirable to analyse ancient coins which 

 had not been buried in the earth. 



32. The economical extraction of Sulphate of Soda from Sea-Water. 

 — On the 7th October 1844:, M. Balard read a paper to the French Aca- 

 demy of Sciences on the means of extracting from sea-water the sulphate 

 of soda, in suiRcient quantity for all the purposes of commerce, without 

 having recourse to the present expensive process. Hitherto it has been 

 found impracticable to obtain the sulphate of soda from sea-water in 

 abundance ; but M. Balard calculates theoretically, that from an evapo- 

 rating surface of 200 hectares (about 500 English acres), 2,500,000 

 kilogrammes might be produced in one year, although practically the 

 amount would not be more than 600,000 kilogrammes. We extract 

 from the paper of ^I. Balard, a passage shewing the causes which 

 have hitherto prevented such results, and his means of remedy : — " When 

 two salts differ in their acid and their base, a double decomposition is 

 possible, and the presence of one salt may favour the solubility of the 

 other. When two salts have, on the contrary, the same acid or the same 

 base, and the double decomposition is no longer possible, the same pheno- 

 menon does not take place. The solubility of one of the salts is diminished 

 by the presence of the other, except in the case of the formation of a 

 double salt. Thus, the hydrochlorate of magnesia impedes the solubility 

 of sea-salt, because it is an hydrochlorate, and that of the sulphate of 

 magnesia, because it is a salt of magnesia. It favours, on the contrary, 

 the solubility of sulphate of soda, because, probably, in this case, the 

 double decomposition takes place. The solubility of the sulphate of soda 

 is, on the contrary, diminished by the presence of sea- salt, because it is 

 a salt of soda. The practical conclusion is simple. Since the hydro- 

 chlorate of magnesia impedes the solubility of the sulphate of magnesia 

 and of the sea-salt, between which the decomposition is to be effected ; 

 and, on the contrary, favours the solubility of the sulphate of soda which 

 is to be precipitated, it must be removed. Since, on the other hand, 

 sea-salt impedes the solubility of sulphate of soda, and favours the pre- 

 cipitation of the product to be isolated, it must be added. To extract 

 from the mother- water the sulphate of magnesia, to eliminate the chloride 



