Equivalents of Potash^ Soda, and Silver* 327 



The introduction of this change, however, would render 

 necessary a like change in the received atomic weight 

 of sodium, with the oxide of which in anhydrous sulphate 

 of soda (Thenardite) that of silver in sulphate of silver is 

 isomorphous. Soda, common salt, and sulphuret of sodium 



must be represented by Na, NaCl, Na. 



Nor can the change stop here. Several years have elapsed 

 since Mitscherlich announced the very interesting fact, that 

 the nitrates of potash and soda were isomorphous respectively 

 with arragonite and calc spar, and that they presented the 

 same cleavages (Pogg.Ann. xviii. p. 173.) ; to which Mar after- 

 wards added that the rhomboids of nitrate of soda possessed 

 the doubly refracting structure in a higher degree even than 

 calc spar [Jahrbuch dei- Chim.und Phys., xix. p. 165). From 

 these observations it was natural to infer that some relation 

 existed between the two alkaline nitrates analogous to the re- 

 lation between the two forms of carbonate of lime ; that like 

 carbonate of lime the nitrates of potash and soda might each 

 be capable of assuming two forms isomorphous each with each, 

 though in ordinary circumstances of temperature, &c. the class 

 of form preferred by each did not correspond ; the nitrate of 

 potash generally aiFecting the right rhombic prism, the nitrate 

 of soda the rhomboidal form. The probability of such a re- 

 lation was strengthened by a comparison of the analyses of 

 chabasie from different localities and by different chemists, 

 from which there appeared strong reason for believing that 

 potash and soda were capable of replacing each other in equi- 

 valent proportions. 



The alums however presented an anomaly. Nothing posi- 

 tive in regard to isomorphism could be inferred from potash 

 and soda occurring indifferently, and in equal atomic propor- 

 tions, in similarly constituted crystals of the regular octohe . 

 dral and cubical forms ; they might occur indifferently in such 

 crystals without being isomorphous : but supposing them 

 really to be isomorphous, there appeared an inconsistency in 

 the alleged existence of 26 equivalents of water in soda alum, 

 while potash alum contained only 24 atoms. This objection 

 has been lately removed by Professor Graham, (London and 

 Edinb. Phil. Mag., vol. ix. p. 26,) who has shown that soda 

 alum when perfectly dry contains only 24 atoms of water, and 

 that the potash and soda alums therefore have the same con- 

 stitution*. 



But all doubt has at length been removed from the relation 

 between the forms of potash and soda by a beautiful observa- 

 [• The identity of constitution of the potash and soda alums had been 

 previously shown we believe by Dr. T. Thomson. — ^Edit.] 



