494 Prof. OcUing on the Natural Groupings 



and consequently with mercury, — 



^?^^^!i = 5-63 5-43Boullay, 5-4Karsten; 



93 + 196 •" ' 



but according to Kopp, 



1693-75 



93 + 245 



= 5 01 5-14 Gmelin. 



It is observable that Kopp makes the oxide of silver to diflfer 

 from the oxides of lead and mercury, and the chloride of mer- 

 cury to differ from the chlorides of lead and silver. It is ques- 

 tionable how far, in a subject confessedly but approximative, 

 Kopp was justified in assigning different atomic volumes to the 

 same element when forming perfectly analogous compounds. 



Chloride of mercury is a somewhat insoluble salt, chloride of 

 lead still more insoluble, chloride of silver one of the most inso- 

 luble salts with which we are acquainted. All three chlorides, 

 however, are much more soluble in solutions of alkaline chlorides, 

 owing to the formation of crystallizable double salts. The mer- 

 curial double salts are well known. Wetzlar and Becquerel have 

 described a very definite chloride of silver and sodium. The sul- 

 phates, iodides, and most other protosalts of the three metals 

 show a marked resemblance in their characters. The metals of 

 this group are remarkable for the facility with which they replace 

 the hydrogen of neutral and faintly acid organic compounds. 

 The only marked discrepancy, in addition to the difference in 

 the atomic heats, consists in the varied behaviour of the oxides 

 and hydrates with solutions of caustic alkali; lead being soluble 

 in potash, silver in ammonia, and mercury in neither solvent. 



This group is associated — 



1. With the sodic group, by the analogy of sulphate of soda 

 to sulphate of silver, and by the circumstance of silver having 

 the same atomic heat as sodium and potassium. 



2. With the calcic group, by the very general isomorphism of 

 plumbic with calcic and barytic salts. 



3. With the ferric group, by the general analogy of cuprous 

 and mercurous salts. 



Moreover, we have several illustrations of a multiple isomoi'- 

 phism of the plumbic with the above three groups. 



Pb^S and Ag^S with Cu^S 



(Sulphide of lead.) Sulphide of silver. Subsulphide of copper. 



*Na NO^ with Ca^ CO^ and Ag^ Sb S^ 



Nitrate of soda. Calc-spar. • Red silver. 



* This last series of empirical formulEe illustrates curiously the'parfty of 

 relations subsisting between those monobasic, bibasic, and terbasic salts 

 which have the same number of oxygen atoms. That this isomorphism is 



