432 Prof. Odling on the Natural Qroupings 



of replacing three atoms of hydrogen to form a salt. Thus we 

 are acquainted with a sulphate and potassio-tartrate of arsenious 

 acid, &c. From even very dilute solutions of the salts of the 

 three teroxides, the metal of each is precipitated most completely, 

 at a boiling temperature, by the introduction of metallic copper, 

 in a manner extremely characteristic. The teroxide of nitrogen 

 is a very volatile liquid, that of phosphorus a very volatile solid, 

 that of arsenic a less volatile solid, that of antimony a solid vola- 

 tile only at a full red heat, that of bismuth a fixed solid, or at 

 any rate one that requires for its volatilization an extremely high 

 temperature. The isomorphism of the four lower members of 

 this group is well marked. The phosphates and arseniates are 

 strictlv isomorphous. The arsenious and antimonious anhydrides 

 are iso-dimorphous. Tlie sulphides of antimony and bismuth are 

 isomorphous. 



The metallic character is most decided in bismuth, is some- 

 what less marked in antimony, is scarcely acknowledged in 

 arsenic, and vanishes in phosphorus. 



The members of this group are associated with the members 

 of the second group by the circumstance, that although their 

 characteristic tendency is to unite with three atoms of hydrogen, 

 or chlorine, or metal, yet they also have the property of uniting 

 with two atoms of metal to form compounds analogous to those 

 of the second group. 



Fe^ S'2* . Spirkise, isomorphous with Fe^ SAs. 



Fe^ S^ . regular system isomorphous with Co^ SAs. 



l^.gq > isomorphous with Ni-As. 



C2 02 analogous to C^ N^ ? 



Nevertheless the bodies H'^N, or amidogen, and Me^As, or 

 kakodyle, are not in any respect analogous to H-0, or water, but 

 bear the same relation thereto that HO, or peroxide of hydrogen, 

 bears to HCl, or chlorhydric acid. We have three typical two- 

 volume molecules, namely HCl, H^O, and H^N. If from each 

 of these we abstract an atom of hydrogen, we obtain the one- 

 volume radicals, CI, HO, and H^N (or Me^As), which in the 

 free state, however, must be repi-esented as the two-volume mole- 

 cules, CI-, H^ 0^, and H'* N'^. The correspondence in specific 

 heat between the first, second, and third groups has been already 

 indicated. Arsenic, antimony, and tellurium are isomorphous. 



The members of the third group are also associated with the 

 members of the first by certain properties in common. We have, 

 indeed, no compound HN or MN analogous to HCl or MCI; 

 but we have numerous bodies which may be regarded as oxidized 



* Graham's ' Elements of Chemistry,' 2nd edit. p. 172. 



