480 



Prof. Odling on the Natural Groupings 



two-volume molecules. Bismuth is not known to form a hydro- 

 genized compound, but the first four members of this group are 

 distinguished from all other elements by their capability of form- 

 ing terhydrides. All these terhydi'idea have an alkaline rather 

 than an acid character. In the case of nitrogen and hydrogen, 

 the formation of the compound NH^ is attended with a conden- 

 sation to the amount of one-half. Phosphorus, arsenic, and 

 antimony correspond in their general properties, and present a 

 marked gradation of character. Nitrogen seems to bear the 

 same relation to the central members of this group, that fluorine 

 and oxygen have respectively to the other members of the first 

 and second groups. The relation of bismuth is analogous but 

 inverse to that of nitrogen. The atomic weights of phosphorus, 

 arsenic, and antimony occur in sequence. Thus — 



P = 31 

 As =: 75 

 Sb = 120 



Sum 226 



226 



= r5-33 



Mean difi"erence = 44 ? 



The atomic weight of nitrogen, or 14, is rather less than half 

 that of phosphorus, while the atomic weight of antimony is more 

 than half that of bismuth, or 213, to precisely the same extent, 



14x2 

 31 



213 

 120 X 2' 



or at least the difference is only equal to '0153. The specific 

 heat of the atoms of phosphorus, arsenic, antimony, and bismuth 

 respectively, is twice the usual specific heat of an atom of metal. 

 In the terhydrides, including a hypothetical terhydride of bis- 

 muth, we find that one, two, o*; three atoms of hydrogen may be 

 replaced by a metal, by chlorine, or by certain organic compounds 

 which in combination play the part of hydrogen. Thus we 

 have, — 



