BETTVEEN THE ATOMIC -WEIGHTS. 247 



atomic weight favors this hypothesis. Vanadate of Lead has been considered isomor- 

 phous with the Phosphate; but as this isomorphism does not rest on any measurement 

 of angles, and as, moreover, the received symbols of the two minerals, Vandinite and 

 Pyromorphite, on whose crystalline forms the isomorphism was detennined, show a 

 very different constitution, I have not given much weight to this fact* The observed 

 atomic weights of the members of this series are almost precisely the same as the the- 

 oretical members, and, with the exception, perhaps, of Molybdenum, there appears 

 to be no instance in which the difference is greater than the amount of possible error. 



The members of the Six Group form a well-characterized family, so that, with the ex- 

 ception of Oxygen, there can be no doubt in regard to the justice of classLfjing them 

 together, and any discrepancies will disappear on considering the group in the light of 

 a series. They form acids containing three and five atoms of Oxygen which are com- 

 pletely homologous, and make two series parallel to that of the elements. They form 

 also a remarkable series of compounds with three atoms of Hydrogen. The idea which 

 has been advanced by some authors, that NH3 is the Nitride of Hydrogen, whUe PH3 is 

 the Hyduret of Phosphorus, or, in other words, that Hydrogen is electro-positive with 

 reference to Nitrogen and electro-negative with reference to Phosphorus and those 

 lower in the series, does not seem to me correct, since the remarkable bases which may 

 be formed from PH3, ASH3, SbHg, and BiHg, by replacing the Hydrogen atoms by or- 

 ganic radicals, seem to indicate that they have the same type as NH3, and are therefore 

 homologues of it. 



The isomorphism of the four lower members of the series is perfect. It has been 

 shown in the table, both by the ciystalline forms of the elements themselves, as well as 

 by those of their coumpounds. In the other series, wherever it was possible, the same 

 double proof has been given. The doubt expressed by G. Rose in regard to the dimor- 

 phism of Arsenic, as I hope to be able to show in a paper soon to be published, has 

 been removed. In one state Arsenic crystallizes in perfect octahedrons of the regular 

 system, and is therefore isomorphous, not only with Antimony and Bismuth, but also, 

 in its allotropic state, with Phosphorus. Isomorphism, as is well known, is not abso- 

 lute, except in fonns of the regular system. The rhombic angles of the crystals of 

 Arsenic, Antimony, and Bismuth are respectively 85° 41', 81° 35', 87° 40', and there- 

 fore conform to the general riile. It will be observed that the angle varies constantly in 

 the same way as we descend in the series. Now, although these few instances do not 

 afford sufficient ground for any general conclusion, yet they show that similar varia- 



* See G. Rose's Mineral System. 



