- 
between the Atomic Weights. 399 
The members of the Six Group form a well characterized fam- 
ily, so that, with the exception of oxygen, there can be no doubt 
in regard to the justice of classifying them together, and any 
discrepancies will disappear on considering the group in the light 
of aseries. They form acids containing three and five atoms of 
oxygen which are completely homologous, and make two series 
parallel to that of the elements. hey form also a remarkable 
series of compounds with three atoms of hydrogen. ‘The idea 
which has been advanced by some authors, that NH: is the ni- 
trid of hydrogen, while PHs is the hydruret 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 PHs, AsHs, SbHs, 
and BiH, by replacing the hydrogen atoms by organic radicals, 
seem to indicate that they have the same type as NHs, 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 crystalline 
forms of the elements themselves, as well as by those of their 
compounds. In the other series, wherever it was possible, the 
sane double proof has been given. The doubt expressed by G. 
Rose in regard to the dimorphism of arsenic, as I hope to be able 
to show ina 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. 
morphism, as is well known, is not absolute, except in forms of 
the regular system. The rhombic angles of the crystals of ar- 
senic, antimony, and bismuth, are respectively, 85° 41’, 87° 35/, 
87° 40), and therefore conform to the general rule. It will be 
course, the amount of possible error. The deviation in the case 
of phosphorus has already been noticed. Oxygen, it must be 
admitted, is not connected with the series from any similarity 
of properties though the phosphids, arsenids, and antimonids, 
Present certain analogies with the oxyds. As has already been 
said, oxygen was placed at the head of this, as well as of © 
last two series, because its atomic weight seemed to be the nu- 
Cleus of all three. 
s 
