178 SEVENTH REPORT—1837. 
forms, why may not the converse hold? why may not different 
substances crystallize in one and the same form? We must 
allow instances to accumulate before we make any serious at- 
tempts at explanation. 
24. It may be proper here to notice a paper by Persoz, in the 
Annales de Chimie et de Physique, No. 1x. p. 145, in which, to 
reconcile the discordant formule of certain substances he sup- 
poses to be isomorphous, he advances the hypothesis that bodies 
unite by equivalent volumes, and not by equivalent atoms ; and 
that compounds may be isomorphous which contain equal 
volumes, either simple or compound. Thus, though the re- 
ceived formule for the sulphates and carbonates RS& RC be 
different, they may be considered alike if we suppose the acids 
to be composed respectively of 2 vols. sulphurous acid + 1 vol. 
oxygen, and 2 vols. carbonic oxide + 1 vol. oxygen, and the 
neutral sulphates and carbonates may be isomorphous. So also 
may the nitrates and hyponitrites (RN & RN) be isomorphous 
since the acids are composed of, 
The nitric . . . of 4 vols. nitrous acid + 1 vol. oxygen 
hyponitric of 4 vols. nitric oxide + 1 vol. oxygen. 
This hypothesis exhibits an unfortunate waste of ingenuity, 
since it has been proposed to explain two supposed cases of ~ 
isomorphism, which have in reality no existence. On the au- — 
thority of Kobell, verified by Voltz, he states that the forms of 
sulphate of barytes (BaO+SO,) and arragonite (CaO+CO,) — 
are identical*, though the inclination of the lateral faces of the — 
Rt Rh Prism in the former is 101° 42’, in the latter, 116° 10’. 
They are, indeed, what Kobell calls homoiomorphous} ; but so 
are numerous other substances, the formule of which it would 
be idle to attempt to reconcile. Because also the nitrate of 
lead (an octohedron) crystallizes without change of form in a 
solution of hyponitrate (PbO+N,O3), he concludes that these 
two salts are isomorphous; and to explain this imaginary 
identity of form between a sulphate and a carbonate, a nitrate 
and a hyponitrate, the hypothesis above stated is had recourse 
to. In the same way he states, that it is impossible to mistake 
the analogy of form between common Borax (NaB +10H) 
* An. de Chim. et de Phys., LX. p. 119. : 
t See Schweigger’s Jahrbuch, vol. iv. p. 410, also Reports of the British — 
Association, vol. i. p. 429. y 
