ON DIMORPHOUS BODIES. 179 
and common soda (NaC + 10H)*, and the octohedral forms of the 
same salts with five atoms of water. Had he been aware of any 
of the real cases of monomorphous groups having discordant 
formule inserted in the list above given, or had he referred 
to them only, his reasoning, however hypothetical, would not 
have been so undeserving of a place in the excellent and elabo- 
rate memoir of which it forms a part. 
25. The chemical constitution of the two metallic sulphurets Seen 
which compose the third of our ¢sodimorphous groups, suggests ° ee 
considerations nearly related to those which have just been de- 
tailed. If they are really unlike in constitution, and are repre- 
sented severally by RS and R,S, then they ought to be included 
in our list of bodies which are like in form but unlike in for- 
mula. It is proper to state, therefore, why they are represented 
as isodimorphous. 
In a former report; I have illustrated the application which 
has been made of the doctrine of isomorphism in determining 
which of several possible multiples of a given number should 
be considered as representing the true equivalent of substances 
in regard to which we have at present no other means of arri- 
ving at a satisfactory conclusion. That alumina, peroxide of 
iron, and oxide of chromium crystallize in the same form, and 
are capable of replacing each other, as in the alums, is con- 
sidered satisfactory evidence that their elementary constitution 
is analogous—that the ratio of the oxygen to the metal is the 
same in all, and that the general formuia R,O, represents the 
‘composition of each. The whole doctrine of replacement, so 
beautifully applied to the elucidation of mineral compounds, de- 
pends on the same principle. No substances have ever yet 
heen observed to replace each other in atomic proportions, and 
without change of form, which are not also represented by the 
| same general formule. ‘The nearest approach to an exception 
“yet known is the fact established by Mosander, that peroxide of 
iron in the titanic irons may replace titaniate of the protoxide of 
iron (Fe,O, may replace FeO +'TiO,) ; but the exception is only 
‘im appearance, for Fe,O, may be represented by FeO + FeO,, in 
which case the formule are still analogous. Ammonia with an 
atom of water and potash are the only substances in our list of 
monomorphous substances with unlike formule which have 
been observed to replace each other, and we have already stated 
* The form of borax is an obliq. Rh. Pr. PM 101° 30’! MM 86° 40’ } Brooke 
That of common soda ditto PM 108° 45' MM 76° 12’ : 
+ Report of the British Association, vol. i. p. 422. 
N 2 
