ON DIMORPHOUS BODIES. 187 
drawn together and built up into regular forms. And as ele- 
mentary or compound bodies belonging to the same natural 
family, though possessing in common many properties, the same 
in kind, yet have them in different degrees, and exhibit them 
under different circumstances, so may we expect crystallizable 
substances, analogous in chemical constitution, and possessing 
like physical properties, toexhibit those properties, indegrees and 
under circumstances specific toeach. Under the same circum- 
stances there may be slight differences between the crystalline 
dimensions as there are between the chemical affinities of two 
bodies ; they may both be dimorphous, but under circumstances 
so widely different as hitherto to have escaped our observation, 
just as certain oxides of chlorine, iodine, and fluorine, which 
we believe to be possible, have hitherto baffled the attempts of 
the most refined manipulation. 
IV. 
33. Of bodies assuming two or more series of unlike physical 
properties, hut of which the crystalline form belonging to each 
series has not yet been determined.—In addition to those sub- 
stances, the dimorphism of which is established by direct mea- 
surement, there is a considerable number, the dimorphism of 
which is rendered exceedingly probable by the fact of their oc- 
curring, in two or more states, physically different. If dimor- 
phism imply a difference in physical properties, as well as in form, 
we may at least be prepared to look for a difference of form 
when marked physical differences present themselves*. 
The following table contains all the substances generally 
known to exhibit such physical differences. 
* Dumas proposes to include all in one group under the name Poly- 
morphous. ‘ Mais pour embrasser tous les phénoménes du méme genre il faut 
dire Polymorphisme sans restreindre a deux le nombre de modifications qu’un 
corps peut présenter, et comprendre dans la méme catégorie toutes les sortes de 
changemens qui peuvent affecter les propriétés physiques.” Lecons sur la 
Philosophie Chimique, p. 303. I think it better, however, to distinguish be- 
tween what we know and what we only suspect; to call those substances in 
which two crystalline forms have been observed certainly dimorphous, those in 
which they have not been observed as probably so. The term polymorphous will 
become necessary as soon as it is established that the same substance does 
erystallize in three or more incompatible forms. 
