Plesiomor- 
phous dif- 
ferences. 
Cause of. 
182 SEVENTH REPORT—1837. 
for reconciling the formule of substances such as those inserted 
in Table III. That an extension of the general conditions ne- 
cessary to isomorphism must by-and-by take place, the num- 
ber of bodies we are already able to insert in this table is suffi- 
cient proof*. 
28. It would be improper to dismiss the consideration of the 
tables of dimorphous and isodimorphous groups without advert- 
ing to the differences in the angular dimensions of the several 
substances comprised by these groups. It is true generally of 
isomorphous bodies, that the angular dimensions of their crystal- 
line forms do not exactly correspond, but only approach to each 
other often very closely, as in the chromate and sulphate of 
potasht, but sometimes differing nearly two degrees, as in same 
of the earthy carbonates. These differences have been much 
dwelt upon, especially by English crystallographers, to some of 
whom they have appeared sufficiently great and constant to 
warrant the rejection of the term iso and the substitution of 
plesio morphism in its stead{. The fact of bodies replacing 
each other is inconsistent with a mere approach in their forms, 
while the circumstance that no constant difference has been 
observed among the forms of the several members of the same 
isomorphous group with different acids or bases, shows, I think, 
satisfactorily, that these differences do not necessarily imply 
unlike forms in the crystalline molecules. If the silicates or 
sulphates of two oxides be almost identical in form, while their 
carbonates differ by more than a whole degree, the difference 
between the forms of the oxides not being constant in the ana- 
logous classes of compounds, may at least have their origin in 
a cause extrinsic to the forms of the substances altogether. 
29. It is well known that Mitscherlich attributed these differ- 
ences to some peculiarity in the chemical affinities, specific to each 
substance or to the several substances entering into a com- 
pound. On this very probable opinion it is unnecessary to 
dwell. He has lately, however, thrown out a suggestion in re- 
gard to the nature of this specific modification of the affinities, 
or rather how it operates, an examination of which will be 
neither uninteresting nor out of place§. 
Supposing the molecules of bodies—their mutually replacing 
equivalents—to be equal in size, and to be placed at like dis- 
tances, the densities of these bodies should be as the weights of 
their equivalents. That the densities are not so related in na- 
* See London and Edin. Phil. Mag., May 1838. 
+ Brooke, Annals of Philosophy, August, 1823, and January, 1824. 
t See Report on Chemistry, Reports of British Association, vol. i. p. 428. 
§ Poggendort’s Annalen, vol. xli. p. 216. 
