430 
SECOND REPORT—1832. 
system of crystallization. Thus specular iron and common 
calc-spar are members of the same homoiomorphous group, 
though the obtuse angle of the former is only 93° 50', while that 
of the latter is 105° 5k It is possible that the same cause 
which produces deviations of one or two degrees in certain sul¬ 
phates and carbonates, may also cause differences of ten or 
twelve degrees in other minerals; but in the present state of 
our knowledge such a supposition is void of all probability. 
The term therefore, though well fitted, apart from theoretical 
views, to express the connexion of forms as belonging to the 
same system of crystallization, has not as yet come into gene¬ 
ral use. 
Cause of these differences .—Mitscherlich accounts for the 
anomalies in question by supposing that the nature of the che¬ 
mical affinity exerted between the acid and base in certain com¬ 
pounds, may have an influence on the dimensions of the cry¬ 
stals they form. If in all the other salts of the phosphoric and 
arsenic acids an absolute identity of form was observed, and 
a slight difference of 1° 24/ found only between the angles of 
the salts of ammonia, are we to reject the general testimony of 
the salts, and to conclude, from the difference in this one case 
only, that the ultimate forms of the two acids are different? or 
are we not rather to suppose that some special cause has inter¬ 
fered to modify the dimensions in this solitary instance ? That 
chemical affinity is the modifying cause cannot be proved, but 
we know it to be capable of producing similar effects. For, 
“ if chemical affinity,” says Mitscherlich*, “have an influence 
on the dimensions, or axes of the crystals on which the dimen¬ 
sions depend, then this influence must cease entirely when the 
axes are alike ; and such is the case. If oxide of lead or stron- 
tian produce a primitive form different from barytes, when they 
combine with the same substance, and if this dissimilarity be 
due to the unlike form of the bases themselves, and not to such 
a modifying power, then, since barytes in combination with 
nitric acid gives a salt which crystallizes in regular octohaedrons, 
the nitrates of lead and strontian should take a form which de¬ 
viates from it; but all the three nitrates crystallize in the same 
form.” The probability therefore at present is, that the acids 
and bases comprehended under each so-called isomorphous 
group, have like forms, and that chemical affinity in certain cases 
is capable of modifying the form of their crystalline compounds. 
Influence of temperature on crystalline forms .—That these 
differences in form are notinconsistent with perfect isomorphism 
in the acids or bases, is further proved by the very interesting 
*Kortg. Vetens. Acad. Handlingar, 1821, p. 48. 
