REPORT ON CHEMISTRY. 
431 
observations of Mitscherlich*, confirmed by Fresnel and Rud- 
berg,—that the angles of the same crystallized substance vary 
in dimension with the temperature. Crystals belonging to the 
regular system expand equally in every direction; other crystals 
expand more in one direction than in another, showing a ten¬ 
dency to approach to the nearest regular form. Thus the an¬ 
gle in calc-spar varies between 32° and 212° , the obtuse an¬ 
gle diminishing, and the form approximating to the cube. We 
have only to suppose, therefore, that there is a certain zero point 
below the mean temperature of the atmosphere, at which the 
forms of all similar compounds—carbonates and sulphates, for 
example—of the isomorphous groups are identical; and that the 
law of expansion by heat, for certain of them, differs from that 
obeyed by the others,—and we have a cause adequate to account 
for all the phenomena of plesiomorphism. The rate of deviation 
in calc-spar, however, is too slow to permit us, without further ex¬ 
periments, to attribute to this cause the entire deviation observed 
in the carbonates ; but if change of temperature can produce 
such alterations, it becomes more probable that variations in 
chemical affinity can produce similar and greater differences. 
Isomorphism of Potash and Ammonia + two atoms of water . 
•—Before leaving the subject of isomorphism, one other very 
important observation of Mitscherlich must be adverted to. 
the several groups of isomorphous bodies given above, with one 
exception, every member of each group is supposed to contain 
the same number of elementary atoms. Tne application of the 
doctrine to the determination of atomic weights, of which seve¬ 
ral illustrations have been given, and to the unravelling of the 
analytical results obtained from complicated mineral substances, 
depends entirely upon this supposition. In all cases it is sup¬ 
posed that the atoms of different elements replace each other 
in equal numbers. But Mitscherlich has found that when the 
salts of ammonia and potash are isomorphous, the potash salt 
is anhydrous, while that of ammonia contains two atoms of 
water. Thus (N + 3 H) + 2 (H + O) is isomorphous with Po -f- O, 
and could they replace each other, eight atoms must take the 
place of two. In like manner, the nitrates of potash and soda 
having respectively the forms of calc-spar and arragonite, if 
we suppose potash, soda, and lime to consist each of one atom 
metal and one atom oxygen, we have nitric replacing carbonic 
acid,—six atoms N + 5 O replacing three C+20. 
Dimorphism. —Native sulphur and sulphur deposited from a 
solution in bisulphuret of carbon, crystallizes in octohaedrons 
with rhombic bases; but when it is melted and allowed to cool 
* Ann ale s de Chimie , tom. xxxii. p. 11J. 
