} 
ON DIMORPHOUS BODIES. 191 
35. Differences of a less permanent and definite kind are ex- 
hibited by various substances, as by some of the metallic oxides 
at different temperatures, which obscurely point to a second state 
analogous to that we are now considering as belonging to them 
also. Thus the protoxide of lead PbO when cold is of a pale 
yellow, when hot of a bright red; the scales of litharge often 
retain this hue at common temperatures. 
It would be premature at once to explain this and similar ap- 
pearances by a supposed dimorphism ; they are deserving how- 
ever of a close attention, and though obscure at present, the 
study of them may lead us to new results. 
36. Many compound, especially saline, substances, when ex- 
posed to the air or slightly heated, undergo a change analogous 
to that we are now considering, due, however, not to a mere 
change in the arrangement of the molecules, but to an alteration 
also in the chemical constitution. When a crystal of sulphate 
of zinc with seven atoms of water is heated under alcohol it as- 
sumes a new form, but it loses at the same time an atom of 
water ; the same is said also to be the case with sulphate of 
magnesia. The blue acetate of copper with six atoms of water if 
heated to 90° or 100° F. changes without apparent change of 
form into the green acetate with one atom of water. On ex- 
amination, however, the apparently unchanged crystal is found 
to consist of a congeries of minute crystal of an entirely differ- 
ent form*. The mellate of ammonia, according to Wohler, un- 
dergoes an equally striking change by simple exposure to the air. 
One of the most curious facts of this description is that observed 
by Herman in regard to the chloride of lithium. When this 
salt is allowed to deliquesce in the open air large four-sided 
prisms are formed. If one of these prisms be taken up in the 
fingers, and then laid on blotting paper, it becomes opaque at the 
point of contact, and the opacity gradually spreads over the 
| whole crystal. If now moved it falls into a powder, which 
again deliquesces in the air and crystallizes}. Changes of this 
kind connected with loss of water are no doubt very numerous. 
37. An appearance observed by Biot, in reference to grape- 
sugar, appears worthy of a place in the present section. He 
States}, that the juice of the grape, before it has been crystal- 
lized, causes the plane of polarization of a polarized ray passed 
through it to deviate towards the left, while after crystallization 
its solution causes the same ray to deviate towards the right. 
_| By crystallization the chemical constitution is unaltered (?), 
* Wohler, Poggendorff, Annal., xxxvii. p. 166. 
+ Pog. Annal., xv. p. 480. t Taylor’s Scientific Memoirs, i. p. 596. 
