ON DIMORPHOUS BODIES. 215 
substances represented by unlike formule. (See Table III.) 
They seem to point to a modification of received opinions. 
8. In cases of reputed pseudomorphism to examine minutely 
the circumstances under which the changed crystals occur, and 
the nature of the crystals themselves ; some of them may prove 
to be cases of dimorphism. (40.) 
9. To observe by the aid of the microscope or otherwise the 
change which fusible substances undergo in the different stages 
of cooling after solidification. Some (5. 6. Table IV.) sub- 
stances appear in cooling to pass through, as it were, interme- 
diate forms which they cannot retain, before they reach that 
state of crystalline arrangement which is proper to the stationary 
temperature. If one substance be known to exhibit such trans- 
- formations, to inquire if all substances represented by the same 
formulee may exbibit them. 
10. What difference of molecular arrangement, as indicated 
by the optical properties, exists in the viscid state of melted 
sulphur compared with the limpid states it assumes at a higher 
and at a lower temperature (47)? Are analogous phenomena, 
differences of colour, density, fluidity, &c., observable in other 
fluids at different temperatures ? Can any other gases exhibit- 
ing like changes be added to the solitary example of nitrous 
acid? (48.) 
In connexion with this subject every accurate measurement 
of a crystal, every nice determination of the hardness or density 
of a well crystallized specimen, and above all every careful 
analysis of specimens previously measured and weighed is of 
great value. For though not immediately available in clearing 
up any obscure or disputed point, they will form a sound basis 
for future reasonings, will indicate new analogies among cry- 
stalline compounds, and will gradually lead us forward to wider 
generalizations. 
Durham, 1838. 
