200 SEVENTH REPORT—1837. . 
ratures, their second form in many being produced, in some being 
stable, only at higher temperatures. Thus the crystals of sul- 
phur from fusion gradually become opaque, and appear to change 
internally to minute individuals of the common form. The yel- 
low biniodide of mercury even more rapidly changes into the 
red. ‘The change of form? undergone by the bichromate of pot- 
ash and the double sulphate of potash and copper, and of colour 
by the protoxide of lead, the oxide of zinc, the binoxide of mer- 
cury, titanic acid, and other oxides, generally takes place before 
they arrive at the ordinary temperature of the atmosphere. What- 
ever be the way in which heat acts, therefore, it is obviously an © 
important agent in the exhibition of the one or the other form 
by dimorphous bodies. 
By an elevation of the temperature, more or less great, the 
first form is changed into the second, in sulphur, disulphuret of 
copper, the biniodide and bichloride of mercury, arsenious acid, 
oxide of antimony, carbonate of lime, carbonate of magnesia, 
sulphate of nickel, bisulphate of potash, seleniate of zinc, and 
probably the garnet. Of these substances, however, the new 
form assumed is permanent in all, with the exception of sulphur 
and the biniodide of mercury. 
Common charcoal readily assumes the form of graphite at a 
temperature below that at which cast iron melts ; of the tem- 
perature at which diamond is formed we as yet know nothing. 
46. The phenomena attendant on the production of the several 
forms renders it extremely probable that they are specific in 
each substance to specific ranges of temperature,—that the form 
assumed depends upon whether the substance is allowed to 
crystallize within the one range or the other,—that at tempera- 
tures near the limit of each range a very slight cause will set the 
particles in motion, for the production of either form as in the 
biniodide of mercury,—and that at greater distances from this 
limit, either above or below the temperatures to which it belongs, 
the form is permanent only because the particles have not the 
power of moving, being coerced as in suddenly cooled glass 
(Rupert’s drops), and requiring time as in sulphur, or the aid of 
heat as in arragonite, or in the process of annealing glass and 
metals, to enable them to overcome the restraint and to assume 
the other form. 
Connected as these phenomena appear to be with certain © 
ranges of temperature, they cannot be ascribed to the agency of — 
heat as a cause, otherwise the presence of this agent in greater 
or less intensity should produce similar effects on all crystalli- 
zable bodies ; they must rather be attributed to some peculiarity 
in the molecular constitution of the substances by which they 
