190 SEVENTH REPORT—1837. 
To this list glass has some claim to be added. Its physical 
properties when annealed, and when suddenly cooled, are known 
to be very different, and in the second of these states it is said by 
Guérard* to be possessed of double refraction. As itis doubt- 
ful, however, how far any specimens of glass used in the arts may 
be considered as definite chemical compounds, we cannot as yet 
draw any certain conclusions from their properties in different 
circumstances. Common charcoal and graphite are also sup- 
posed by some chemists to be modifications of carbon sufficiently 
distinct to awaken the suspicion that this substance may assume 
even a third crystalline form. 
34. The appearances presented by the bichromate of potash 
when cooling from fusion, and by the double sulphate of potash 
and copper, are very interesting. In both cases the change com- 
mences, as in the yellow crystals of biniodide of mercury, at 
one edge of the mass, and gradually spreads over the whole. As 
in the biniodide, the changed is in all probability ahetoromorphous 
state, and the same will, I think, prove true of all the substances 
contained in the present table. They are necessarily placed apart 
in the present state of our knowledge till their forms in the 
changed condition shall have been determined. 
The chance, so to speak, of their proving dimorphous is much 
strengthened by the analogy in constitution between the bisul- 
phate of potash, which is known to assume two unlike forms, and 
thedouble sulphate in the table. The formula of the one KS + HS 
is the exact counterpart of that of the other KS + CuS, the 
copper in the latter replacing the hydrogen in the former. Led 
by this analogy, I have sought for the same phenomena in other 
compounds of the same class. Sulphate of potash fuses rea- 
dily at a bright red heat with the anhydrous sulphates of zine and 
of nickel, but on cooling the same change does not present it- 
self, at least under the same circumstances. Under conditions 
slightly varied we may expect all the compounds represented by 
the general formula RR+RR to occur in two states. physic- 
ally different. 
* Pog. Annal., xxxviii. p. 233. 
+ The probability of the change in question being connected with dimorphism 
is strengthened by a recent observation of Mr. Talbot, (Lond. and Edin. Phil. 
Mag., Feb. 1838, p. 149) that a thin film of nitre, on solidifying from fusion, 
undergoes, when the temperature falls to a certain point, a change quite analo- 
gous to that exhibited by the bichromate and double sulphate in the table, and, 
as in those substances, diffusing itself from a point over the whole mass. In 
nitre the appearance is no doubt connected with the two forms it is known to 
assume. 
