ON DIMORPHOUS BODIES. 201 
are displayed, being merely developed under certain thermal 
conditions. 
- 47. The changes that take place in solid bodies at different tem- 
peratures, whether in form or in colour, are in general easily ob- 
served. In liquids, on the contrary, changes in the molecular 
arrangement are not so obvious, though there is little reason to 
doubt that they frequently take place. Of this fact melted sul- 
phur presents the most striking illustration with which we are 
acquainted. At 230° F. it is very fluid; at 430° F. viscid and 
tenacious; and again at 480° F., and upwards, of great fluidity. 
Changes of a different kind are exhibited by hyponitrous acid 
(NO,), which at 60° F. is of a green colour, while at — 4° F. 
it is wholly colourless*. On the other hand, a solution of iodide 
of starch, which at 200° F. is colourless, becomes blue as it cools. 
These differences can only arise from some change in the mole- 
cular arrangement induced by, or consequent upon, the change 
of temperature, precisely as in the case of some of the solid sub- 
stances above describedt. Analogous phenomena have not yet 
been observed in other fluid bodies, either because the change of 
position in the molecules takes place at temperatures to which 
fluids are not often exposed, or because it is not often accompa- 
nied by changes in the physical properties, such as can be readily 
observed :—it may be also because they have not hitherto been 
looked for. It is not unlikely that liquids, whether permanent 
or obtained by fusion, would at different temperatures differently 
affect the course of a prolonged ray if tested by the beautiful 
‘method of Biot. 
48. Even in gaseous bodies the relative position of the molecules 
does not appear to be the same at every temperature. ‘The va- 
pour of nitrous acid (NO,), at the temperature of 100° F., is 
of a deep red, while at 212° it is black and opaque} (Brewster). 
It may indeed be said that in this case decomposition takes 
* Mitscherlich, Lehrbuch der Chemie, vol. i. p. 342. 
+ In the Lecons sur la Philosophie Chimique par M. Dumas, which has 
come into my hands since the text went to press, is a paragraph (p. 305) almost 
werbatim with the above. Headds, ‘‘C’est sans doute aux mémes influences qu’il 
faut rapporter la propriété que l’eau posséde d’avoir un maximum de densité 
4 4°C, au lieu de continuer a se contracter 4 mesure qu’elle se refroidit.”—p. 
$36. He seems to have been unaware of the property observed by Sir D. 
Brewster in the vapour of nitrous acid, as in resuming the facts he had stated, 
he says, “‘ Vous voyez qu’on arrive a conclure que dans les gaz l’influence de 
Ja forme des molecules parait nulle oupresque nulle; qu’elle semble au contraire 
ee dans les solides, et qu'elle se fait également sentir dans les 
uides.” 
1 According to Sir David Brewster, a tube, filled with the red vapour at 100° 
and sealed, becomes black when heated to 212° F. i 
