1854.] OF THE ROYAL INSTITUTION. 449 
WEEKLY EVENING MEETING. 
Friday, May 26, 
Sir Henry Hoxzanp, Bart., M.D., F.R.S., Vice-President, 
f in the Chair. 
B. C. Bropiz, Esq., F.R.S. 
On Melting Points. 
WueEn the temperature of certain substances is raised, they pass 
from the solid to the fluid and from the fluid to the gaseous con- 
dition. These transitions are attended with the absorption of heat. 
There are other bodies which by elevation of temperature undergo a 
transformation of a different kind. Thus, when liquid phosphorus 
is heated, in such a manner that its change into the gaseous con- 
dition is prevented, at a certain temperature it becomes solid, and 
passes into the red modification ; these allotropic changes also are 
invariably attended with evolution or absorption of heat. 
Considerable anomalies are found in the statements which dif- 
ferent experimenters have made as to the melting point of sulphur. 
The cause of these discrepancies lies in the facility with which the 
allotropic condition of sulphur is altered by heat. The melting 
point of octohedral sulphur lies very close upon the point at which 
it undergoes a change into the oblique-prismatic condition. When 
this sulphur has been melted, it passes more or less completely into 
a third allotropic form. For these reasons, the melting point taken 
was never that of a pure sulphur. However, by certain precautions 
in experimenting, the true melting points of sulphur have been 
ascertained. 
The experiment cannot be made in the usual manner of taking 
a melting point, namely, by placing a thermometer in the fluid sub- 
stance, and observing the point of solidification. Fluid sulphur 
is always a mixture of more than one modification. The experiment 
is made by placing minute fragments of sulphur in thin glass-tubes, 
immersing the tubes in a bath of dilute sulphuric acid, and observing 
the temperature of the fluid at the melting of the substance. Ex- 
periments thus conducted have shewn that the melting point of octo- 
hedral sulphur is 114°5°C. and of the oblique sulphur 120°C. This 
latter sulphur is obtained in a pure condition by heating the octo- 
hedral sulphur at a temperature below its melting point, from 100° 
to 110°C. This change invariably takes place when the sulphur 
is exposed, even momentarily, to this temperature in a state of 
powder. 
The solidifying point of melted sulphur varies according to the 
