450 NOTICES OF THE MEETINGS [May 26, 
temperature to which it has been raised in the melted condition. 
Powdered sulphur, carefully melted so as not to raise its temperature 
above one degree beyond its melting point, will solidify precisely at 
its melting point, 120°C. If, however, the temperature be raised 
to 300°C. it will solidify at about 110°C. The cause of this dif- 
ference is, that the sulphur in the latter case always contains a large 
portion of a third modification, namely, the viscid form of sulphur. 
There are some remarkable anomalies in melting points which do 
not so readily as the above admit of explanation. Under certain 
circumstances drops of sulphur will remain in the liquid condition 
at a temperature far below the true point of solidification, and solidify 
instantaneously when touched. The same is the case with phos- 
phorus, Water contained ina capillary tube may be immersed without 
freezing ina mixture cooled to—110°C. The same experiment may 
be made with a considerable quantity of water if the surface be pro- 
tected by a thin layer of wxther. In these cases the water instantly 
freezes by agitation or by touching the surface with a solid body. 
Similar observations have been made in the crystallization of certain 
salts. A solution of sulphate of soda made at 30° or 40°C. will 
not crystallize on cooling, provided the fluid be not disturbed, but 
instantly crystallizes on touching the surface with a wire. This 
phenomenon does not take place with all solutions. A solution of 
nitre crystallizes normally. There is also a difference of degree in 
this property. A solution of borax will remain in an open flask in 
the supersaturated condition, and crystallizes only on violent agitation. 
An experiment was shewn by which a connexion was established 
between this class of facts and those of which mention was first 
made. Two tubes were exhibited, each containing the same quantity 
of sulphur dissolved in the same quantity of bisulphide of carbon. 
One tube had crystallized in the normal manner, the other had de- 
posited no crystals. The sulphur had in both tubes been dissolved 
at the same time and in the same manner. But the tube in which 
the sulphur did not crystallize had been exposed to a higher tem- 
perature than the other tube. In this case therefore it was evident 
that the cause of the supersaturated condition was an alteration of 
the substance induced by heat. On breaking the point of the tube 
and agitating the fluid with a wire, the sulphur instantly crystallized. 
The analogy was pointed out of the sudden alteration of this con- 
dition by agitation and contact, and the decomposition which many 
chemical substances, such as the iodide of nitrogen, undergo by 
similar causes. 
[B. C. B.] 
