They are mostly colourless liquids which boil without decomposition, or solids of low melting point.
It is to be noted that although the correlation of melting-point with constitution has not been developed to such an extent as the chemical significance of other physical properties, the melting-point is the most valuable test of the purity of a substance, a circumstance due in considerable measure to the fact that impurities always tend to lower the melting-point.
The overheating curve of rhombic sulphur extends along the curve AC, where C is the melting-point of monoclinic sulphur.
When zinc is placed on the lead (heated to above the melting-point of zinc), liquefied and brought into intimate contact with the lead by stirring, gold, copper, silver and lead will combine with the zinc in the order given.
It has in general one value for the powdery metal as obtained by reduction of the oxide in hydrogen below the melting point of the metal, another for the metal in the state which it assumes spontaneously on freezing, and this latter value, in general, is modified by hammering, rolling, drawing, &c. These mechanical operations do not necessarily add to the density; stamping, it is true, does so necessarily, but rolling or drawing occasionally causes a diminution of the density.
