THE NATURE OP CHEMICAL PHENOMENA. 279 
very remarkable change of property, and the change is as 
marked in allotropy as in combination. There is in combina- 
tion always a greater or less evolution of heat. In the formation 
of allotropes in some cases certainly, and in all probability in 
all cases there is also an evolution of heat. Compounds can usually 
be reconverted into the bodies from which they were obtained by a 
sufficiently high temperature, heat being absorbed in this reversal. 
Similarly, the allotropes of oxygen, phosphorus, and carbon, 
under the influence of heat, manifest a tendency to revert to the 
commoner form, and, it is fair to infer, absorb heat in doing so. 
Lastly, I may remark that I know no reason for supposing that 
the law of the indistructibility of matter is not followed in these 
transformations. In short, it is difficult to find any point in 
which the formation of allotropic forms of elements can be shewn 
to differ from the combination of dissimilar elements. Hence 
one is brought to the conclusion that the phenomena are of the 
same order. Just as hydrogen and oxygen can form two combi- 
nations— water and hydrogen peroxide — so oxygen can form two 
combinations, viz., common oxygen and ozone. 
The fact that in the formation of compounds two or more 
elementary substances are concerned, whilst in the formation 
of allotropes only one elementary substance is concerned, does 
not seem to be a sufficient reason for looking on the two cases as 
of an essentially different nature, since the phenomena are so 
strikingly similar. 
