THE NATURE OF CHEMICAL PHENOMENA. 
277 
the form of heat during their combination, and an equal amount 
of energy must be supplied to effect its reversal. That is, "water 
has less energy stored in it than hydrogen and oxygen, and, 
therefore, since it has not as much energy as these bodies it 
cannot properly be said to contain them. It contains the 
matter of them undoubtedly, as there is no change of weight, 
but not the substances as we know them. It is desirable, 
therefore, to remember the statement that water is a compound 
of hydrogen and oxygen is only true in the sense that the dis- 
appearance of these gases in certain proportions is always 
accompanied by the appearance of a corresponding quantity of 
water, and vice versa. 
I now pass to the second question. Certain of the elements 
are peculiar in that they occur in several different forms, which 
are said to be allotropic. Such, for example, are phosphorus and 
oxygen and carbon. 
I wish to make a few remarks on this point, because I 
have noticed that there is often an impression left on the minds 
of those who have studied the elements of chemistry that the 
question of allotropy is somehow a peculiar matter, of which it 
is possible to know the facts, but that at present a satisfactory 
explanation is not forthcoming. 
The element oxygen ordinarily occurs as a colourless, trans- 
parent, odourless gas, of considerable activity, combining readily 
with a number of the other elements, sometimes needing the aid of 
heat to start the action, sometimes iiot. When perfectly pure, 
dry oxygen is well cooled and submitted to the action of the silent 
electric discharge, a portion of it changes to a grey-blue gas, 
which, when compressed, if kept cold, yields an indigo-blue liquid ; 
this new substance has a very characteristic odour, producing a 
kind of influenza when smelled, and has far greater oxidizing 
powers than the ordinary form of oxygen. Moreover it is heavier 
than oxygen, equal volumes of the two gases having densities 
