206 PRINCIPLES OF CHEMISTRY 



the volatile substance of decomposing organisms are clearly destroyed 

 or changed not only by ozone, but also by many powerfully oxidising 

 substances, such as chlorine with water, potassium permanganate, and 

 the like. 14 



Thus in ozone we see (1) the capacity of elements (and it must 

 be all the more marked in compounds) of changing in properties with- 

 out altering in composition ; this is termed isomerism ; 15 (2) the 

 capacity of elements for arranging themselves in molecules of 'different 

 densities ; this forms a special case of isomerism called polymerism 

 (3) the capacity of oxygen for appearing in a still more intense and 

 energetic chemical state than that in which it occurs in ordinary 

 gaseous oxygen ; and (4) the formation of unstable equilibria, or 

 chemical states, which are expressed both by the ease with which ozone 

 acts as an oxidiser and in its capacity for decomposing with explo- 

 sion. 16 



Hydrogen peroxide. Many of those properties which we have seen 

 in ozone belong also to a peculiar substance containing oxygen and 

 hydrogen, and called hydrogen peroxide, or oxygenated water. This 

 substance was discovered in 1818 by Thenard. When heated it is 

 decomposed into water and oxygen, evolving as much oxygen as is 

 contained in the water remaining after the decomposition. That 

 portion of oxygen by which hydrogen peroxide differs from water be- 

 haves in a number of cases just like the active oxygen in ozone, which 

 distinguishes it from ordinary oxygen. In H 2 O 2 , and in O 3 , one atom 

 of oxygen acts in a powerfully oxidising manner, and on separating out 



14 The oxidising action of ozone may be taken advantage of for technical ends ; for 

 instance, for destroying colouring matters. It has even been employed for bleaching 

 tissues and for the rapid preparation of vinegar, although these methods have not yet 

 received wide application. 



15 Isomerism in elements is termed allotropism. 



16 A number of substances resemble ozone in one or another of these respects. Thus 

 cyanogen, C.^N.^, nitrogen chloride, &c., decompose with an explosion and evolution of 

 heat. Nitrous anhydride, N./) 3 , forms a blue liquid like ozone, and in a number of cases 

 oxidises like ozone. Bed phosphorus is to white phosphorus, in a certain sense, what 

 oxygen is to ozone, and in other respects the reverse ; this is also a case of allotropism. 

 Thus a chemical analogy is diffused in different and most varied directions, and it is only 

 after an acquaintance with the diverse relations of substances that an idea can be formed 

 of the complexity of chemical changes, whilst their general system is still wanting; that 

 is to say, there is nothing analogous to and explaining the correlation of liquid to 

 gaseous substances. But there is reason to think that in this case also an explanation 

 will arise with the accumulation of data, as we see from the fact that the conception of 

 dissociation explained in the simplest manner a number of chemical relations which 

 without it were not at all clear. It should be here observed that the transition 

 between oxygen and ozone under the conditions of a silent discharge forms a reversible 

 reaction which is subject to the conception of dissociation, whilst, exempt from the 

 conditions of a silent discharge, the passage of ozone into oxygen is not reversible, and 

 forms an instance of decomposition in the strictest sense. 



