212 PRINCIPLES OF CHEMISTRY 



just as hydrochloric acid is HC1 and sodium chloride NaCl. Hence the 

 aqueous radicle HO is a compound radicle, just as chlorine, Cl, is a 

 simple radicle. They give hydrogen compounds, HHO, water, and HC1, 

 hydrochloric acid ; sodium compounds, NaHO and NaCl, and a whole 

 series of analogous compounds. Free chlorine in this sense will be 

 C1C1, and hydrogen peroxide HOHO, which indeed expresses its 

 composition, because it contains twice as much oxygen as water. 



Thus in ozone and hydrogen peroxide we see examples of very 

 unstable, easily decomposable (by time, spontaneously, and on contact) 

 substances, full of the energy necessary for change, 28 capable of 

 being easily reconstructed (in this case decomposing with the evolu- 

 tion of heat) ; therefore they are examples of unstable chemical 

 equilibria. If a substance exists, it signifies that it already presents a 

 certain form of equilibrium between those elements of whicli it is built 

 up. But chemical, like mechanical, equilibria exhibit different degrees 

 of stability or solidity. 29 



28 The lower oxides of nitrogen and chlorine and the higher oxides of manganese 

 are also formed with the absorption of heat, and therefore, like hydrogen peroxide, act in 

 a powerfully oxidising manner, and are not formed by the same methods as the majority 

 of other oxides. It is evident that, being endowed with a richer store of energy (acquired 

 in combination or absorption of heat), such substances, compared with others poorer 

 in energy, will exhibit the greatest diversity of cases of chemical action with other sub- 

 stances. 



29 If the point of support of a body lies in a vertical line below the centre of gravity, the 

 equilibrium is entirely unstable. If the centre of gravity lies below the point of support, 

 the state of equilibrium is very stable, and a vibration may take place about this posi- 

 tion of stable equilibrium, as in a pendulum or balance, which ends in the body passing 

 to its position of stable equilibrium. But if, keeping to the same mechanical example, 

 the body be supported not on a point, in the geometrical sense of the word, but on a 

 small plane, then the state of unstable equilibrium may be preserved, unless destroyed 

 by external influences. Thus a man stands upright supported on the plane, or several 

 points of the surfaces of his feet, having the centre of gravity above the points of support. 

 Vibration is then possible, but it is limited, otherwise on passing outside the limit of 

 possible equilibrium another more stable position is attained about which vibration 

 becomes more possible. A prism immersed in water may have several more or less 

 stable positions of equilibrium. It is the same with the atoms in molecules. Some 

 molecules present a state of more stable equilibrium than others. Hence from this simple 

 comparison it will be already clear that the stability of molecules may vary considerably, 

 that one and the same elements, taken in the same number, may give isomerides of different 

 stability, and, lastly, that there may exist states of equilibria which are so unstable, so 

 ephemeral, that they will only arise under particularly special conditions such, for 

 example, as certain hydrates mentioned in the first chapter (see Notes 57, (57, and others). 

 And if in one case the instability of a given state of equilibrium is expressed by its 

 instability with a change of temperature or physical state, then in other cases it is 

 expressed by the case of decomposition under the influence of contact or of the purely 

 chemical influence of other substances. However clearly the greater or less stability 

 of the elementary structure of substances be depicted to us in these general considera- 

 tions, still at present there is no possibility of presenting them in a sufficiently con- 

 crete form to enable purely mechanical conceptions to be applied to them; that is, 

 to subject them to mathematical analysis, and to master the subject to such an extent 



