GENEEAL. AND PHYSICAL CHEMISTRY NEKNST. 251 



the equilibrium. For special cases of heterogeneous equilibria there 

 comes into play the so-called " phase rule,"' which expresses in prin- 

 ciple that in every case fixed (stable) equilibria correspond to given 

 conditions of temjDerature, pressure, and concentration. This rule 

 is therefore rather a reliable formula than a theory proper, and that 

 is Avhy from many sides we are warned not to exaggerate its value. 

 More important from a theoretical standpoint is the demonstration 

 in chemical compounds of two sorts of stability. One is the apparent 

 stability which is characterized by the fact that its speed of decom- 

 position is very slow (examples : Nitric oxide, hydrogen peroxide, and 

 most organic compounds), and the other, the true stability, which is 

 characterized by the fact that the equilibrium depends on a quantita- 

 tive formation of the substance considered apart from its components. 



Electrochemistry and photochemistry are governed by laws closely 

 related to those of thermochemistry. Although the study of the 

 latter of these two fields has presented, up to the present time, great 

 difficulties in the way of theoretical investigation, Faraday's law, 

 which establishes the proportion between chemical transformation 

 and the quantity of electricity passed through a system in a given 

 time and which thus makes possible the calculation of the electric 

 energy necessary for a given change, has provided an accurate 

 foundation for the application of thermodynamics to electrochemistr3\ 

 Also, by continuing the special conception which gave rise to the 

 theories of osmotic pressure and electrolytic dissociation, a simple 

 conception of electrochemical processes has been developed. It has 

 at the same time become apparent that electrical forces unquestionably 

 play a great part, not only in electrochemical phenomena, but also in 

 many purely chemical reactions. 



Thus we are brought to the problem of the nature of chemical 

 forces. Although this question does not perhaps possess the funda- 

 mental importance that is often attributed to it, nevertheless it should 

 be briefly considered. It can be treated here still more briefly be- 

 cause we are obliged to admit that during the period under considera- 

 tion there has been no answer to this question which really teUs us 

 anj^thing more than we can see with our own eyes. It seems reason- 

 ably certain that we should admit the existence, not only of electrical 

 and therefore polar forces, but of nonpolar natural forces somewhat 

 of the nature of Newtonian gravity. "\Mien fluorine and potassium 

 unite to form a salt, the colossal affinity between the two elements de- 

 pends at any rate in part on the affinity of the fluorine for negative 

 electricity and of the potassium for positive electricity ; but when we 

 find in the ordinary nitrogen molecule two atoms of nitrogen united 

 in a combination, perhaps of equal stability, it would appear that in 

 the case of as complete an identity as presented by two atoms of 

 nitrogen the action of polar forces should be entirely excluded. The 



