THERMODYNAMIC AL SYSTEM OF GIBBS 179 



Gibbs describes the effects which may cause a system to 

 undergo changes of this kind in the following terms : 



"Such a result will probably be produced in a fluid mass by 

 contact with another fluid which contains molecules of all the 

 kinds which occur in the first fluid (or at least all those which 

 contain the same kinds of matter which also occur in other sorts 

 of molecules), but which differs from the first fluid in that the 

 quantities of the various kinds of molecules are entirely deter- 

 mined by the ultimate composition of the fluid and its tem- 

 perature and pressure. Or, to speak without reference to the 

 molecular state of the fluid, the result considered would doubt- 

 less be brought about by contact with another fluid, which 

 absorbs all the proximate components of the first, *Si, ... <S„, 

 independently, and without passive resistances, but for which 

 the phase is completely determined by its temperature and 

 pressure and its ultimate composition (in respect at least to the 

 particular substances just mentioned). By the absorption of 

 the substances Si, ... /S„ independently and without passive 

 resistances, it is meant that when the absorbing body is in equi- 

 librium with another containing these substances, it shall be 

 possible by infinitesimal changes in these bodies to produce the 

 exchange of all these substances in either direction and inde- 

 pendently. An exception to the preceding statement may of 

 course be made for cases in which the result in question is 

 prevented by the occurrence of some other kinds of change; in 

 other words, it is assumed that the two bodies can remain in 

 contact preserving the properties which have been mentioned. 



"The term catalysis has been apphed to such action as we are 

 considering. When a body has the property of reducing 

 another, without limitation with respect to the proportion of 

 the two bodies, to a phase of dissipated energy, in regard to a 

 certain kind of molecular change, it may be called a perfect 

 catalytic agent with respect to the second body and the kind of 

 molecular change considered." 



