354 LEADING AMERICAN MEN OF SCIENCE 



deals with "the sociology of chemistry." Previously chemists had 

 been absorbed in the recognition of chemical substances as indi- 

 viduals and in studying their transformations, but Gibbs dis- 

 cusses their behavior in the presence of each other. He shows 

 under what conditions of temperature and pressure different sub- 

 stances, and the same substances in different states, can exist 

 together and what effect changes of these conditions will have 

 upon the composition of such mixtures. Chemists used to confine 

 their attention as much as possible to those reactions that went in 

 one direction and resulted in a practically complete change into 

 new compounds. They regarded incomplete and reversible reac- 

 tions with the same aversion as pre-Darwinian botanists did, 

 varieties which did not fit into their system of classification. Now 

 chemists find the most interesting and most common reactions are 

 those that proceed only partially in one direction when they are 

 checked by the opposite tendency arid an equilibrium established. 

 The study of the effect of the conditions, such as temperature, 

 pressure and relative amount of the components, upon such an 

 equilibrium is one of the most fruitful lines of investigation now 

 being carried on. 



The best known of the formulas of this paper is that called 

 " Gibbs' Phase Rule." It is usually expressed in this form: 



F = C+ 2-P. 



Where P denotes the number of phases (or distinct and separa- 

 ble masses of matter, such as chemical elements or compounds or 

 solutions or mixtures of gases), C denotes the number of compo- 

 nents (or chemical substances forming the constituents) and F 

 denotes the number of degrees of freedom (or the number of the 

 three variable factors, temperature, pressure and volume, which 

 must be arbitrarily fixed in order to define the condition of the 

 system). For example, let us take water alone, in which case C = i. 

 If we have water only in the form of a gas (steam or vapor), there- 

 fore in one phase, P = i; therefore F = 2, that is, we must give two 

 of the variables, say, the pressure and the temperature, before we 

 can know the third, the volume. If we have water in contact 



