40 ELISHA MITCHELL SCIENTIFIC JOURNAL 



shortcomings over-balance its merits. Despite all hostile crit- 

 icism, the periodic law is today the basis of classification for 

 all advanced work in inorganic chemistrj* and its influence is 

 marked even in the teaching of elementary chemistry. The 

 periodic law enables us to correlate, more or less well, a large 

 number of facts in regard to the properties of the elements 

 and of their compounds. Yet people were slow in recogniz- 

 ing the full bearing of the statements that "the properties of 

 the elements are a periodic function of their atomic weights" 

 and I do not know what would happen if the law were being 

 advanced today for the first time. There is a mathematical 

 sound about the words "periodic function" which I fear might 

 stamp the law as "too theoretical for the chemist." 



While the Periodic Law enables the chemist to predict what 

 the properties of certain substances will be, it concerns itself 

 with these only and has nothing to do with the methods of 

 preparation and separation in use by the chemist. This gap 

 is filled by the second great simplifying generalization, the 

 Phase Rule. So loug as we are considering only the cases 

 most studied by the chemist, those involving changes of con- 

 centration, pressure and temperature, we may state the phase 

 rule in a relatively simple form: "When passive resistances to 

 change are eliminated, the degrees of freedom of the system 

 are two less than the difference between the number of phases 

 and the number of components." Each chemically and physi- 

 cally distinct mass in the system constitutes a phase. Thus 

 we may have the vapor phase, one or more liquid phases, and 

 one or more solid phases. With ether and water we get two 

 liquid phases; red and yellow phosphorus are two solid phases; 

 so are ice and salt; while sodium sulphate decahydrate is only 

 a single phase. 



The statement that "the degrees of freedom are two less 

 than the difference between the number of phases and the 

 number of components" does not sound like a very important 

 one. I can remember the time when I thought that people 

 made a good deal of unnecessary fuss over the phase rule. It 



