No one can write on thermodynamics without being deeply in?uenced by Willard Gibbs and Max Planck. The writings of the former will always be the admiration (and sometimes the de spair) of the student of thermodynamics on account of the extreme rigor and completeness with which he discusses the subject, while Planck in his Lectures on Thermodynamics has given a treatment which is a model of conciseness, accuracy and logic. It has been my endeavor to write a book which, in addi tion to being accurate, logical and sufficiently rigorous, will fur nish the student with numerous examples of the application of the principles of the science. It is with this object in view that four chapters have been devoted to the phase rule and its applications.