Excepting the chapter on Phases, this book represents what I have been in the habit of teaching the Senior Class in the Chemical Course at Rutgers College. I find lectures very unsatisfactory. The subject needs much thought, and the student does not seem able to get the proper material for this from notes, unless the lectures are made mere dictations. Besides, Physical Chemistry has attained such development that it is well to put a certain part of it in permanent text book form now, for whatsoever changes in our views time may bring, certain ideas we now have will not change. These fundamental ideas I have tried to put into this little book. Of course, I have also included some theories and ideas that later on may have to be rejected. That cannot be helped.