Introduction

The Ising ferromagnet is therefore relatively simple to study, and we will devote considerable attention to it in these notes. In other words for the most part omit discussion of what are called first order phase transitions in the Ehrenfest classification. The phenomena which we will describe are often called critical phenomena and occur at second order phase transitions, although this name is inappropriate, because the Ehrenfest classification is not correct. We will see that the equation of state of a physical system near to a critical point obeys what appears to be an analogue of the law of corresponding states, encountered in the equation of state (such as that due to Van der Waals) for a fluid.