Equality various possibilities, depending upon

So far in this book we have been concerned almost entirely with the properties of perfect crystals of ice. The properties discussed were, in fact, not structure-sensitive and both the theory and necessarily the measurements apply equally to real crystals. In the remaining chapters, however, we shall examine attributes of ice which do depend sensitively upon the perfection of the particular crystal involved. There are many imperfections which occm in real crystals—surfaces, impurities, dislocations, vacancies and so on — with which we shall necessarily be concerned but in the present chapter we focus attention on the various possible kinds of point defects. 

---