Since this series was started in 1954 a new volume has come out about every third year. The number of pages per volume has increased slowly but surely. While the number of chapters per volume has decreased considerably, their average length has more than doubled. Longer contributions have been invited deliberately but an increasing number of authors have considerably exceeded the space reserved for them. It would seem that, if the character of the series is to be preserved, the average chapter should not grow much longer.
The chapters devoted to liquid helium, to metals including superconductors, and to magnetism together with nuclear orientation, each generally take up about 30 percent of the total space. Other subjects, such as molecular physics, thermometry and semiconductors complete the remaining 10 percent. In the present volume liquid helium occupies less space than in the preceding ones. This was accidental; one main contribution to this volume was invited but has not been forthcoming. I feel that after 56 years, liquid helium remains the most enigmatic and, at the same time, one of the most investigated substances; its large share of space in the series is justified.