1. W. H. Keesom, Helium, p. 174, Elsevier, Amsterdam (1942).
2. C. F. Squire, Low Temperature Physics, p. 23, McGraw-Hill Book Co., New York (1953).
3. K. W. Taconis, J. J. M. Beenakker, A. O. C. Nier and L. T. Aldrich, Physica 15, 733 (1949). (See footnote on page 738.)
4. H. A. Kramers, Physica 15, 971 (1949).