1. J.F.Allen, 1952, (p. 66).
2. See H. Kamerlingh Onnes, 1908, (esp. p. 18), where the first liquefaction of helium is described with great precision, in full detail.
3. H. Kamerlingh Onnes, 1911a, (see p. 4).
4. H. Kamerlingh Onnes, 1913e, (p. 327).
5. H. Kamerlingh Onnes, 1913d, (p. 55).