When the temperature is 2.I9 ยฐ above the absolute zero, liquid helium passes through the phase of metamorphosis into so-called helium II. The author, in his research into the nature of liquid helium II, shows that its viscosity is at least one billion times smaller than the viscosity of water; thus helium I1 may be called a flowing liquid, i.e., a liquid capable of flowing without viscosity. Basing upon this exceptional fluidity of helium II, the author offers the following explanation of that great diathermancy which has been observed by Keesom. The creation of powerful convectional currents is stipulated by special thermo-hydrodynamic forces which were uncovered by Allen and Johnson in a powder, saturated with liquid helium II, but the author discovered them also in free helium I I where they always appear when a difference in temperatures exists. These currents can easily be demonstrated in a small apparatus which resembles the "Sengerov's \Vheel." This apparatus, submerged into liquid helium I I, begins to rotate under the influence of slight heating.
Further studies of these currents have shown existence of an exceptional case of direct transition of heat into kinetic energy of the movement of the liquid. The experiment has shown that this phenomenon is thermodynamically convertible.
\Vhile forcing helium II under pressure through a narrow slit, the temperature can be lowered several tenths of a degree. In view of the fact that this exceptional ability of helium If, theoretically, should be preserved to the lowest temperatures, an experimental method is being developed making it possible to approach absohlte zero as near as it is desired. A complete explanation of the super-fluidity of helium I1 based on the quantum theory was given by Tisso amt Landau. This explanation is close to the modern theory of super-conductivity. According to this theory, helium II is considered as a mixture of two liquids: the first with zero entropy and the other, normal helium I. These liquids can exist in the mutual counter current. The quantum theory makes it possible quantitatively to describe these phenomena and the results obtained coincide well with experimental data.
Studies of the phenomena of super-fluidity of helium I 1 open great perspectives in the field of physics of low temperatures.