Four experiments on liquid helium 3 and helium 4 at temperatures below 0.1K are proposed: 1) Solubility and diffusion- coefficient measurements to test some recent predictions of Fermi Liquid theory. 2) Measurement of the Kapitza thermal conductance and of the transmission of transverse spin waves across the interface between superfluid A and B phases of helium 3, and 3) a related experiment involving helium 3/helium 4 mixtures, these two experiments being designed to determine the transmission probabilities of helium 3 quasiparticles across the interfaces. 4) A test of recent theory by measuring the reflection probability of rotons at the free surface of liquid helium 4. The liquid state of helium (and hydrogen), which can only be produced at extremely low temperatures, close to absolute zero, furnishes an example of a "quantum fluid" where Nature shows us an example of quantum mechanics operating on a "bulk" (or "macroscopic") scale. Such a fluid evidences very unusual properties and offers a fertile ground for research into the fundamental properties of matter. The present experiments are designed to examine some of those properties in depth and thus deepen our understanding of this strange realm of the physical world.
