HEAVY-FERMION SUPERCONDUCTIVITY (in at least some materials) is mediated by the magnetic interactions among the charge carriers. In conventional low-temperature superconductors, electrons pair up via the mediation of vibrations of the underlying solid (set in motion by virtue of the electron's negative charge as it passes through a lattice of positive ions whose motion, in turn, attracts another electron); equivalently, one can imagine the vibrations to be organized into particles called phonons. In heavy-fermion materials, by contrast, the electrons in question are inner-shell electrons which, because they are tightly bound, move as if they were heavier (by a factor or 100 or more) than normal electrons. Now physicists at the University of Cambridge have shown that superconductivity in Ce-Pd-Si and Ce-In compounds arises from the magnetic interactions of the electron spins. Instead of ripples of lattice distortions (phonons) doing the binding, it is ripples in the density of spins that are the cause of the pairing. Thus in these materials magnetism and superconductivity (sometimes regarded as antithetical) may both result from spin density waves. This "magnetic glue" might also have a role in high-temperature superconductivity. (Nature, 2 July 1998.)