The Salt Finger Experiments of Jevons (1857) and Rayleigh (1880)

Abstract

Over a century before Melvin Stern discovered salt fingers, W. Stanley Jevons performed the first salt finger experiment in an attempt to model cirrus clouds. Remarkably, he seemed to realize that a more rapid diffusion of heat relative to solute played a role in the experiments. However, he oversimplified the physics and incorrectly assumed that the ?interfiltration of minute, thread-like streams? was a general result of superposing heavy fluid over light fluid. Interestingly, Lord Rayleigh became aware of these experiments more than two decades later. Here newly discovered evidence is presented that Rayleigh repeated the Jevons experiments at the Cavendish laboratory in Cambridge in April 1880. The results led him to initiate the study of buoyancy effects in fluids by formulating several stability problems for a stratified, but nondiffusive, fluid. He thus discovered the expression for the buoyancy frequency of internal waves and the convective phenomenon now known as the Rayleigh?Taylor instability. His neglect of diffusion meant that he missed an opportunity to discover double-diffusive convection; though given the limited knowledge of fluid physics at the time, this is understandable. The historic record shows a tortuous intellectual path in which observations of clouds led to an inappropriate experimental demonstration of salt fingers that inappropriately motivated the theoretical discovery of the frequency of internal waves, which was ignored until well into the next century.