A gas that sinks in a liquid-the first helium experiment published by Kamerlingh Onnes.

The year 1906 was a year of many remarkable achievements in the laboratory of Kamerlingh Onnes and his staff. They put into operation a hydrogen liquefier producing 4 l h(-1), followed by a liquid-hydrogen cryostat for work below 21 K. They prepared their first sample of 2 l of helium gas. Pressure-volume-temperature measurements down to 54 K were in progress for hydrogen, and initiated for helium, in order to obtain an estimate for the critical temperature of helium; this estimate was urgently needed for design of a helium liquefier. Towards the end of 1906, Kamerlingh Onnes performed the first helium experiment at liquid-hydrogen temperature: a test of the phase behaviour of a mixture of hydrogen and helium. In the process, he discovered what he termed the barotropic effect: at increasing pressure, the helium-rich vapour phase sank to the bottom, having become heavier than the hydrogen-rich liquid phase. This paper describes the experiment and the resulting flurry of activity by Van der Waals, Kamerlingh Onnes, Keesom and Van Laar, all trying to understand and model the curious phase behaviour, as well as earlier relevant work by Van der Waals, Korteweg, Kuenen and Van Laar.