In the paper by K. F. Bonhoeffer and P. Harteck published in 1929, on para-hydrogen, no mention is made of the refractive index of the new modification, and, so far as we can find, no experiments on this subject have hitherto been published. It may, therefore, be presumed that, on the hypothesis which led to the discovery of the para-form, no change of index was expected; and, on any theory, it seemed probable that the change would be very small. It appeared worth while, however, to make use of a Jamin interferometer already set up for other purposes to test this point experimentally and the following paper records the result of the investigation. Para-hydrogen was prepared by adsorbing ordinary hydrogen, purified by previous adsorption in palladium, in charcoal at the temperature of liquid oxygen (—183°C.). After 1 1/2 to 2 hours the gas was allowed to warm up, and should then consist of approximately 50 per cent, ortho- and 50 per cent, para-hydrogen. In order to test whether this wTas actually so Schleiermacher’s method of measuring the thermal conductivities of the two forms, as described in Bonhoeffer and Harteck’s paper, was followed. In their case a Wollaston wire (0.01 mm. diameter) was stretched in a narrow cylindrical tube immersed in liquid hydrogen, and the change in the resistance of the wire, which was electrically heated to 200° absolute, was observed when para- was substituted for ordinary hydrogen at the same pressure (40 mm.). In these circumstances the resistance varied between 111.85 ohms for ordinary hydrogen to 106.25 for hydrogen containing 99.7 per cent, para-hydrogen.
The liquefaction of helium by an adiabatic method