14). The hydrogen under pres sure, after passing through a cylinder containing solid potash, enters the apparatus by the pipe a, which is of solid drawn cop per of fin. bore. Passing inside the casing o, the pipe divides, one branch forming the coil in the chamber d, and continuing as g to form one of the twin coils in the chamber k. The other branch e leads through c to form the coil f, and continues as the other of the twin coils in k. Be low k the tubes are wound closely together to form the regener ator coil pp, and unite in the valve q, which is operated by the rod ss passing right through the liquefier in the casing r, and operated by wheel t. The lower part of the apparatus is en closed in the vacuum vessel II. An exhaust pump is con nected with the outlet v from the chambers k and d, which are connected by a constricted neck. By this means liquid air or oxygen can be drawn from the vessel r.; through the pipe h into k, where it evaporates under reduced pressure, the vapour passing upwards through the chamber d. All the compressed hydrogen passes through the twin coils in k and is cooled at least to --150°, and economy of liquid air is effected by using its vapour to cool that fraction of the incom ing hydrogen which passes through the coil in d. After expanding at the valve q the unliquefied hydrogen passes upwards through the interstices of the regenerator coil, then between the vacuum vessel and the outside of the chamber k, and through the inter stices of the coil f, cooling that part of the hydrogen entering the apparatus by this route, before it enters the other of the twin coils in g. The expanded hydrogen leaves the apparatus by the pipe u. At ten is shown the rubber ring and gland which connect the glass vacuum vessel with the metal parts of the apparatus.
When liquid forms in the bot tom of the vacuum vessel 1, it can be allowed to run into the receiver by allowing gas to es cape from the cock x. Liquid hydrogen can be manipulated without difficulty in a vacuum vessel, preferably surrounded with an outer vessel containing liquid air. If it is poured into an open glass vessel it evaporates very rapidly; and while the vapour can be ignited, solid air condenses and crusts the bot tom of the vessel. A litre of the liquid weighs only 6o grammes.
When the pressure is reduced to 55 mm. of mercury the hy drogen solidifies; the solid hav ing a glassy appearance and melt ing at Pressure raises the melting point by per at mosphere.
Liquefaction of Helium.— The liquefaction and solidification of helium was ultimately achieved by Onnes in July 1908, and his final experiments will now be de scribed with the aid of a diagram of the apparatus (fig. 15). The principle was identical with that of the apparatus used for lique fying hydrogen; and just as the hydrogen in the liquefier was cooled to liquid air temperature before entering the regenerator coil, the helium was pre-cooled to the temperature ( —258° C) of liquid hydrogen boiling m vacuo. The helium under a pressure of 3o atmospheres entered from the compression apparatus at A, the stream dividing so as to pass through two coils B and B', which were cooled externally by the currents of cold hydrogen and helium leaving the apparatus. The
coils in B and B' unite at their lower ends, and the pipe through which the helium flows passes to the top of the apparatus, where it again divides, forming two separate coils which pass through the chambers C and C', through which flow the cold hydrogen and helium. The coils again unite at the bottom of the chambers CC', and from D a single coil enters the chamber E, into which liquid hydrogen is drawn from the vacuum vessel shown on the left of the apparatus through the valve H. A vacuum is main tained in E by an exhaust pump connected with the outlet of the chamber B, the hydrogen gas under low pressure following the path indicated by the arrow. The compressed helium now enters the regenerator coil F, expands at the valve K, passes back through the interstices of the regenerator, through the chambers C' and B' to the outlet. A single vacuum vessel surrounded the whole of the lower part of the apparatus, and the liquid helium collecting in this vessel was forced (in later experiments), into the receiving vessel shown on the right of the apparatus. This was surrounded by a series of concentric vessels containing in order liquid hydrogen, liquid air and alcohol. The boiling point under atmospheric pressure was found later to be —268.9° C.
Onnes immediately set to work to attempt to solidify helium by boiling the liquid under reduced pressure. On the day on which helium was first liquefied (July 1908) the first attempt was made, the pressure being reduced to about 7 mm. Finally using a battery of condensation pumps, the ultimate method for producing a high vacuum, when a pressure of 0.013 mm. and an estimated temperature of 0.82° absolute was reached, the "helium still remained a thin liquid." But it was not for Onnes to achieve the final triumph, which was accomplished by his colleague and successor, Prof. Keesom. Just as the melting point of hydrogen is raised by increasing the pressure, it was possible that by increasing the pressure on the helium the melting point might be brought within the range of temperature already attained. Helium was therefore compressed into a glass tube (a metal con trivance was first used) containing a piece of iron rod, which could be moved by a magnet outside the apparatus. The glass tube was cooled to a series of very low temperatures, and, though it was impossi ble to differentiate visually between solid and liquid hydrogen, it was quite clear that under certain conditions of temperature and pressure the piece of iron became fixed in the tube. Thus, on June 26, 1926, every known substance had been liquefied and solidified. The relationship of these pressures and temperatures is shown in the diagram. It will be seen that the curve does not meet the vapour pressure curve for the liquid, which lies too close to the horizontal axis to be represented. It is quite possible, as the Dutch physicist Roozeboom had pre dicted in the year 1901, that the curves do not meet and that helium remains a liquid in contact with its vapour, however close the temperature may approach to the absolute zero.