Various circumstances combined to prevent Dewar from actually carrying out the operation thus foreshadowed, but his anticipations were justified and the sufficiency of the method he indicated practically proved by Dr H. Kamerlingh Onnes, who, working with the splendid resources of the Leiden cryogenic laboratory, succeeded in obtaining helium in the liquid state on the 10th of July 1908. Having prepared 200 litres of the gas (160 litres in reserve) from monazite sand,[2] he cooled it with exhausted liquid hydrogen to a temperature of 15 or 16° abs., and expanded it through a regenerative coil under a pressure of 50 to 100 atmospheres, making use of the most elaborate precautions to prevent influx of heat and securing the absence of less volatile gases that might freeze and block the tubes of the apparatus by including in the helium circuit charcoal cooled to the temperature of liquid air. Operations began at 5.45 in the morning with the preparation of the necessary liquid hydrogen, of which 20 litres were ready by 1.30. The circulation of the helium was started at 4.30 in the afternoon and was continued until the gas had been pumped round the circuit twenty times; but it was not till 7.30, when the last bottle of liquid hydrogen had been brought into requisition, that the surface of the liquid was seen, by reflection of light from below, standing out sharply like the edge of a knife against the glass wall of the vacuum vessel. Its boiling-point has been determined as being 4° abs., its critical temperature 5°, and its critical pressure not more than three atmospheres. The density of the liquid is found to be 0.015 or about twice that of liquid hydrogen. It could not be solidified even when exhausted under a pressure of 2 mm., which in all probability corresponds to a temperature of 2° abs. (see _Communications from the physical laboratory at the University of Leiden, 1908-1909_). The following are brief details respecting some of the more important liquid gases that have become available for study within recent years. (For argon, neon, krypton, &c., see ARGON.)