figures, falls to -115° (Dewar). Even the evaporation of liquids of common occurrence, under low pressures easily attainable with an air-pump, may produce low temperatures, which may be again taken advantage of for obtaining still lower temperatures. Water boiling in a vacuum becomes cold, and under a pressure of less than 4·5 mm. it freezes, because its tension at 0° is 4·5 mm. A sufficiently low temperature may be obtained by forcing fine streams of air through common ether, or liquid carbon bisulphide, CS_{2}, or methyl chloride, CH_{3}Cl, and other similar volatile liquids. In the adjoining table are given, for certain gases, (1) the number of atmospheres necessary for their liquefaction at 15°, and (2) the boiling points of the resultant liquids under a pressure of 760 mm. C_{2}H_{4} N_{2}O CO_{2}, H_{2}S (1) 42 31 52 10 (2) -103° -92° -80° -74° AsH_{3}, NH_{3} HCl CH_{3}Cl C_{2}N_{2} SO_{2} (1) 8 7 25 4 4 3 (2) -58° -38° -35° -24° -21° -10° [28] Natterer's determinations (1851-1854), together with Amagat's results (1880-1888), show that the compressibility of hydrogen, under high pressures, may be expressed by the following figures:-- _p_ = 1 100 1000 2500 _v_ = 1 0·0107 0·0019 0·0013 _pv_ = 1 1·07 1·9 3·25 _s_ = 0·11 10·3 58 85 where _p_ = the pressure in metres of mercury, _v_ = the volume, if the volume taken under a pressure of 1 metre = 1, and _s_ the weight of a litre of hydrogen at 20° in grams. If hydrogen followed Mariotte's law, then under a pressure of 2,500 metres, one litre would contain not 85, but 265 grams. It is evident from the above figures that the weight of a litre of the gas approaches a limit as the pressure increases, which is doubtless the density of the gas when liquefied, and therefore the weight of a litre of liquid hydrogen will probably be near 100 grams (density about 0·1, being less than that of all other liquids).