_Oxygen._--Liquid oxygen is a mobile transparent-liquid, possessing a faint blue colour. At atmospheric pressure it boils at -181.5° C.; under a reduced pressure of 1 cm. of mercury its temperature falls to -210° C. At the boiling point it has a density of 1.124 according to Olszewski, or of 1.168 according to Wroblewski; Dewar obtained the value 1.1375 as the mean of twenty observations by weighing a number of solid substances in liquid oxygen, noting the apparent relative density of the liquid, and thence calculating its real density, Fizeau's values for the coefficients of expansion of the solids being employed. The capillarity of liquid oxygen is about one-sixth that of water; it is a non-conductor of electricity, and is strongly magnetic. By its own evaporation it cannot be reduced to the solid state, but exposed to the temperature of liquid hydrogen it is frozen into a solid mass, having a pale bluish tint, showing by reflection all the absorption bands of the liquid. It is remarkable that the same absorption bands occur in the compressed gas. Dewar gives the melting-point as 38° absolute, and the density at the boiling-point of hydrogen as 1.4526. The refractive index of the liquid for the D sodium ray is 1.2236. _Ozone._--This gas is easily liquefied by the use of liquid air. The liquid obtained is intensely blue, and on allowing the temperature to rise, boils and explodes about -120° C. About this temperature it may be dissolved in bisulphide of carbon to a faint blue solution. The liquid ozone seems to be more magnetic than liquid oxygen. _Nitrogen_ forms a transparent colourless liquid, having a density of 0.8042 at its boiling-point, which is -195.5° C. The refractive index for the D line is 1.2053. Evaporated under diminished pressure the liquid becomes solid at a temperature of -215° C., melting under a pressure of 90 mm. The density of the solid at the boiling-point of hydrogen is 1.0265.