An improved form of apparatus for the fractionation is represented in fig. 9. The gases to be separated, that is, the least volatile part of atmospheric air, enter the bulb B from a gasholder by the tube _a_ with stop-cock _c_. B, which is maintained at a low temperature by being immersed in liquid hydrogen, A, boiling under reduced pressure, in turn communicates through the tube _b_ and stop-cock _d_ with a sparking-tube or tubes _f_, and so on through _e_ with a mercurial pump. To use the apparatus, stop-cock _d_ is closed and _c_ opened, and gas allowed to pass from the gasholder into B, where it is condensed in the solid form. Stop-cock _c_ then being closed and _d_ opened, gas passes into the exhausted tube _f_, where it is examined with the spectroscope. The vessel D contains liquid air, in which the tube _e_ is immersed in order to condense vapour of mercury which would otherwise pass from the pump into the sparking-tube. The success of the operation of separating all the gases which occur in air and which boil at different temperatures, depends on keeping the temperature of B as low as possible, as will be understood from the following consideration:-- The pressure _p_, of a gas G, above the same material in the liquid state, at temperature T, is given approximately by the formula B log p = A - ---, T where A and B are constants for the same material. For some other gas G´ the formula will be B1 log p1 = A1 - ---, T and p B1 - B log --- = A - A1 + ------, p1 T Now for argon, krypton and xenon respectively the values of A are 6.782, 6.972 and 6.963, and those of B are 339, 496.3 and 669.2; so that for these substances and many others A - A1 is always a small quantity, while (B1 - B)/T is considerable and increases as T diminishes. Hence the ratio of _p_ to _p_1 increases rapidly as T diminishes, and by evaporating all the gases from the solid state, and keeping the solid at as low a temperature as possible, the gas that is taken off by the mercurial pump first consists mainly of the substance which has the lowest boiling point, in this case nitrogen, and is succeeded with comparative abruptness by the gas which has the next higher boiling point. Examination of the spectrum in the sparking-tube easily reveals the change from one gas to another, and when that is observed the reservoirs into which the gases are pumped can be changed and the fractions stored separately. Or several sparking-tubes may be arranged so as to form parallel communications between _b_ and _e_, and can be successively sealed off at the desired stages of fractionation. [Illustration: FIG. 10.]