_Analytic Uses._--Another important use of liquid gases is as analytic agents, and for this purpose liquid air is becoming an almost essential laboratory reagent. It is one of the most convenient agents for drying gases and for their purification. If a mixture of gases be subjected to the temperature of liquid air, it is obvious that all the constituents that are more condensable than air will be reduced to liquid, while those that are less condensable will either remain as a gaseous residue or be dissolved in the liquid obtained. The bodies present in the latter may be separated by fractional distillation, while the contents of the gaseous residue may be further differentiated by the air of still lower temperatures, such as are obtainable by liquid hydrogen. An apparatus such as the following can be used to separate both the less and the more volatile gases of the atmosphere, the former being obtained from their solution in liquid air by fractional distillation at low pressure and separation of the condensable part of the distillate by cooling in liquid hydrogen, while the latter are extracted from the residue of liquid air, after the distillation of the first fraction, by allowing it to evaporate gradually at a temperature rising only very slowly. In fig. 8, A represents a vacuum-jacketed vessel, containing liquid air; this can be made to boil at reduced pressure and therefore be lowered in temperature by means of an air-pump, which is in communication with the vessel through the pipe _s_. The liquid boiled away is replenished when necessary from the reservoir C, _p_ being a valve, worked by handle _q_, by which the flow along _r_ is regulated. The vessel B, immersed in the liquid air of A, communicates with the atmosphere by _a_; hence when the temperature of A falls under exhaustion below that of liquid air, the contents of B condense, and if the stop-cock _m_ is kept open, and _n_ shut, air from the outside is continuously sucked in until B is full of liquid, which contains in solution the whole of the most volatile gases of the atmosphere which have passed in through _a_. At this stage of the operation _m_ is closed and _n_ opened, a passage thus being opened along _b_ from A to the remainder of the apparatus seen on the left side of the figure. Here E is a vacuum vessel containing liquid hydrogen, and _d_ a three-way cock by which communication can be established either between _b_ and D, between _b_ and _e_, the tube leading to the sparking-tube _g_, or between D and _e_. If now _d_ is arranged so that there is a free passage from _b_ to D, and the stop-cock _n_ also opened, the gas dissolved in the liquid in B, together with some of the most volatile part of that liquid, quickly distils over into D, which is at a much lower temperature than B, and some of it condenses there in the solid state. When a small fraction of the contents of B