and poured into water) is placed in a two-necked Woulf's bottle (Fig. 29), and a quantity of sulphuric acid, previously diluted with six times its volume of water, is introduced by means of the funnel. A brisk action sets in, and hydrogen is rapidly disengaged. After the lapse of a few minutes the air within the apparatus will be swept out by the hydrogen, when the gas may be collected over water in the pneumatic trough.

The hydrogen so obtained is never absolutely pure; it is liable to contain traces of arsenuretted hydrogen, sulphuretted hydrogen, phosphoretted hydrogen, oxides of nitrogen, and nitrogen. The nitrogen is derived from the air, which finds its way through joints in the apparatus, and also from air dissolved in the acid. There is no known process for removing this impurity. The other gases are due to impurities in the zinc and the sulphuric acid, and can be removed, if required, by passing the hydrogen through a series of tubes containing absorbents (see page 210).

Absolutely pure sulphuric acid, even when diluted with water, has no action upon perfectly pure zinc.

Scrap iron may be substituted for zinc, but the hydrogen so obtained is much less pure, and is accompanied by compounds of carbon (derived from the carbon in the iron), which impart to the gas an unpleasant smell; the reaction in this case is the following:

Hydrochloric acid can be employed in place of sulphuric acid with either zinc or iron, the reaction then being :

Zn+2HCl=ZnCl2 + H2

(6.) Hydrogen in a high degree of purity is conveniently prepared in small quantity by the electrolysis of water acidulated with sulphuric acid (see page 207).

(7.) Hydrogen is disengaged when certain metals, such as zinc, iron, magnesium, and aluminium, are boiled with an aqueous solution of potassium or sodium hydroxide. Thus, in the case of zinc, when this metal in the form of filings is boiled with a solution of potassium hydroxide, hydrogen is evolved, and a compound of zinc, potassium, and oxygen remains in solution, namely, potassium zinc oxide, thus:

2KHO+ZnH2+ Zn K2O2.

(8.) Hydrogen is also obtained by heating alkaline oxalates, or

formates, with either potassium or sodium hydroxide, with the simultaneous formation of an alkaline carbonate; thus with sodium oxalate :

Na,C2O4+2Na HO=H2+2Na,CO3.

=

Properties.-Hydrogen is a colourless gas, and has neither taste nor smell. It is the lightest known substance, being 14.435 times lighter than air. Its specific gravity is 0.0693 (air 1). One litre of the gas at o° C., and under a pressure of 760 mm. of mercury (ie. the standard temperature and pressure) weighs 0.0896 gramme; or I gramme of hydrogen at the standard temperature and pressure occupies 11.165 litres.

On account of its extreme lightness, hydrogen may be poured

upwards from one vessel to another. If a large beaker be suspended mouth downwards from the arm of a balance and counterpoised, and the contents of a jar of hydrogen be poured upwards into the beaker, the equilibrium of the system will be disturbed, and the arm carrying the beaker will rise.

The lightness of hydrogen can also be shown by causing a stream of the gas to issue from a tube placed in such a position that its shadow is cast upon a white screen by means of a powerful electric light. When the gas is streaming from the tube, its upward rush will be visible upon the screen as a distinct shadow, caused by the difference between the refractive power of air and hydrogen (Fig. 30).

Hydrogen is inflammable and burns with a non-luminous flame, the temperature of which is very high. The product of the combustion of hydrogen is water, and if a jet of the gas be burned beneath the apparatus seen in Fig. 31, considerable quantities of water may be collected in the bulb. In the act of combustion the hydrogen combines with the oxygen of the air, forming the oxide of hydrogen, namely, water:*

If hydrogen be mixed with the requisite quantity of air, or oxygen, and a light applied to the mixture, the combination of the two gases takes place instantly with a violent explosion; hence the necessity of carefully expelling all the air from the apparatus in which hydrogen is being generated before applying a flame to the issuing gas.

Hydrogen will not support the combustion of ordinary combustibles; thus, if a burning taper be thrust into a jar of the gas, the hydrogen itself will be ignited at the mouth of the jar, which must be held in an inverted position, but the taper will be extinguished; on withdrawing the taper it may be reignited by the burning hydrogen.

FIG. 31.

Although hydrogen is not poisonous, it is incapable of supporting animal life owing simply to the exclusion of oxygen. When mixed with air and inhaled, it raises the pitch of the voice

* From this fact the name Hydrogen (signifying the water producer) is derived. + Baker has recently shown (Jour. Chem. Soc., April 1902) that if the two gases are perfectly pure and dry, they may be strongly heated without uniting. In these experiments a coil of silver wire suspended in the gases was heated by means of an electric current until the silver melted, that is, above 1000°; but no chemical union of the oxygen and hydrogen took place, although the ordinary temperature of explosion is 615° (V. Meyer). Baker has also shown that if a mixture of these two gases, which has not been specially dried, be exposed to sunlight, combination slowly takes place; whereas with the perfectly dry gases no measurable combination occurs.

M

almost to a falsetto. The same effect may be seen by sounding a pitch-pipe, or organ-pipe, by means of a stream of hydrogen instead of ordinary air, when it will be noticed that the note given out is greatly raised in pitch.

Hydrogen is very slightly soluble in water. It was formerly believed that this gas formed an exception to the rule that the solubility of gases in water diminishes with rise of temperature, and it was supposed that the solubility of hydrogen was constant between the temperatures o° and 25°. More recent experiments have shown that this is not the case. The solubility of this gas, as determined by W. Timofejeff (1890), is seen in the table on p. 113. Hydrogen was first liquefied on May 10, 1898, by Dewar. Prior to this time it had never been obtained as a coherent or static liquid-that is, a liquid with a meniscus-although momentary indications of its liquefaction had been obtained by Olszewski as far back as 1895. The critical temperature of hydrogen (namely, -238°) being below the lowest point obtainable by the rapid ebullition of liquid oxygen or air, no external refrigerating agent is available which is capable of cooling the gas below its critical point, and therefore of causing its liquefaction. By an extension of the principle of self-cooling explained on p. 76, however, namely, by causing a jet of the gas previously cooled to -205° to continuously escape from a fine orifice under a pressure of 180 atmospheres, Dewar has succeeded in collecting considerable quantities of liquid hydrogen in specially constructed vacuum-jacketed vessels.

The

Liquid hydrogen is clear and colcurless as water, thus disposing of the theory once advocated that if obtained in the liquid state hydrogen would be found to exhibit metallic properties. boiling-point of the liquid is 253° (Dewar), at which temperature air is immediately solidified. Thus, if a tube sealed at one end, but freely open to the air at the other, be immersed in liquid hydrogen, the cooled end of the tube quickly becomes filled with solidified air. Similarly, oxygen is frozen to a pale-blue solid.

The specific gravity of liquid hydrogen is about 0.07; that is to say, it is only about 14th the density of water, or about 14 c.c. of the liquid weighs only 1 gramme. By its own rapid evaporation liquid hydrogen has been frozen to a white solid mass, which melts at - 257°; and by the rapid evaporation of this solid a temperature of 260° has been obtained, which is the lowest degree of cold ever reached. By means of liquid hydrogen as a refrigerating

agent, the more recently discovered gas Helium has also been liquefied (see p. 269), hence all the known gases have now been condensed to the liquid state.

Occluded Hydrogen, or Hydrogenium.-Certain metals, such as iron, platinum, and notably palladium, possess the property when heated of absorbing a large quantity of hydrogen, and of retaining it when cold. Graham found that at a red heat palladium absorbed, or occluded, about 900 times its own volume of hydrogen, while even at ordinary temperatures it was able to absorb as much as 376 times its volume.* Graham concluded that the hydrogen so occluded assumed the solid form, and was alloyed with the palladium, and to denote the metallic nature of the gas he gave to it the name hydrogenium. From later experiments of Troost and Hautefeuille, it seems probable that a definite compound of hydrogen and palladium exists, of the composition of Pd2H.

After its absorption of hydrogen the metal presents the same appearance as before, although some of its physical properties have become slightly modified; thus it is more magnetic than ordinary palladium, and its electric conductivity is considerably reduced.

In view of our present knowledge of the entire absence of any metallic characters in liquid or solid hydrogen (gained, however, entirely since Graham's time), the view that this is an alloy is no longer tenable, as this term is only strictly applicable to the union of metals.

The absorption of hydrogen by palladium is readily seen by making a strip of palladium foil the negative electrode in an electrolytic cell containing acidulated water, the positive pole being of platinum. Oxygen will be evolved from the latter electrode, while for some time no gas will be disengaged from the surface of the palladium, the hydrogen being completely absorbed by the metal. During the absorption of hydrogen the palladium undergoes an increase in volume: Graham observed the increase in length of a palladium wire to be equal to 1.6 per cent.

According to Neumann and Strientz (Zeitschrift für Analytische Chemie, vol. 32), one volume of various metals in a fine state of division is capable of absorbing the following amounts of hydrogen :