HYDROGEN (from v8utp, water, and yewdw, to generate) is a chemical element which in the free state occurs in volcanic gases, but exists for the most part in combination with oxygen as water. As its elemental characteristics and the chief compounds of hydrogen have already been described in the article CHEMISTRY, vol. v. pp. 478, 483, 491, 499, 544, &c, it is here only necessary to refer to the liquefaction of gaseous hydrogen.
At the close of 1877 and in the beginning of 1878, not only hydrogen, but all the so-called permanent gases, were reduced to the liquid state, an achievement the more remarkable as it was the result of the simultaneous but entirely independent labours of two distinguished physicists, M. Cailletet of CMtillon-sur-Seine and M. Pictet of Geneva. The experiments of the former, who clearly demonstrated the possibility of liquefying acetylene, carbonic oxide, hydrogen, methane, nitric oxide, nitrogen, and oxygen, are described in detail in the Annates Je Chimie et cle Physique, ser. 5, vol. xv. p. 132 ; those of M. Pictet, who operated only upon oxygen and hydrogen, are detailed in the same journal, ser. 5, vol. xiii. p. 145. The instrumental means employed by them were very dif-ferent, as will be evident from the following description.
M. Cailletet's apparatus is represented in the annexed sketch (fig. 1). The gas under experiment is contained in a stout glass tube of narrow bore of the form shown in fig. 2.

-Cailletet's Apparatus for Liquefaction of Gases.

Fig. 2.
; the tube A with liquid sulphur dioxide, car-
To fill this tube with gas, both ends being open, a globule of mercury is first introduced at the lower curved extremity ; the tube is then placed in a nearly horizontal position, the curved extremity is connected with the holder containing the gas, or with the apparatus in which the gas is being evolved, by means of caoutchouc tubing, and a current of the pure dry gas is passed through the tube until the air is entirely ex-pelled ; this being effected, the point opposite to the curved extremity is sealed in the blowpipe flame ; the tube is then brought into a vertical position, so that the globule of mercury closes the lower extremity, the caoutchouc tube is withdrawn, and the tube AA thus filled is screwed into its place in the cylinder B. The lower part of the cylinder contains mercury, the upper part water. The pressure is exerted by forcing a plunger into a massive steel cylinder filled with water by means of the screw and wheel seen on the left in fig. 1, the water being forced from this cylinder through the fine coiled tube into the upper part of the cylinder in which the glass tube is fixed ; by rapidly turn-ing the wheel seen on the right of the figure, a valve may be opened, allowing of the escape of the water, and thus the pres-sure on the gas may be sud-denly withdrawn. The pressure to which the gas is submitted is measured by means of either of the manometers seen on the right. The gas may be cooled by surrounding
bon dioxide, or nitrous oxide, and the deposition of ice on the out-side of the cylinder containing the refrigerating liquid is prevented by covering it with a glass cylinder or bell jar, under which is placed some desiccating material. By strongly compressing a gas in this apparatus, and then suddenly relieving it from pressure, an enor-mous reduction in its temperature is effected, owing to the sudden expansion of the gas, and it is under these circumstances that lique-faction takes place in cases where pressure alone is ineffective. On submitting hydrogen to a pressure of nearly 300 atmospheres, and then suddenly withdrawing the pressure, M. Cailletet observed the formation of a fine mist in the interior of the tube ; the experiments of Andrews and his own previous observations had shown that this result afforded incontestable proof of the presence of liquid, if not of solid, particles.

The general disposition of M. Pictet's apparatus is seen in fig. 3. It is of a mueh more complex character, but permits of the ex-periment being made on a comparatively very large scale. The condensation of the gas is effected in a copper tube of narrow bore,

connected at the one end with a manometer capable of indicating pressures up to 800 atmospheres, and with a fine steel stop-cock, and at the other with a very strong wrought-iron vessel in which the hydrogen is generated by heating a perfectly dry mixture of potassium formate with potassium hydroxide ; these two substances enter into reaction in accordance with the equation HCOOK + HOK = K2C03 + H2, the hydrogen being evolved in a perfectly regular manner when the temperature is maintained at 225°. The dimensions of the condensation tube are—internal diameter -001 metre, external diameter '015 metre, length 4'16 metres ; and the retort has a capacity of 1659 cubic centimetres. The charge introduced consisted of 500 grammes potassium hydroxide and 1251 grammes potassium formate. The condensation tube is surrounded with a wider tube containing condensed carbon dioxide or nitrous oxide', these tubes are enclosed in the lower box seen in the sketch.' The upper box contains a tubular arrangement in which carbon dioxide or nitrous oxide from the gasholder is reduced to the liquid state. The pumps are such as are used in ice-making machines, one of each pair being an exhausting, the other a condensing pump; one pair of these pumps is employed in condensing and volatilizing the sulphur dioxide, and the other in condensing and volatilizing the carbon dioxide or nitrous oxide. Figs. 4 and 5 show the appa-ratus in vertical section and in plan.
In working the apparatus, in the first place liquid sulphur dioxide from the reservoir seen between the first pair of pumps is charged into the outer tube of the condenser in the upper box, and is caused rapidly to evaporate by the action of the exhausting pump in connexion with the tube, while the other pump serves to recondense it in the reservoir, which is a kind of tubular boiler provided with a cold water circulation. The temperature of the liquid sulphur dioxide is thus reduced to - 65° C.; on allowing the carbon dioxide or
nitrous oxide to enter the inner tube of the condenser thus cooled, it begins to liquefy under a pressure of about 4 atmospheres, and the condensation of the whole of the gas is effected at a pressure of from 4 to 7 atmospheres, the temperature of the liquid sulphur dioxide fluctuating between - 65° and - 50°. The liquid carbon dioxide

atmo-spheres ; on then opening the stop-cock, the orifice being illuminated by a powerful electric light, an opaque jet of a highly characteristic steel-blue tint was seen to issue forth. At the same moment, a sharp hissing noise like that produced on plunging a red-hot bar of iron into water was heard, and simultaneously a highly
-0
characteristic rustling noise on the ground, recalling the sound of small seed falling. Moreover, the jet was not continuous as in the ease of oxygen, but intermittent, each projection of matter being attended with the peculiar rustling noise alluded to; in fact, owing to the great reduction in temperature, due to the sudden volatilization of the liquid, portions became solidified in the tube. On closing the stop-cock after several seconds, the pressure being still 370 atmo-
leres, the pressure fell to 320 atmospheres, then slowly rose to 330 atmospheres, and remained constant for several minutes ; on now opening the stop-cock again, a jet issued of extremely short duration, accom-panied with the violent projection of solid particles, tut then nothing escaped although the pressure was 315 atmospheres, an indication that the hydrogen had solidified in the interior of the tube. On ceasing to aspirate from the tube surrounding the condensation tube, and thus allowing the temperature of the latter to rise somewhat, discharges took place with increasing frequency, until after about a quarter of an hour the pressure fell to zero.
Pictet thus succeeded in liquefying and solidifying hydrogen on January 10th, 1878, Cailletet having demonstrated its liquefaction on December 30th previously.

== FURTHER TEXT TO BE ADDED ==


Footnote

The statement in the article CHEMISTRY, vol. v. p. 479, that hydrogen has never been liquefied, was true at the date of publication.