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the spheroidal state: it is only when the cylinder has cooled considerably that the ethylene comes into direct contact with it; a violent boiling of the liquid shows that this has taken place. Henceforward a more abundant stream of ethylene may be poured in until it fills the vessel m above the cylinder a; afterwards the cock h is closed, and the smaller pump which is connected with the vessel m by means of the tube i must be kept working without interruption, by which the temperature of the ethylene is continually lowered; and when it has fallen below the critical temperature of the gas contained in the flask c (oxygen or air), the cock of this flask is slowly opened. The gas enters the cooled cylinder a under the pressure indicated by the manometer b, and becomes liquid speedily enough, in consequence of which the index of the manometer shows a constant fall; when it becomes stationary, the cylinder a is wholly filled with the liquefied gas. When this has been done, the bottle c is closed, and by slowly opening the cock d the liquefied gas is poured into the glass vessel placed underneath, which is secured from external heat by its triple walls. Whilst the liquid oxygen is being poured from the cylinder a, the pressure descends to 20 atm., and remains at this point as long as any liquid oxygen remains in the cylinder: it is only when there is no more that the pressure becomes less than 20 atm. As the liquefied gas comes under the ordinary atmospheric pressure, a considerable part of it resumes the gaseous state, and only half or a third of the liquid remains in the glass vessel after having cooled down to its boilingtemperature. In order to prevent the collected liquid being blown out by the powerful jet, the thin copper tube through which the stream flows is closed beneath, and provided with four lateral openings.
I mentioned above that the temperature of ethylene in the vessel m must be lowered by pumping to less than the critical temperature of the gas we wish to liquefy; but it is not necessary to measure the temperature of the ethylene. It is sufficient to measure its pressure. According to my computations (3), the following relation exists between the pressure and the temperature of the liquefied ethylene :
The critical temperatures, also according to my reckonings, are for oxygen -118°.8 C.; for air —140° C.; for nitrogen, -146° C. The pressure of ethylene must accordingly be lowered to 100-40 millim. in order to liquefy oxygen; to liquefy air, to 20 millim., and to liquefy nitrogen, to 10 millim. The pressure to which ethylene is subjected in the vessel m is indicated by the metallic vacuometer k, for which in more exact experiments a mercury manometer may be substituted. The pressures I have stated above for ethylene are maximum pressures at which it is still possible to bring about the liquefaction of large quantities of the corresponding gases; but it is generally necessary to take care that the pressure of the liquid ethylene be lowered to the minimum obtainable by the pump serving for the experiment. The inore we lower the temperature of the ethylene, the sooner the cylinder a is filled with the liquefied gas, and the more liquid we obtain by pouring out the liquefied gas under the ordinary pressure. To lower the temperature of a considerable quantity of ethylene to 146° or 150° C., which temperature is absolutely necessary for the liquefaction of large quantities of air and nitrogen, a large air-pump with rapid and powerful action is required. When in 1890 I used a smaller pump, which drew out two litres of gas at each complete double stroke, I could only liquefy oxygen in the described apparatus ; but when in the following year 1 brought from Burckhard's factory, Basle, an excellent sliding valve-pump, six times larger than the preceding one, and working with great speed and perfection, I was enabled in the same apparatus easily to obtain at once 200 cub. centim. of liquid air. It is true that I never tried to liquefy nitrogen in large quantities, but I believed it unnecessary, taking into account my former experiments with nitrogen (6). I had already examined the properties of liquid and solid nitrogen, and showed that the use of liquid nitrogen as a frigorific agent is of no greater advantage than that of oxygen or air. However, considering that by using both of my pumps the pressure of ethylene in the vessel m is easily lowered to 10 millim. and the temperature to -150° C., I can decidedly affirm, that all so-called permanent gases, except hydrogen, may be liquefied in my apparatus. When we want to obtain such a considerable rarefaction of ethylene, the compressing tube of the larger pump must be connected with the exhausting tube of the smaller one, whereby the effect of the larger pump is exceedingly increased.
It need hardly be said that the processes connected with the liquefaction of large quantities of gases, as the liquefaction of ethylene in the cylinder f, the charging of the cylinder c
with oxygen or air, and the working of both the larger and smaller pump, must be accomplished not by hand, but by means of a gas-motor of 1-3 H.P.
The indispensable condition for such experiments to be successful, is the purity of the gases to be liquefied; the liquid carbon dioxide, used as a frigorific agent, must be free from moisture; the ethylene, oxygen, and air must be completely dry and free from carbonic acid. A small amount of carbon dioxide in oxygen or air renders these gases turbid and opaque when liquefied: a slight quantity of moisture may freeze and stop up the narrow tubes which join together the component parts of the apparatus, and thus frustrate the experiment, prepared with so much trouble. In order absolutely to purify oxygen and air from water and carbon dioxide, there must be put into the bottle a, before it is charged, 1 kilog. of potassium hydroxide in thin sticks, that will in a few days completely absorb the moisture and carbonic acid which may be produced whilst the gas is being forced into the flask, in consequence of the action of the condensed oxygen on the leather piston of the pump.
The quantities of liquid oxygen and air I got by means of the apparatus described were quite sufficient for carrying out my experiments on the liquefaction of hydrogen and the examination of the optical properties of liquid oxygen, which I shall shortly describe. On that account I did not think it necessary to increase the dimensions of the apparatus (which, however, it would have been easy to do), the more so because, after having liquefied the first 200 cub. centim. of oxygen or air, the operation may be repeated every 15 minutes, on an equal quantity of gas, as long as the store of liquid ethylene suffices, and the pressure in the bottle c does not fall below 60 atm. in this case other cylinders, containing the whole charge of the corresponding gas, should take the places of c and f.
I now pass to the description of the experiments I have executed by means of the apparatus described, either by myself or working in conjunction with Prof. Witkowski.
On the Absorption Spectrum and the Colour of
[Notice published in German in the Bulletin International de l'Académie de Cracovie, January 1891, and in Wiedemann's Annalen, 1891, vol. xlii. p. 633.]
In my earlier investigations I found four absorptionbands in the spectrum of liquid oxygen, corresponding to the
*Wiener Akademie Berichte, xcv. p. 257.
wave-lengths 628, 577, 535, and 480. Messrs. Liveing and Dewar, who at a later date examined the absorption spectrum of gaseous oxygen in a long steel tube under a strong pressure, found the same four absorption-bands in the visible part of the spectrum, and, besides them, in the utmost red two others, corresponding to the lines A and B of the solar spectrum, which were also observed by Egoroff and Janssen.
The apparatus I have described enabled me to repeat my former experiments and to examine more exactly the absorption spectrum of a thicker layer of liquid oxygen in the utmost red.
The liquid oxygen was poured out of the liquefying apparatus into a thin-walled glass tube, the lower end of which was soldered and closely fixed into three glass vessels one outside of the other, to preserve it from external heat. The column of oxygen was 30 millim. thick and about 50 millim. deep. In this glass tube the oxygen remained at its boiling temperature (-181°-4 C.) under atmospheric pressure, in sufficient quantity for the experiment, during more than half an hour, though it was strongly heated by a Drummond's lime-light, concentrated on it by means of a collecting-lens : this light was used to produce the absorption spectrum. In examination of the spectrum I used a universal spectroscope of Krüss, with a prism of Rutherford. Besides the four known absorption-bands, the experiment also proved the existence of a fifth band, corresponding to the solar line A: it is somewhat blurred, but can be seen distinctly enough if a red glass is put between the source of light and the slit in the spectroscope. This band appeared feebler than the absorption-bands which correspond to the wave-lengths 628, 577, and 480, but stronger than the absorption-band at 553. With this relatively slight dispersion, the band A could of course not be decomposed into lines. And this time too I was unable to perceive any absorption corresponding to the solar B.
The experiments in 1883 made out liquid oxygen to be a colourless fluid, for but small quantities of it were then obtained. Since then I have several times observed that oxygen, when liquefied in wider tubes about 15 mm. thick, appears of a bluish colour. During my experiments already alluded to, in which for the first time a relatively considerable quantity of liquid oxygen was collected in a glass vessel, its bluish colour appeared quite distinctly. The oxygen was prepared from potassium chlorate and manganese dioxide; to ascertain that it contains no traces of ozone from which the * Phil. Mag.  xxvi. p. 286 (1888.)
bluish colour might be derived, it was carefully tested for that substance. Paper moistened with potassium iodide and starch, exposed for several hours to the action of the oxygen used, was not coloured at all; and when the oxygen was made to pass through a solution of potassium iodide and starch the result was the same. It remained for several weeks in an iron flask, in contact with solid potassium hydroxide, and was by this means completely purified from carbonic acid, vapour of water, and chlorine. After these experiments, there is no doubt that liquid oxygen, seen in layers of about 30 millim., possesses a distinctly bluish colour. This colour is, moreover, quite in agreement with the absorption spectrum of oxygen. It was rather strange that a colourless liquid-as it was hitherto thought to be-should give such a pronounced absorption spectrum, in which the absorptions in orange, yellow, and red are preponderant; but after the bluish colour of liquefied oxygen was proved, this apparent contradiction no longer exists.
I may conclude with a word or two about the colour of the sky. There exist so many hypotheses on that point, that I scarcely venture to add one more. But the simplest theory, in my opinion, would be to ascribe that colour to the principal component part of our atmosphere, which-at least in a liquid state-is blue.
On the Critical Pressure of Hydrogen.
[These researches were published in Polish, in the Reports of the Cracow Academy 1891, vol. xxiii. p. 385; a short German abstract therefrom was printed in the Bullet. Întern. of the same Academy. The following description is taken from the first-named source, and is explained by figs. 2 and 3.]
In my former researches, undertaken in 1884 and 1885, I showed that hydrogen cannot be liquefied even by employing the lowest obtainable temperatures and a high pressure, reaching to 190 atm. ; and that it is only during the sudden expansion from a high pressure that a greater or less trace of liquefaction can for an instant be seen. This depends on the temperature of the frigorific medium, as well as on the initial pressure of expansion. As cooling agents there were employed :-oxygen boiling under atmospheric pressure (t= -18104) and in vacuo, reaching to 9 millim. (t=-211°•5) ; also air boiling under atmospheric pressure (t=-191°4) and in vacuo at 10 millim. (t=-220°), as well as nitrogen boiling under atmospheric pressure (t=-194°4) and in vacuo at 60 millim. (t=-213°).