verflüssigten Sauerstoffs und Stickstoffs." In this apparatus I liquefied all the gases spoken of, in quantities varying from a few to a good many cubic centimetres of liquid, and determined their critical temperatures and pressures, as well as their boiling-points under atmospheric pressure. I succeeded in solidifying four of these gases, viz., nitrogen (6), carbon monoxide (6), nitric oxide (7), and marsh-gas (7), by lowering the pressure to several millimetres of mercury, and determined their freezing-points and the corresponding pressures of solidification. I also showed that liquid oxygen and air boiling in vacuo at a pressure of 4 millimetres of mercury do not freeze though their temperatures are lowered to below -211° and 220° C. By diminishing the pressure of solid nitrogen to 4 millimetres of mercury, I obtained a temperature reaching -225° C., the lowest that has ever been obtained and measured (6). With the same apparatus I also made a series of experiments with reference to the liquefaction of hydrogen, submitting it to a pressure reaching 180 atm., and at the same time cooling it down to -211° and even -220° C., by means of liquid ethylene and liquid air boiling in vacuo (2). I also showed that the critical temperature of hydrogen is below -220°C. In the same apparatus which served for the liquefaction of hydrogen I liquefied a mixture of two volumes hydrogen and one volume oxygen, and thus obtained a liquid which was in thin layers colourless and transparent (8). 1 likewise determined the specific gravity of oxygen, nitrogen, and methane at the boiling-points of these gases (15). The same apparatus was also of use for examination of the absorption spectrum of liquid oxygen and air, and showed that liquid oxygen in layers not thicker than 7 millim. absorbs light very strongly; also that it gives, among others, two strong absorption bands, corresponding in position to two absorptions of the solar rays, which are due to the oxygen in the air (13). Using liquid oxygen as a cooling agent, I obtained pure ozone in the shape of a dark-blue liquid, easily and violently exploding, and of which I determined the boiling-point (12). Besides the above-mentioned gases, I have examined another series with regard to their behaviour at low temperatures, especially those which had not yet been examined in this respect, or which had been examined without success. I first solidified the following gases and determined their meltingpoints: chlorine (10), hydrochloric acid (10), hydrofluoric acid (11), phosphine (11), arsine (10), stibine (11), ethylene (12), [silicon tetrafluoride (10)]. Moreover, I determined * Does not melt, but evaporates in solid state. * the boiling-points of ethane (17), propane (17), and hydrogen selenide (19), as well as their critical temperatures and pressures. The following liquids were solidified for the first time by me-methyl (14), ethyl (14), amyl alcohol (10), ethyl ether (10), carbon bisulphide (14), and phosphorous chloride (PCI) (14). In order to measure very low temperatures, I used exclusively a hydrogen thermometer: the bulb was plunged in the liquefied gas itself; only exceptionally a few not very low temperatures were measured with the carbon-bisulphide thermometer. Wishing to ascertain how far gas-thermometers may be used to measure very low temperatures, I compared thermometers filled with different gases, and especially the hydrogen thermometer with the nitrogen, oxygen, and nitric-oxide thermometers, immersing them in liquid ethylene, gradually cooled to -151° C. It was proved by this that the three last-mentioned thermometers indicated temperatures not very different from those indicated by the hydrogen thermometer, even at temperatures much lower than the critical temperatures of the corresponding gases. This experiment proved at the same time that nitric oxide does not change its molecular weight, corresponding to the formula NO, even at a temperature so low as 147° C. Profs. Victor Meyer and Daccomo, disregarding the results of my investigations, performed a similar experiment (Liebig's Ann. d. Chem. ccxl. p. 326), but they cooled the nitric oxide with solid carbon dioxide and ether down to -70° C. only. As, according to my experiments on the liquefaction of hydrogen, its critical temperature lies below -220° C., it may be admitted that its coefficient of expansion does not, even at -220° C., differ much from the coefficient of gases at ordinary temperatures, and that hydrogen is the only body which can be used in a thermometer for measuring very low temperatures. The determinations of the temperatures by measuring the quantity of heat taken away from a given body, a silver ball for instance, is not precise; for, as Zakrzewski showed, the specific heat of silver changes in the interval of 0° to -100° C. by about 3 per cent.; so that the temperature which is thus determined must differ from the true one to some not inconsiderable extent. Thermoelectric thermometers, or thermometers based upon the variation of the electric conductivity of metals at low temperatures, can be used only in the limits between which they have been compared with the hydrogen thermometer; every extrapolation may lead to great mistakes. An excellent thermo * * Bullet. Intern. of the Acad. of Cracow, April 1891, p. 146. meter, based on the alteration of the electric conductivity of a thin iron wire, has been constructed by Prof. Witkowski*, my fellow-worker on the optic properties of liquid oxygen. Liquefaction of Large Quantities of Oxygen and Air. [Description of the apparatus given in the Bulletin International de l'Académie des Sciences de Cracovie, June 1890, under the title, "K. Olszewski, Transvasement de l'Oxygène liquide."] Though I have simplified and improved my former apparatus for liquefying gases to such a degree that I have been able to show the liquefaction of oxygen to a numerous auditory during the lecture, yet it leaves much to be desired as regards the practical application of liquefied gases as cooling agents. By means of my former apparatus I was able to obtain only small quantities of liquefied gases; a greater diameter could not be given to the glass tubes used for the purpose, because they would not resist the high pressure which is necessary for liquefaction. Besides this, the use of glass tubes exposed to high pressures is always attended with some danger: it often happens that tubes tested for 60 atm. sometimes burst during the experiment at 40 atm., or even at a lower pressure. I proved long ago (6) that liquid oxygen is the best cooling agent; for it easily gives the temperature of -211.5 C. if the pressure is lowered to 9 millim. of mercury, and it does not freeze even at the pressure of 4 millim. To obtain considerably larger quantities of liquid oxygen for the purpose of applying it as a frigorific agent, it was necessary, instead of brittle glass, to use a substance endowed with more resisting-power, even though not transparent, and to find means to pour the liquid oxygen into a glass vessel. My new apparatus excludes the inconveniences of the former one, and renders it possible to preserve the liquid oxygen a longer time under the ordinary atmospheric pressure. A flask of wrought iron, 5 litres in capacity (such as is used to hold liquid carbon dioxide), containing oxygen under a pressure of 80 atm., is joined by a narrow copper tube to the upper end of a steel cylinder tested at a pressure of 200 atm. This cylinder, having a capacity of 30-100 cub. centim., according to the quantity of oxygen which we wish to liquefy at a time, is immersed in liquid ethylene, of which the temperature may easily be lowered to -14 C. by means of an *Bull. Intern. of the Acad. of Craco air-pump. The lower end of the cylinder is joined by a narrow copper tube to a little stopcock, through which the oxygen, liquefied in the cylinder, can be poured down into an open glass vessel kept cool by the surrounding air. Owing to this isolation, liquid oxygen contained in the open vessel evaporates but very slowly; and when after some time its quantity has considerably decreased, a new portion which has been liquefied in the meantime can be led down into the vessel by turning the cock. This may be continued until the store of ethylene serving to cool the cylinder and the amount of oxygen in the iron flask are exhausted. 240 g. of liquid ethylene suffice to keep the oxygen liquid at the atmospheric pressure for half an hour. By connecting the glass vessel which contains the liquid oxygen with a good air-pump, its temperature can easily be lowered to -211° C. Thus was solved the problem of liquefying considerable quantities of oxygen without the slightest danger. This decides me to resume my former experiments concerning the liquefaction of hydrogen; and I hope thereby to obtain more successful results. To this description, which I have given in a literal translation from the above-mentioned Bulletin International, I subjoin a diagram (fig. 1) representing a section of my apparatus, which I shall shortly explain. But I must remark that in the same year (1890), when proceeding to my experiments on the liquefaction of hydrogen, I doubled the dimensions of the apparatus without changing anything in its construction. The dimensions that I shall afterwards give refer to the enlarged apparatus. The steel cylinder a, of a capacity of 200 cub. centim., has its upper end connected by means of a thin copper tube with a metallic manometer b, and an iron bottle c 10 litres in capacity, containing dry oxygen or air under a pressure of 100 atm., the lower aperture of the cylinder a being connected by a very thin copper tube with the little cock d, which serves to let out the liquefied oxygen or air. The cylinder a is placed in a glass vessel m with double or treble walls, which serves to receive the liquid ethylene, of which the iron flask fis the reservoir. This flask (3 litres in capacity) is shaped like a siphon, and contains about 1 kilog. of liquid ethylene. The ethylene, liquefied and cooled in the flask ƒ by means of ice and salt, passes, after the cock is turned, into the condenser g filled with a mixture of ether and solid carbon dioxide. To lower the temperature of this mixture −78° C.) Phil. Mag. S. 5. Vol. 39. No. 237. Feb. 1895. to 100° C., the tube n is connected with a smaller doubleaction pump which at every movement of the piston draws out about 1 litre of gas. When the pressure in the condenser g is lowered to 50 millim. Hg, the vessel m is connected, by turning the cock i, with a large vacuum-pump provided with sliding-valves, and at the same time, by slowly turning the cock, the ethylene, already considerably cooled, is let down into the vessel m. The liquid ethylene, on entering the vessel m, at first evaporates quickly, and the vapour is forced back into a gasometer by means of the large pump alluded to, and may again be employed as a frigorific agent for a subsequent experiment. Owing to the very great difference between the temperatures of the steel cylinder and the liquid ethylene, the latter does not at first touch the cylinder directly, but is separated from it by a thin layer of vapour, being in |