[1] Lecture delivered before the Society of Arts, from the Journal of the Society.
A RESEARCH ON THE LIQUEFACTION OF HELIUM.[1]
My experiments on the liquefaction of helium were carried out with a sample of that gas, sent to me by Prof. Ramsay from London, in a sealed glass tube holding about 140 c. cm. I take this opportunity of rendering him my most sincere thanks. In his letter Prof. Ramsay informed me that the gas had been obtained from the mineral clevite, and that it was quite free from nitrogen and other impurities, which could be removed by circulation over red hot magnesium, oxide of copper, soda lime, and pentoxide of phosphorus. The density of the gas was 2.133 and the ratio of its specific heats (Cp/Cv) 1.652, the latter figure indicating that the molecule of helium was monatomic, as had already been found to be the case with argon. Prof. Ramsay further informed me that the gas was only very slightly soluble in water, 100 c. cm. of water dissolving scarcely 0.7 c. cm. of helium.
From the results of my earlier experiments I had been led to expect that it would be only possible to liquefy helium at a very low temperature; the small values obtained for the density and solubility of the gas, together with the fact that its molecule is monatomic, indicating a very low boiling point. For this reason I did not consider it necessary to use liquid ethylene as a preliminary cooling agent, but proceeded directly to conduct my experiments at the lowest temperature that could be produced by means of liquid air. The apparatus employed in these investigations is figured in the accompanying diagram.
The helium was contained in the glass tube, c, of the Cailletet's apparatus, C. The tube, c, reached to the bottom of a glass vessel, a, which was intended to contain the liquid air. The vessel, a, was surrounded by three glass cylinders, b, b' and b", closed at the bottom and separated from one another. The outer vessel, b", was made just large enough to fit into the brass collar, o, which supported the lid, u, of the apparatus. The tube, a, fitted into an opening in the center of the lid; the tube, t, connected with an apparatus delivering liquid oxygen, passed through a hole on the right. The vessel, b, was also connected with a mercury manometer and air pump by means of a T tube, p, v, one arm of which passed through the third hole in the lid of the apparatus. The tube, a, was closed by a stopper, through which passed the tube, c, of the Cailletet's apparatus, a tube connected with the drying apparatus, u, u', and one limb of a T tube, by means of which the manometer and air pump could be put in connection with the interior of the vessel. The lower part of the whole apparatus was inclosed in a thick walled vessel, e, containing a layer of phosphorus pentoxide.
By turning the valve, k, the vessel, b, could be partially filled with liquid oxygen, which, under a pressure of 10 mm. of mercury, boiled at about -210° C. Almost immediately the gaseous air began to condense and collect in the tube, a; a supply of fresh air was constantly maintained through the drying tubes, u and u', which were filled with sulphuric acid and soda lime respectively. When the quantity of liquid air ceased to increase, the tap on the U tube, u, was closed, the T tube, p' v', was connected with the manometer and air pump, and the liquid air was made to boil under a pressure of 10 mm. of mercury. In order to protect the liquid air from its warmer surroundings, a very thin, double wall tube, f, reaching to the level of the liquid in the outer vessel, was placed inside the tube, a. When, as in some of my experiments, liquid oxygen was used in the inner vessel, this part of the apparatus was dispensed with.
Using the apparatus I have just described, I carried out two series of experiments, in which liquid air and liquid oxygen were employed as cooling agents. The tube of the Cailletet's apparatus was thoroughly exhausted by means of a mercury pump, and then carefully filled with dry helium. In the first series of experiments the helium, confined under a pressure of 125 atmospheres, was cooled to the temperature of oxygen boiling, first under atmospheric pressure (-182.5°), and then under a pressure of 10 mm. of mercury (-210°). The helium did not condense under these conditions, and even when, as in subsequent experiments, I expanded the gas till the pressure fell to twenty atmospheres, and in some cases to one atmosphere, I could not detect the slightest indication that liquefaction had taken place. The first time that I compressed the gas I had, indeed, noticed that a small quantity of a white substance separated out and remained at the bottom of the tube when the pressure was released. Possibly this may have been due to the presence of a small trace of impurity in the helium, but it could not have constituted more than 1 per cent. of the total volume of the gas.
In the second series of experiments I employed liquid air, boiling under a pressure of 10 mm. of mercury. The helium was first confined under a pressure of 140 atmospheres, and then allowed to expand till the pressure fell to twenty atmospheres, or, in some cases, to one atmosphere. The results of these experiments were also negative, the gas remained perfectly clear during the expansion, and not the slightest trace of liquid could be detected. The boiling point of liquid air was taken, from my previous determination, to be -220° C. (Comptes Rendus, 1885, p. 238). This number cannot, however, be taken as a constant, as the liquid air, boiling under reduced pressure, becomes gradually poorer in nitrogen. Further, the quantity of nitrogen lost by the liquid air on partial evaporation varies not only with the rate of boiling, but even according to the manner in which it has been liquefied.