is convenient and more effective to cause the deposition of a mercury mirror over the surface of the inner vessel (by leaving a little liquid mercury in the lower part of the double-shaped flask), instead of silvering as previously described. Under such conditions the mercury instantly distils and forms a brilliant mirror all over the surface of the inner vessel. The fact that mercury has a very high refractive index and is a bad conductor of heat are factors of importance in retarding the conveyance of heat. After the mercury mirror has been formed any further increase in the thickness of the film can be prevented, and at the same time the vacuum improved by freezing the excess of liquid mercury in the lower part of the vessel. The vacuum vessels described equally retard the loss as well as the gain of heat, and are admirably adapted for all kinds of calorimetric observations. The future use of these vessels in thermal observations will add greatly to the accuracy and ease of conducting investigations. The double spherical form of vacuum vessel is excellent for showing that the elevation or depression of a given volume of air a few feet causes an increase or diminution of volume, due to the small change of atmospheric pressure. The volume of air in the inner sphere is guarded from any sudden change of temperature by the surrounding highly vacuous space. This is only one of the many uses to which such receivers can be put. In making vacua, many other substances have been examined along with mercury, but they have not given equally satisfactory results. Sulphur would occur to any one as a substance that might replace mercury, seeing the density in the form of vapour, and also the latent heat of vaporisation, are nearly identical; and it has the further advantage of being a solid at ordinary temperatures. The sulphur vacua have, however, FIG5. so far not been an improvement, chiefly because traces of organic matter are decomposed by the sulphur, giving sulphuretted hydrogen and sulphurous acid, gases which are dissolved by and remain in the sulphur. When the surface of such a sulphur vacuum is cooled with liquid oxygen in the manner previously described, a faint crystalline deposit occurs, only it takes a much longer time to appear than in the case of the mercury vacuum. If a similar vessel is boiled out, using phosphorus as the volatile substance, the application of liquid oxygen to the surface causes instant deposition. Thus it can be proved sulphur and phosphorus distil at ordinary temperatures just like mercury. An investigation as to the electric conductivity of metals, alloys, and carbon at low temperatures has been undertaken in conjunction with my friend, Professor J. A. Fleming, D.Sc., F.R.S. The experiments are made by means of a resistance coil shown in Fig. 5, consisting of a piece of notched mica coiled with the fine wire to be tested, and of stout insulated copper-rod connections. The coil and connection are immersed in liquid oxygen contained in a vacuum test-tube, and the temperature of -200° C. can be reached by exhausting the oxygen by means of a powerful airpump. The results point to the conclusion that absolutely pure metals seem to have no resistance near the zero of temperature as indicated by the above curves (Fig. 6) obtained by experiment. With alloys there is little change in resistance, as indicated in the curves (Fig. 7). The conductivity of carbon decreases with low temperatures, and increases with high ones. At the temperature of the electric arc, carbon appears to have no resistance. The optical constants of liquid oxygen, ethylene, and nitrous oxide have been so far determined, and in this matter my colleague, Professor Liveing, has been associated with me in the conduct of this work. The results obtained are given in the following table, and tend to confirm the Law of Gladstone as being applicable to such substances: SPECIFIC The determination of the refractive index of liquid oxygen, at its boiling-point of 182° C., presented more difficulty than would have been anticipated. The necessity for enclosing the vessel containing the liquid in an outer case to prevent the deposit of a layer of hoar-frost which would scatter all the rays falling on it, rendered manipulation difficult; and hollow prisms with cemented sides cracked with the extreme cold. It was only after repeated attempts, involving the expenditure of a whole litre of liquid oxygen on each experiment, that we succeeded in getting an approximate measure of the refractive index for the D line of sodium. The mean of several observations gave the minimum deviation with a prism of 59° 15' to be 15° 11' 30", and thence = μ 1.2236. The density of liquid oxygen at its boiling-point of -182° C. is 1.124, -1 and this gives for the refraction-constant, " 1989, and for = d the refraction-equivalent 3.182. This corresponds closely with the refraction-equivalent deduced by Landolt from the refractive indices of a number of organic compounds. Also it differs little from the refraction-equivalent for gaseous oxygen, which is 3.0316. This is quite consistent with the supposition that the molecules of oxygen in the liquid state are the same as in the gaseous. If we take the formula FIG. 8. for the refraction-constant we find the value of it for liquid oxygen to be 1265, and the The optical projection of vacuum vessels having the shape of a double test-tube are very suitable for lecture illustration. As the critical point of oxygen is some thirty degrees higher than nitrogen it is easier to liquefy, and, consequently, becomes the most convenient substance to use for the production of temperatures about - 200° C. Liquid nitrogen, carbonic oxide, or air can conveniently be made at the ordinary atmospheric pressure, provided they are brought into a vessel cooled by liquid oxygen boiling under the pressure of about half an inch of mercury. A simple arrangement for this purpose is shown in Fig. 8. The inner tube contains the liquid oxygen under exhaustion, surrounded by a vacuum vessel, the interior space between the inner tube and the vacuum vessel being connected with a receiver containing the gas which is to be liquefied. If the object is to collect liquid air, the inner air space is left quite open, no precautions being needed to free the air from carbonic acid or moisture, because under the conditions such substances are solids, and only cause a slight opalescence in the liquid, which drops continuously from the end of the inner tube and accumulates in the vacuum vessel. If the air supply is forced to bubble through a little strong sulphuric acid, the rate of condensation and the relative volume of gas and liquid can be observed. Liquid air boils at the temperature of 190° C., giving off substantially pure nitrogen. As the nitrogen boils 10° C. lower than oxygen, after a time the liquid alters its composition and boiling point, finally becoming pure oxygen. During the evaporation the liquid air changes very remarkably in colour, passing from a very faint blue to a much deeper shade. The changes can be traced best by the marked increase in the width of the absorption bands of liquid oxygen. If air, collected in the above manner in a vacuum vessel, is isolated from a rapid heat supply by immersing the vessel in liquid oxygen, and then a powerful air-pump brought to act upon it, after a time it passes into the condition of a clear, transparent, solid ice. Nitrogen solidifies, under such conditions, into a white mass of crystals, but all attempts to solidify oxygen by its own evaporation have failed. Such liquids as air and oxygen, we should anticipate, would be especially transparent to heat radiation, seeing they are very diathermic substances in their gaseous state. The thermal transparency of liquid oxygen can be shown by passing the radiation from the electric arc, as shown in the diagram, through a spherical FIG. 9. K H W N vacuum vessel filled with clear filtered liquid, thereby concentrating the rays at a focus and igniting a piece of black paper held there. In this experiment the oxygen lens has a temperature of - 180° C., yet it does not prevent the concentrated radiation reaching a red heat at the focus. At such low temperatures as boiling oxygen and air all chemical action ceases. If some liquid oxygen is cooled to -200° C., and a glowing piece of wood inserted into the vessel above the liquid, it refuses to burst into flame, because of the low pressure of the vapour. An interesting experiment may be made by immersing an electric pile, composed of carbon and sodium, into liquid oxygen, when almost immediately the electric current ceases. The gaseous oxygen coming from the liquid must be exceedingly pure and dry, and as it has been alleged two chemical substances require the presence of a third one in order that they may combine, it was interesting to ascertain if a substance like sulphur would continue to burn after ignition in such an atmosphere. Sulphur placed in a small platinum vessel that had just been heated to redness, was raised to the boiling point, and in the act of combustion lowered into a vacuum vessel containing liquid oxygen. The com |