from the bottle, and being provided with a screw plug in the bottom, it may be opened from time to time (fig. 208). When the apparatus is to be used, the tube containing the gas is lowered into its place (the collar sitting upon a leather washer placed upon the shoulder within the bottle) and is securely held down by a nut, which passes over the narrow part of the collar, and engages in the thread upon the neck of the bottle. If the gas to be liquefied does not require more than about 150 atmospheres pressure, the tube need not be protected by any outer jacket, as all such guard tubes render it more difficult to see the experiment, and if an image of the tube is being projected upon the screen they greatly distort the image. When it is needful to surround the tube, either for safety, or in order to cool or warm the gas, the jacket should be a tube made by cementing flat glass together in the form of a square tube, or long square cell, fig. 210; in this way the image will be the least obscured. As the pump is put into action the mercury will be forced up into the narrow part of the glass tube, and the liquid will make its appearance as a small column floating upon the mercury. If the pressure required to liquefy the particular gas under consideration be not greater than about 200 atmospheres, the operation may be performed entirely by the use of the long lever handle of the pump; the side piston which is actuated by the small wheel, being FIG. 210. only intended to enable the experimenter to exert additional pressure upon the gas, when the use of the lever becomes uncomfortably hard work. To release the pressure, water is allowed to escape by turning the second wheel. If this be done gently the liquefied gas will be seen to boil off slowly into gas; if done suddenly the liquid will instantly flash off into gas, thereby developing a degree of cold which in many cases causes the formation of a cloud of either the liquefied or solidified gas. With this apparatus, and without the application of any re U frigerating mixtures, a number of gases may be reduced to the liquid state, gases whose critical points are not below the ordinary temperature, and which require not more than about 150 atmospheres for their liquefaction.1 632. When the critical point 2 of a gas is only a few degrees below the ordinary temperature, it is possible to obtain it in the liquid state without the use of any refrigerators, by cooling it by sudden expansion. Ethylene is a good illustration to employ. When this gas is compressed even to 150 atmospheres, no liquefaction takes place, but if the pressure be slightly but suddenly released, by a rapid movement of the wheel, the internal temperature will be lowered below the critical point (about +10°C.). and the gas will be liquefied. By a little manipulation with the pump, even a gas whose critical point is as high as that of carbon dioxide (31.9°C.) may be made to show a similar result; by very rapidly compressing this gas its temperature may be raised above this point, and no liquid will be produced even at 150 atmospheres, but on a momentary release of the pressure, the temperature falls and the gas is condensed. The critical point of a gas, e.g. carbon dioxide, may also be shown by surrounding the tube with the square glass cell, into which, warm water is poured, having a temperature a few degrees above the critical point. On compressing the gas with a pressure greatly above what would be required at ordinary temperatures, it will be seen that there is no liquefaction taking place. If the warm water be run out and replaced by cold water, the liquid state is at once assumed by the gas. The liquefaction of gases whose critical points are so low that powerful refrigerating means have to be employed may also be effected by the Cailletet apparatus, but of slightly modified form. In this case the gas tubes are much longer, and therefore the steel bottle proportionately deeper than that above 1 See Table XXXII, in the Appendix. 2 See Table XXXI. in the Appendix. described. The capillary end of the tube containing the gas is bent twice at right angles, so that the end may be dipped down. into the refrigerating liquid. Fig. 211 shows the apparatus for the liquefaction of oxygen with the arrangements for producing the requisite reduction of temperature. The degree of cold necessary for this experiment is obtained by the evaporation of liquid ethylene. The ethylene is contained in the steel cylinder E, which is connected to a fine copper worm A A'. The worm is contained in an inverted narrow glass bell, and, passing out through a cork at the base, is so bent that the liquid ethylene may be delivered into the small glass tube F. In order to prevent the deposition of hoar frost upon the outside of this tube, it is contained in the glass jar G, provided with a close-fitting cover, and in which a quantity of strong sulphuric acid has been placed in order to dry the air within it. A second copper worm B B′ is also contained in the glass bell; this is connected at B with a hydrogen supply, while its lower extremity is attached to a glass tube D which dips into the tube F. The end of the gas tube o FIG. 211. containing the oxygen is then brought into the tube F, as shown in the figure. The glass vessel containing the two worms is about two thirds filled with ether which has been cooled to -20°C. in a freezing mixture of ice and calcium chloride. Solid carbon dioxide is then thrown in until the temperature falls to -70°C. When this point is reached, a quantity of liquid ethylene is allowed to pass through the worm until the tube F is about three parts filled. The valve of the ethylene reservoir is then closed, and a stream of dry hydrogen gas passed through the worm B B'. The stream of cold and dry hydrogen bubbling through the liquid so increases the rapidity of the evaporation, that the temperature quickly falls below the critical point of oxygen, when a pressure of from 50 to 100 atmospheres (as registered by the Bourdon gauge upon the pump) will suffice to effect the liquefaction of the gas, the liquid collecting in the cooled limb of the tube 0. The hydrogen, delivered from a cylinder of compressed gas, may be dried by being passed through a large glass vessel, having a capacity of 8 or 10 litres, filled with fragments of quicklime. LANTERN ILLUSTRATIONS Not only are there a great many experiments which may with advantage be shown to an audience by means of the lamp, but there are others which can only be rendered visible by projection upon a screen. The necessary apparatus for such lantern experiments as are described in the foregoing chapters, and the best methods for its disposition, may be described under the following heads-viz. the light, the lantern, and the lenses. 1. The source of light.-Practically this will be either the limelight or the electric light, although in the absence of these, and in a small room, for some experiments, a good duplex or triplex oil lamp can be made to do duty fairly well. (a) The limelight can be produced by causing the flame of either burning hydrogen and oxygen, coal-gas and oxygen, or ether and oxygen to play upon a small cylinder of lime, and thereby raise it to incandescence; these different flames being called respectively the oxy-hydrogen, oxy-coal gas, and the oxyether flames. The form of jet employed for the oxy-hydrogen flame is one in which the two gases mix in a small chamber, packed with wire gauze, and the mixture is inflamed at a platinum nozzle. Fig. 212 The two gases are shows such a jet. The oxy-hydrogen flame being very fine, the spot of light produced when it impinges upon the lime has the advantage of being smaller than that obtained by either the oxy-coal gas or oxy-ether flames; from the nature of this flame, however, it very rapidly bores little holes. into the lime. This 'pitting' of the lime is attended with considerable risk to the nearest lens, especially when a short focus condenser is being used, for if the position of the lime be not fre FIG. 212. quently altered by a slight rotation of the cylinder, the flame will rebound from its surface and strike the lens. It is always advisable to guard the lens by interposing between it and the flame either a piece of sheet glass or thin mica. (The clear sheets of mica used for covering photographs are well suited for this purpose.) There are two kinds of limes usually supplied for the limelight, known as 'hard' and 'soft;' for the oxy-hydrogen flame the hard limes are to be preferred. The gas bags of former days have now been almost entirely superseded by iron or steel bottles containing the separate gases under pressure. In using these bottles, it is a great advantage to |