VIAL OF THE FOUR ELEMENTS.--This instrument is represented by pl. 2, fig. 27. Take a tube drawn out at one end, obstruct the canal two inches from the extremity, and twist the contracted part into a ring. Draw out the other end of the tube, introduce the proper liquids, remove the point of the tube, and seal it. The liquids generally employed for filling the vial of the four elements are, 1. Mercury; 2. A very concentrated solution of carbonate of potash; 3. Oil of turpentine; 4. Alcohol. A portion of air is also allowed to remain in the tube. * * * * * WATER HAMMER.--Pl. 2, fig. 18, is a representation of this instrument. Choose a tube of a good diameter, and with thick sides; seal it at one end and draw it out at the other. Blow a bulb at the base of the contracted part; then, having put a quantity of water in the tube, let it boil therein, to expel the atmospherical air. When you imagine that all the air has been expelled, and that nothing remains in the tube but steam and water, seal the open point. When you have to seal a tube in this manner, you should be careful to draw out the extremity of the tube somewhat abruptly, and leave a very small opening, so that it shall be sufficient to expose the point to the jet of a candle blown by a mouth blowpipe, to have the sealing completely and suddenly effected. You can afterwards round this sealed part by turning it in the flame of the lamp, provided, however, that you have preserved a sufficient thickness of glass at the sides of the point. If you omit to take this precaution, the pressure of the atmosphere, acting with great force on the softened glass when it is unsupported by the partial vacuum within the tube, is capable of producing such a flattening, or even sinking in of the matter, as could not subsequently be rectified; except, indeed, by heating simultaneously the liquid contained in the tube and the glass to be mended, which is an operation of a very delicate description. * * * * * WELTER’S SAFETY TUBES.--After having closed a tube at one end and drawn it out at the other, give it the curvature exhibited by plate 3, fig. 18. Pierce it then laterally, in the middle of the part _a b_, and solder there the extremity of a tube, to the other end of which a funnel has been soldered: it is necessary that the funnel be closed by a cork. The soldering being terminated, a bulb must be blown and the tube bent in S, in the manner shewn by the figure. Then open the closed end, and cut off the contracted point. THE ART OF GLASS-BLOWING. V.--_Graduation of Chemical and Philosophical Instruments._ OF THE SUBSTANCES EMPLOYED IN THE PREPARATION OF THESE INSTRUMENTS. Before proceeding to the subject of _graduation_, it is necessary to say a few words respecting the substances which are generally employed to fill a variety of instruments, particularly barometers and thermometers.