GAY LUSSAC’S BAROMETER.--Take a tube which is very regular in the bore, four-tenths of an inch in diameter, and thirty-five inches and a half in length. Seal one of its extremities and draw out the other; then cut the tube at about two-thirds of its whole length from the sealed end, and reunite the two pieces by means of a capillary tube soldered between them, the whole being kept in a line. See pl. 2, fig. 1. Pierce laterally the part of the tube which is drawn out, at some inches from the base of the point, and force the margin of the hole into the interior of the tube, by means of a conical point of metal, in such a manner as to form a little sunk funnel, of which the orifice must be very small. After having introduced the proper quantity of mercury into the instrument, boil it, and assist the disengagement of the bubbles of air by agitating a fine iron wire within the tube. Then remove the part of the tube which was drawn out, by sealing the end of the wide part. Give to the whole instrument the curvature indicated by pl. 2, fig. 3. * * * * * BUNTEN’S BAROMETER.--This instrument differs from the preceding but in one point, namely, that the capillary tube is formed of two soldered pieces, of which the one, passing into the other, is terminated by a capillary point. This arrangement is exhibited by pl. 2, fig. 2. * * * * * BAROMETER PIERCED LATERALLY FOR DEMONSTRATIONS.--Take a tube thirty-nine inches long, with thick sides, and two-tenths of an inch internal diameter. Seal it at one end, and choke it at the distance of eight inches therefrom. Pierce a hole in the tube about twelve or sixteen inches from the choked part, and solder to the hole an additional piece, which can be closed by a cork or covered by a piece of bladder. The instrument is represented by pl. 2, fig. 15. * * * * * BELL-GLASSES FOR EXPERIMENTS.--These are pieces of tube sealed at one end, and widened or bordered at the other. They are extremely useful, and much employed in chemical experiments. They also supply the place of bottles for preserving small quantities of substances. Sometimes they are required to be straight, as pl. 3, fig. 12. Sometimes they need to be curved, as pl. 3, fig. 29. This is particularly the case when they are to be employed as retorts, for which purpose the sealed part should be made thin. Pl. 3, fig. 6, exhibits a retort with a tubulure. * * * * * BLOWPIPE.--We shall give in this article an account of the various pieces of glass which form part of the blowpipe described in the early part of this work. See pl. 1, fig. 19. The beak C, which is employed with the candlestick, is merely a bent tube, at the extremity of which a bulb is blown. The bulb is terminated by a point, the thickness of the sides of which is augmented by turning it for a long time in the flame.