bend with soot by holding it in the yellow flame and do not lay the tubing on a cold surface while it is hot. Rotate the tubing while heating it, especially at first, so as to prevent cracking by uneven heating. Apply the heat continuously and do not attempt to bend the tubing until the heat has rendered it perfectly pliable. Then remove from the flame and bend, being careful while doing so to keep the bend round; if the curve is so sharp that the tubing is flattened at any point the passage of liquid or gas will be interfered with, also such bends are brittle and break easily. Procedure: Use a wing-top burner. Secure a yellow flame. Keep this flame for thin-walled tubing, FIG. 33. HOLDING GLASS TUBING IN THE FLAME. but for that with a thick wall, change to a colorless flame as soon as the tubing is slightly coated with soot at the part which is to be bent. Hold the tubing between the thumb and fingers and rotate it in the flame until it is pliable; then remove it from the flame and bend it into the required shape. It is well to keep the tube on an asbestos mat while bending it. To draw out glass tubing.-Rotate the central portion of the tubing in the flame until it is soft. Remove from the flame. Then make traction on both ends until the points are of the desired diameter. Let the glass become perfectly cold and then cut it as previously directed. Tubing prepared in this way can be used for pipets. To make glass stirring rods.-Heat and draw the tubing as directed in the preceding paragraph, but FIG. 34. GLASS TUBE DRAWN TO A POINT, READY TO CUT. instead of cooling and cutting the point, draw the glass apart and fuse the ends in the flame until there is no opening. This will result in two tubes each closed at one end. If a bulb is wanted at the end of a rod, rotate the closed end in the flame until it is soft, remove it from the flame, and, at once, blow into the open end. Continue blowing until a bulb the desired size forms. Such rods are fairly good substitutes for solid glass ones. Filtering Filtering is a process by means of which finely divided solids are removed from liquids. It consists Ооо FIG. 35. FOLDING FILTER PAPER TO FIT Funnel. in making liquids pass through a substance that is sufficiently porous not to interfere with the passage of The liquid matter, but dense enough to prevent or impede the passage of solids suspended in the liquid. methods of filtering will be discussed in Chapter XV. FIG. 36. FILTERING. In the laboratory, filtering is usually done with filtering paper. This is folded as shown in Fig. 35, and placed in a funnel. When folding the paper, be careful not to press it at the point or it will tear. Place the paper in the funnel with three thicknesses on one side and one on the other. Do not allow the paper to project above the funnel and fit it into the funnel in such a manner that its entire surface is supported, otherwise the weight of the liquid may tear the paper. When the liquid to be filtered is thick, it is usually better to moisten the paper with distilled water before pouring in any of the liquid. Also, it is well to pour the liquid down the side of a rod held with its point upon the single layer of paper, as in Fig. 36. To introduce a powdered substance into a narrow necked flask or a test tube.-Fold a strip of paper as shown in Fig. 37, place the powder in one end of the trough thus made; introduce the paper into the tube, turn the paper so that the powder will be deposited where desired; withdraw the paper. Slide heavy pieces in carefully; a flask is easily broken. Weighing When weighing chemicals, put them on a piece of paper or in some utensil, as a watch crystal, beaker, etc.; do not put them on the unprotected pan of the scales for many chemicals will corrode or otherwise injure the pan. The weight of this paper or utensil must be either noted, recorded, and afterward deducted from the total weight of utensil and chemical, or else an object of equal weight must be placed in the other pan. The weights are put in the center of the right-hand pan. Pupils should form the habit of handling weights with forceps, for when small amounts are being weighed, even moisture from the fingers will interfere with an absolutely correct result. Measuring Liquids are measured in graduated glass measures, pipets, and burets. When using measures, hold the measuring glass so that the lowest point of the meniscus1 is on a level with the eye. To use a pipet, hold it between the thumb and the middle finger of the right hand; place the pointed end in the liquid and the other end in the mouth and suck up the liquid nearly to the top of the tube, but be very careful not to draw it into the mouth; while removing the tube from the mouth, quickly place the index finger over the top. Then, by slightly releasing the pressure of the finger over the opening, either allow the number of drops required to escape, or, if the pipet is a graduated one, allow the liquid to drop from the tube, back into the bottle, until the top of its column is on a line with the mark of the quantity required. Then hold the pipet over the utensil into which the liquid is to be put and release the finger over the opening; the pressure of the air entering the tube will force the liquid from it. When using a buret, pour the liquid to be FIG. 38. measured into the buret tube, place a recepBURET tacle under it, and, carefully, release the pressure of the stop cock on the rubber tubing until the amount of liquid required has passed into the receptacle. The upper surface of a fluid column, as that of mercury in a thermometer. The lowest point, i. e., in the center, is considered the correct height of the column of liquid. |