pur

fit tightly into the flask. A vulcanised india-rubber stopper is much more durable for this and most other chemical poses; it is perforated by a sharp well-wetted cork-borer, or by a wetted round file. Two pieces of glass-tubing rather longer than would be required for the tubes a and b are then bent (6) into the form shown in fig. 22. The ends of the tubes are, if necessary, cut off (5) to the right length, and their sharp edges rounded by holding them in the Bunsen flame or in the tip of the blowpipe flame, or by rubbing them with the face of the file.

Two parallel holes are then bored in the cork by means of a round file or with a proper sized cork-borer (8); the holes must be rather smaller than the glass tubes, and must not run into one another or to the outside of the cork. They are slightly enlarged, if necessary, by the round file. Into these holes the tubes a and b are then pushed with a twisting motion; if the holes have been made of the proper size the tubes must enter somewhat stiffly, but without requiring much pressure. If the holes have been carelessly made too large, the tubes may often be made to fit by slipping upon them little pieces of narrow india-rubber tubing, or by putting the pieces of india-rubber tubing into the holes in the cork before fitting in the glass tubes.

Upon the upper end of a is fitted a piece of small indiarubber tubing about an inch and a half in length, into the other end of which is pushed a short jet (c) made by drawing out a piece of glass tubing in the flame (fig. 15 p. 29); its nozzle may be contracted, if necessary, by holding it perfectly dry in the flame for a short time. The neck of the flask should then be bound round with twine like the handle of a cricket bat, or tightly covered with a piece of flannel; this prevents the fingers from being burnt when water has been boiled in the flask and its neck has become heated by the steam.

The wash-bottle, when thus fitted up, is filled about twothirds with distilled water. Tap-water should never be kept in the wash-bottle.

A fine stream of water may then be obtained from the jet (c) by blowing down the tube (b); this stream serves for

washing precipitates and for other purposes. A larger stream is obtained by inverting the bottle, when the water will flow out from the end of the tube (b), air entering meanwhile by the tube (a).

Hot water is frequently required for washing precipitates, to obtain this the wash-bottle is placed upon a piece of wire gauze supported on a tripod-stand, and is heated by the Bunsen flame issuing from a rose-burner, as shown in fig. 10 (p. 22).

14. Cleaning Apparatus.-It is indispensable to the success of an analyst that all glass and porcelain apparatus should be scrupulously clean before being used.

Test-tubes, beakers, and porcelain dishes are washed in a stream of tap-water by using the test-tube cleaner (fig. 23 b). Sometimes a little hot hydrochloric acid is requisite to remove stains, and occasionally it is necessary to heat a little strong sulphuric acid in a vessel in order to cleanse it from grease or other impurities; a little hot caustic potash or ammonia may also often be used with advantage. In fact in removing a substance from a vessel to which it strongly adheres, the student should always consider what the substance is and then remove it by a liquid in which it is easily soluble. Each article, after having been well washed and rinsed with tap-water, should be placed upside down in the small wicker

FIG. 23.

basket to drain, or, if required for immediate use, should be rinsed out with a little distilled water. It should be borne in mind that apparatus must be washed as soon as possible after use, as after standing the surface is usually much more difficult to cleanse. The brush must be cautiously moved as it reaches the bottom of a test-tube, since as the glass is very thin the brush is easily pushed through it.

Test-tube Brush.-It will be found that the piece of sponge at the end of the test-tube cleaner (fig, 23 a) is not well adapted to cleanse the bottom of test-tubes and boiling tubes. A much more efficient end is given to the brush by removing the sponge and bending back the end of the wire stem upon itself at a point just above

where the hairs commence (fig. 236). By slightly curving the part of the stem carrying hairs, the brush will better adapt itself to curved surfaces such as those of porcelain dishes.

Test-tubes, whilst in use, are placed in the test-tube stand; boiling tubes may be supported in the wicker-basket, or in a specially made stand. When washed they should be placed to drain mouth downwards in the wicker-basket.

Glass funnels should have their narrow stems cut off to within about half an inch from the shoulder; the sharp outer edge must then be removed by rubbing it with the face of a triangular file. The inside of the narrow neck is best cleaned by washing it with a common tobacco-pipe cleaner, inserted from the shoulder of the funnel so as not to cut the hairs by the edge at the other end of the neck.

15. Platinum foil and wire are cleansed by boiling them in bydrochloric acid, and rinsing off the acid with water; the wire should then be strongly heated for some time in the blowpipe flame until, on being dipped into pure strong hydrochloric acid, it no longer colours the Bunsen flame. If the tip of the wire cannot be thus cleansed it should be cut off. Commercial platinum often contains Barium, and the wire made from it therefore gives a green colour to the flame: such wire is useless for flame coloration tests and for spectrum analysis.

It is best to keep the foil or wire in a small beaker or dish, containing strong hydrochloric acid diluted with sufficient water to prevent it from fuming; the platinum will thus be ready for use after having been rinsed with water.

16. Before putting by apparatus it should be made a rule to wash all glass and porcelain which is not in actual use, and place it in the wicker-basket to drain; the basket is put away with its contents.

All iron apparatus should be carefully dried, and must be kept in a dry place to prevent rusting. On uo account must metal apparatus be kept in the wicker draining basket.

17. Heating Glass and Porcelain.-A few general precautions should be observed in heating glass and porcelain vessels, to guard against cracking them.

A vessel containing a liquid must never be heated by the flame above the level of the liquid inside.

A dry hot vessel must be allowed to cool before pouring in any liquid, or placing it on a cold surface. See also (18, 19). 18. Porcelain dishes are generally used for boiling liquids; they are supported on a tripod or retort stand, and may be safely heated by the naked flame. Porcelain crucibles are used for containing solid bodies which are to be strongly heated; they are supported on a pipe-clay triangle placed upon the ring of a retort-stand or upon a tripod-stand: the flame should not be allowed to play steadily at once upon the bottom of the crucible so as to heat it suddenly, but should be constantly shifted by moving the burner, until the porcelain is hot. The crucible should also be allowed to cool slowly on the triangle, as contact with a cold body is very apt to crack it: the crucible and its cover whilst hot are handled by the crucible tongs.

19. Glass vessels require to be heated more cautiously than porcelain: a large naked flame must never be allowed to play for any length of time on any one part of the glass surface. In heating a test-tube or boiling tube, this "local heating" is prevented by holding the tube obliquely with the lower part in the flame (fig. 1) and moving it gently up and down, or by constantly turning it round on its axis. Test-tubes are too narrow for boiling liquids in, the liquid being very apt to boil over. Small quantities may be boiled and larger quantities heated short of boiling in a test-tube, but the broader "boiling tubes" are best suited for this purpose. Large glass flasks, such as the wash-bottle, are most safely heated by placing them on a piece of wire gauze on a tripodstand (fig. 10, p. 22) and heating with a rose-burner; in some laboratories a sand-bath is available, the flask is then heated by being placed on the surface of hot sand. Test-tubes, if not full of liquid, can be held by the neck whilst being heated, if the tube be held obliquely so that the fingers are not over the flame. All risk of burning the fingers is avoided by bending round the neck of the tube a slip of folded paper or of leather, and pinching the ends together close to the tube.

SECTION III.

ANALYTICAL OPERATIONS.

THE student, before commencing the analytical reactions, must become familiar with the operations which are constantly employed in Analytical Chemistry. The processes will be most readily and perfectly understood by reading the following descriptions, and performing the experiments given in illustration of them.

20. Solution. Many solid substances, if stirred or shaken with water, gradually "dissolve" in that liquid; salt and sugar may be mentioned as examples. Other liquids may be employed instead of water, and if they cause the solid substance immersed in them to partially or entirely become liquid and mingle uniformly with the liquid, they are said to "dissolve" the solid, the liquid thus obtained being called a "solution" of the solid, and the liquid which dissolves the solid being termed the "solvent." Thus water, when shaken with sodium chloride (common salt), dissolves it, yielding solution of sodium chloride; and water is therefore called a solvent for sodium chloride. Further, a solid which dissolves in a liquid is said to be "soluble" in that liquid, if it does not dissolve it is said to be "insoluble."

The process of solution is more rapid when the solid substance is employed in the state of powder than when it is in large lumps, since a larger surface of the substance is thus exposed to the solvent by the powder; it is also much hastened by heating the solvent, which causes a more rapid circulation of the liquid over the solid, and also frequently increases the solubility of the substance.

Two kinds of solution may be distinguished. "Simple solution" occurs when a substance dissolves in a liquid without alteration in composition, the solution possessing there