ing on, add to some tap-water contained in a test-tube several drops of nitric acid and some silver nitrate solution, and notice that the water becomes milky; a separate portion in another test-tube will also be found to become milky on addition of some ammonia and ammonium oxalate solution. These changes are due to the presence in the water of certain solid substances dissolved in it. If the distilled water from FIG. 11.

the flask be examined in the same way it will remain clear, showing that these substances have been removed by distillation.

Tests for Distilled Water.-Good distilled water should remain quite clear when to separate portions of it are added solutions of ammonium oxalate, silver nitrate, barium chloride, and ammonium sulphide these tests prove the absence of calcium, chlorides, sulphates, lead, and iron respectively. It should also leave no residue when evaporated.

B. Nitric Acid.-Exp. 32.-Clean the retort from Exp. 31 by rinsing out the bulb with a little dilute hydrochloric acid, and then thoroughly with water; let it drain for a few minutes, and place in it some solid potassium nitrate (nitre); then pour upon this, through a funnel placed in the neck (or

tubulure), a quantity of strong sulphuric acid about equal in weight to the nitre used, and allow the acid to drain off the neck (in case a non-tubulated retort is employed), by supporting it for some time in an upright position. Proceed to distil just as in the preceding experiment; a thick yellow oily liquid will trickle down the neck of the retort into the flask; this liquid is somewhat impure nitric acid, and will be found to fume strongly in the air :-

KNO3 + H2SO4 = HNO3 + KHSO4.

Test for Nitric Acid.-Place some small pieces of copper in a test-tube, pour upon them a little of this nitric acid; on warming gently reddish-brown fumes will appear in the tube. (See Exp. 22, p. 18). This property of giving red fumes when treated with copper is often used as a test for nitric acid.

SECTION II.

PREPARATION AND USE OF APPARATUS REQUIRED IN ANALYSIS.

Notes.-Refer to the list in paragraph (494),* which shows what apparatus is required for each student, and mark all apparatus as directed in the note at the end of the list.

FIG. 12.

1. The Bunsen-Burner.-By far the most useful lamp for general heating purposes is the Bunsen-burner (fig. 12). It is a gas lamp so constructed that coal gas entering through the tube a may be burnt mixed with a proper proportion of air which flows in through the holes at the foot of the burner; the oxygen of the air which is thus mixed with the gas burns the carbon in the interior of the flame. Accordingly the luminosity, which is believed to depend upon the existence in the flame of unburnt carbon or carbon-compounds, is destroyed. The Bunsen-flame is therefore useful because it deposits no soot upon any cool object. which is being heated in it, and also because it is much hotter, owing to the more complete combustion of the gas, than any of the ordinary flames. Its high temperature, non-luminosity, and colourless appearance also render it very valuable for producing flame colorations, as will be seen hereafter. The lamp should be provided with some means for partly or entirely closing the air-holes when requisite; this is usually effected either by a loose perforated ring which is

a

* Numbers occurring in the text which are enclosed in brackets refer to the paragraphs commencing in this section. For convenience in reference, paragraph numbers are printed in thick type, and at the head of each page will be found, enclosed in square brackets, the numbers of the paragraphs it contains.

slipped over them, or by making the tube b turn round on its long axis. When a small flame is being employed, the air must be partly shut off, else the flame "burns below."

When the lamp is to be used, a piece of tightly-fitting india-rubber tubing 5-16ths of an inch in diameter is pushed over the end of the tube a, and the other end of the tubing is slipped over the tube which supplies gas to the working bench, the gas-tap is turned on, and as soon as the gas is distinctly smelt issuing from the tube b, it is lighted. The flame should be almost perfectly colourless, and give scarcely any light. Occasionally the gas will burn at the bottom instead of the top of the tube b; this usually happens when the flame is turned down very low, or the supply of gas is insufficient; also when the quantity of air admitted through the holes is too great, or the burner is lighted too soon after turning on the gas-tap. It may be at once detected by looking through the holes in the base of the lamp, when the luminous flame will be seen burning from the small gas jet inside; the flame produced whilst the gas is "burning below" is also characterised by being long and somewhat luminous, and by emitting a very unpleasant smell. In such a case the flame should be at once extinguished by pinching the india-rubber tubing close to the burner, and the gas should be relightedafter escaping for a few seconds: the momentary stoppage of the gas-stream by suddenly striking the india-rubber tube upon the bench with the hand will also frequently cause the flame to rise to the top of the burner without extinguishing it. Closure of the air holes serves the same purpose, but this must be done with care, as after the flame has burnt below for a short time the metal becomes too hot to be touched. The risk of the gas burning below is prevented by slipping upon the top of the burner a tightly fitting thimble, with a hole in its top somewhat less in diameter than that of the burner.

2. For diffusing heat over a large surface, the "roseburner" is very useful; it is a small perforated cap c (fig. 12) made of iron, which, when placed upon the top of the tube b, yields a small circle of flames; the rose-burner is put on

and removed by grasping a small iron peg in its centre with crucible-tongs; it is of course very hot after being removed, and should never be handled, or placed upon wood, until it is cool.

3. The flame of a spirit-lamp is occasionally employed instead of the Bunsen-flame, but for general purposes the spirit-lamp should only replace the Bunsenburner where coal-gas cannot be obtained.

The spirit-lamp (fig. 13) consists of a glass vessel containing

FIG. 13

methylated-spirit, into which dips a cotton wick supported by means of a brass, or better, a stoneware wick-holder. When not in use the wick should be always covered with the glass cap to prevent evaporation of the spirit. If the spirit is tolerably free from resinous matter its flame will be nonluminous, and will deposit no soot upon a cold object.

4. The blowpipe is frequently used to produce a small but very hot flame from the flame of a gas-lamp, spirit-lamp, or candle, by blowing through it a fine stream of air from the mouth.

FIG. 15.

FIG. 14.

b

In order to obtain the "blowpipe flame" from the flame of a Bunsen-burner, the blowpipe is held by the right hand, with its finely pierced tip a (fig. 14), resting on the edge of the burner, and just inside the flame (fig. 15); the mouth-piece b is then taken between the lips, and after blowing out the cheeks to their full extent, the air contained in them is forced out through the jet a, and causes a small pointed

a

tongue of flame to issue from the side of the gas-flame. The