EXPERIMENTS TO BE DONE BY EACH PUPIL

I. Observation of the action of heat supplied by the gradual application of a Bunsen flame to a dry test-tube containing a little of the substance.

Any of the following or other substances may be tried: Mercuric oxide, red lead, lead nitrate, potassium chlorate, potassium nitrate, ammonium nitrate, ammonium chloride, mercuric iodide.

Gases evolved may be tested for as follows: (1) Notice colour, appearance of fumes, odour. (2) Apply lighted wax taper first at the mouth of the tube, then pushed inside.

(3) Hold in the gas strips of red and blue litmus paper, moistened with water.

(4) Hold within the tube a drop of clear lime water at the end of a narrow glass tube, then gently suck for a moment at the open end of the tube so as to draw up the drop.

This series of tests is not intended to enable the pupil to identify the gas, for that is a matter of comparatively small importance at this stage. After observations have been correctly recorded, the teacher may suggest to the pupil other experiments which appear to him desirable, such as the collection of the gas in bulk or further examination of the residue.

Some of the substances mentioned in the above list will afford opportunities of testing the genuineness and completeness of the student's work. For

example, he may have read or learned that mercuric oxide gives off oxygen and mercury when heated, but unless he has tried the experiment or seen it tried, he could not guess that the powder would become black while hot, and that at a high temperature it would give a yellowish-coloured gas and a small yellow sublimate upon the sides of the tube, owing to the trace of nitrate which the commercial red oxide invariably contains.

II. Crystallisation of salts from water, and recognition of crystalline form, or at least differentiation of one sort of crystal from another.

For example, make strong solutions of potassium nitrate and ammonium chloride in hot water, and pour into separate watch-glasses. On cooling the long prisms of the nitre are easily distinguished from the fern-leaf forms of the sal ammoniac. Similarly alum, lead nitrate, and barium nitrate will yield regular octahedrons and dodecahedrons, and intermediate forms distinct enough to be readily visible and easily sketched. Other salts which crystallise from hot water are potassium chlorate, sodium nitrate, magnesium and zinc sulphates, copper sulphate, chrome alum, potassium dichromate, &c.

III. Precipitates to be distinguished by differences of density or consistence. The following are white precipitates differing in character: solution of alum mixed with ammonia gives a gelatinous precipitate; calcium chloride with carbonate of

ammonia a flocculent precipitate which becomes sandy on heating or after standing; calcium. chloride with dilute sulphuric acid a mass of minute crystalline needles; barium chloride with dilute sulphuric acid a fine powder some of which may remain suspended a long time; silver nitrate with a chloride a white precipitate which curdles and on exposure to daylight assumes a purple tint, &c.

In like manner arsenious sulphide, lead chromate, zinc chromate, stannic sulphide, silver iodide are examples of precipitates which have a yellow colour, but differ in tint and density.

IV. The action of strong sulphuric acid upon substances resulting in the evolution of gas.

The following examples will serve : common salt, potassium nitrate, red lead, potassium iodide, sodium sulphite, sodium formate,

A few grams of the substance may be placed in a test-tube and strong sulphuric acid added in quantity sufficient to make a fluid paste. If heat is applied it will be well to caution young students as to the corrosive nature of the liquid, and direct them to turn the open mouth of the tube away from their own faces and from the persons of their neighbours. The evolved gas may be tested as already described under Section I.

In addition to such exercises as the foregoing, specimens of well-crystallised compounds such as potassium iodide, alum, sugar, zinc sulphate, or of minerals such as galena, fluorspar, pyrites, calcite,

or specular iron may be given to be drawn and described. And when qualitative analysis is begun, the pupil should be taught in every case to write down, before applying tests, an account of the appearance and more obvious characters of the substance analysed.

CHAPTER II

OBSERVATION-QUANTITATIVE

QUANTITATIVE experiments may be gravimetric or volumetric; that is, they may take the form of measurements of weight or of volume. Contrary to general belief, quantitative work may be done with very simple and inexpensive appliances, and in the hands of even young students results may be obtained which closely accord with theory. It is, however, advisable in the first instance to set exercises to be done independently of theory until a series of experiments has given concordant results, showing that the pupil has acquired sufficient skill to be allowed to test for himself some general law which he has already learnt.

The balance shown in the figure may be obtained of several instrument makers for about 27s. 6d., and a box of weights ranging from 100 grams to I milligram costs 6s.

The simplest quantitative experiments are those in which a gain or a loss of weight of a single object, such as a crucible, has to be recorded. The following are examples of the kind of experi