correct." Experimentally support this statement by determining iron in a solution of ferric chloride, (1) with thiosulphate of sodium standardised against pure iodine, (2) with thiosulphate of sodium standardised against iodine liberated in the cold from an acid solution of a known mass of potassium dichromate to which excess of potassium iodide has been added.

XIII. Prove that air is a mixture and not a compound of oxygen and nitrogen. This will involve, (1) the proof that oxygen and nitrogen are not present in air in atomic proportions; (2) the proof that when oxygen and nitrogen are mixed in the proportion in which they exist in air there is no evidence of chemical change, but that nevertheless the mixture has the properties of air; (3) the proof that "air" dissolved by water has a different composition from that of the air merely in contact with the water; (4) the proof that nitrogen and oxygen in air can be partially separated by atmolysis.

XIV. Determine the coefficient of solubility of sulphuretted hydrogen in water.

APPENDIX III.

TABLES WHICH MAY BE USED AS GUIDES IN EASY QUALITATIVE ANALYSES.

A. Detection of commonly occurring metals when not more than one is present in any group.

I. Test for an ammonium salt by boiling a portion of original with excess of potash and examine gas for ammonia.

II. GROUP I. To original solution add a little hydrochloric acid (if no pp. forms Group I. is absent):

Pp. dissolves, probably

a salt of

As or Sb.

Pp. forms; add more HCIAg

Pp. does not dissolve; add much cold water

Pp. dissolves, Pp. does not dissolve; boil with much water
probably

Ba or Sr,

as chlorides. Pp. dissolves, Pp. does not dissolve; add ammonia

Pb(PbCl).*

III.

GROUP II. Into filtrate from Gr. I., or into original liquid (acidulated by HCl) if Gr. I. is absent, pass sulphuretted

The formulae in brackets shew the compositions of the pps.; thus if lead were present the pp. produced by HCIAq would be PbCl2, this salt is soluble in boiling water.

hydrogen till liquid smells strongly of this gas; then, if pp. forms, warm, dilute, warm again and filter; boil filtrate and saturate it with HS gas; collect any pp. which forms on same filter as before; again boil filtrate and pass in H ̧S; and repeat this treatment until HS ceases to produce any change in the filtrate. Wash pp. thoroughly::

2

(a) Pp. produced by HS is black or dark brown. (HgS, CuS, PbS, SnS, or Bi,S);

Pp. dissolves,
Snous

Digest pp. with warm yellow ammonium sulphide

Pp. does not dissolve; wash pp. and warm it with
dilute nitric acid

(b) Pp. produced by HS is yellow (CdS, As,S,, or SnS ̧);

(e) Pp. produced by H.S is orange-red (Sb,S),

Sb.

Confirm by proving pp. to be soluble in warm (NH4)2S, Aq.

IV. GROUP III. Boil filtrate from Gr. II., after adding a little HNO,Aq, till liquid no longer smells of H,S; add a little more HNO Aq and boil again*, then add a considerable quantity of ammonium chloride and then ammonia in very slight excess

3

Appendix to Group III.: to test for an alkaline phosphate or oxalate:

(a) Test original for, (1) phosphoric acid by ammonium molybdate test; (2) oxalic acid by calcium chloride test, after boiling with large excess of solid sodium carbonate for some time, to remove metals as carbonates, filtering, and acidulating filtrate with acetic acid (s. Table for detection of acids).

(b) Test a portion of filtrate from Gr. II. for barium and strontium by calcium sulphate test; if these metals are absent, test another portion of same liquid for calcium by sulphuric acid and alcohol test (s. Table for Gr. V.). If none of these metals is found, fuse a portion of original solid with Na,CO,, boil fused mass in water, filter, wash residue on filter 3 or 4 times with warm water, then dissolve residue in HCIAq, add excess of NH ClAq and NH,Aq, (filtering if pp. forms)

* The reasons for this treatment are, (1) to remove H,S; (2) to oxidise iron salts from ferrous to ferric, the treatment with HS in pptg. Gr. II. reduces ferric to ferrous salts.

+ Should HCl and HS have produced neither a pp. nor a change of colour, it is better to add NH4ClAq and NH,Aq to a portion of the original liquid to which the reagents for Grs. I. and II. have not been added.

and test for magnesium by sodium phosphate test* (s. Table for Gr. VI.).

V. GROUP IV. To filtrate from Gr. III. add a few drops of ammonium sulphide :

VI. GROUP V.

Mn(MnS) gives amethyst colour in borax

bead, O. F.

To filtrate from Gr. IV. add ammonium carbonate, collect pp., wash it, dissolve in HClAq, and divide liquid into two parts :

* The Mg phosphate or oxalate is decomposed by the Na,CO, MgCO3 is formed and Na phosphate or oxalate dissolves in the wash-water.