It has already been mentioned (p. 20) that Mendelejeff's classification of the elements, although it gives us the outlines of a true natural classification, is not at present perfect, inasmuch as it separates members of natural groups, such as those of the earthy metals. In regard to this classification it must be borne in mind that by it the elements are arranged in groups according to their atomicity, and this is not in all cases determined. A glance at the table (p. 19) will show this, for copper, silver, and gold are there included both in Group I., containing monad metals, and in Group VIII. But the commonest and most stable compounds of copper, such as cupric oxide or cupric sulphide, appear to show that it is a dyad rather than a monad. Silver, also, though it appears like copper in Groups I. and VIII., may also be a dyad,1 while gold forms two series of compounds, in one of which it is monad, and in the other triad. In the classification which I have adopted, I have followed Mendelejeff's tables as modified by Watts, but I have modified them somewhat, in order not to separate metals having a similar physiological action.

Class I. - Monad Metals.2

Group I. - Alkalis - Potassium, Sodium, Lithium, Caesium,

Rubidium, II. - Ammonium.

1 The formula of argentous oxide is Ag4O, and if this formula be correct, and silver be a monad, oxygen must be a tetrad; but if silver be a dyad, argentous oxide may be represented as

Ag-Ag Ag-O-Ag.

(Fownes' Chemistry, by Watts, 12th ed. vol. i. p. 369.)

2 The metals whose names are printed in italics are not officinal.

Class II. - Dyad Metals.

Group I. - Metals of the alkaline earths - Calcium, Strontium,

Barium.

(Appendix.) Metals of the earths - Aluminium, Cerium, Beryllium, Zirconium, Thorium, Lanthanum, Didymium, Yttrium, Erbium. II - Magnesium. III. - Copper, Zinc, Silver, Cadmium. IV. - Mercury. Class III. - Triad Metals. Thallium, Iridium, Gallium. Class IV. - Tetrad Metals. Tin, Lead, Titanium. Class V. - Pentad Elements. Nitrogen, Phosphorus, Arsenic, Antimony, Bismuth, Vanadium, Tantalum, Niobium or Columbium. Class VI. - Hexad Metals. Chromium, Uranium, Tungsten, Molybdenum.

Class VII. - Heptad Metals.

Manganese. - Vide next group.

Class VIII.

Group I. - Iron metals. Iron, Nickel, Cobalt, Manganese. II. - Platinum, Gold.

General Tests for the Acid Radicals in Metallic Salts.-As the same acids occur in the salts of different metals, the tests for their presence are described again and again in the Pharmacopoeias. In order to save repetition, it may be advisable to give here in a tabular form the tests for the different acids. It is to be remembered that the same tests apply to the simple recognition of a metallic salt, and to its detection as an impurity in other substances. The tests are generally applied to solutions of the salt in water.

Salt

Reagent

Reaction

Acetate* . .

Sulphuric acid.

Vapour of acetic acid given off and recognised by its smell.

" . .

Ferric chloride

Deep red colour.

Borate . .

Sulphuric acid.

The saturated solution causes deposit of shining scales, which give a green colour to the flame of alcohol.

Benzoate * .

Dilute solution of ferric sulphate

Flesh-coloured precipitate.

Bromide . .

Disulphide of carbon and chlorine water

If disulphide of carbon be poured into a solution of the salt, the chlorine water added drop by drop, and the whole agitated, the disulphide will acquire a yellow or yellowish-brown colour. (If iodine be present there will be a violet tint.)

Carbonate .

Acid . . .

Causes effervescence.

Bicarbonate .

" . . .

Causes effervescence more abundant than in the case of the carbonate. With solution of mercuric chloride bicarbonates give a white, and carbonates a yellow precipitate.

Citrate * .

Calcium chloride

The solution remains clear, but deposits white precipitate on boiling (calcium citrate being less soluble in hot than in cold water).

" . .

Sulphuric acid and heat

Is charred and evolves the odour of acetic acid.

Chloride . .

Nitrate of silver

White precipitate, soluble in ammonia, insoluble in hydrochloric or nitric acids.

Hypophosphite. .

Heat

Heated in a dry test-tube it evolves phosphoretted hydrogen, which takes fire spontaneously, and burns with a bright flame.

" .

Nitrate of silver

White precipitate, which rapidly turns brown and black.

" .

Hydrochloric acid and mercuric chloride

White precipitate of calomel, and on further addition separation of metallic mercury.

Hyposulphite .

Sulphuric acid

Gives rise to the smell of burning sulphur, and causes white precipitate of sulphur (bisulphite and sulphite give no precipitate).

Iodide . .

Disulphide of carbon and chlorine water

If disulphide of carbon be poured into a solution of the salt, then chlorine water added drop by drop, and the whole agitated, the disulphide of carbon will acquire a violet colour.

" . .

Starch water, starch paste, or gelatinised starch, with chlorine water

Blue colour in the cold, discharged by boiling.

Nitrate . .

Sulphuric acid and copper

Nitrous fumes.

" . .

Sulphuric acid and solution of ferrous sulphate

When sulphuric acid is added to a solution containing a nitrate, and a solution of ferrous sulphate is carefully poured over it, a dark colour appears at the junction of the two liquids.

Salt

Reagent

Reaction

Oxalate* . .

Calcium chloride .

White precipitate. In applying the test to cerium and iron, their salts must be decomposed by boiling with potash or soda. The oxide of cerium or iron is removed by filtration, and the reagent applied to the filtrate, which contains oxalate of potassium or sodium.

Phosphate .

Chloride of ammonium, ammonia, and sulphate of magnesium

White precipitate.

Phosphide . .

Sulphuric or hydrochloric acid

Evolves phosphoretted hydrogen.

Salicylate * .

Ferric salts . . .

Intense violet colour.

Sulpho-carbolate

Ferric chloride . .

Violet colour. This salt can be distinguished from the salicylate by heat, when it gives off inflammable vapours having the odour of carbolic acid.

Sulphate . .

Barium chloride .

White precipitate, almost insoluble in nitric acid.

Sulphide . .

Mineral acids, e.g. sulphuric or hydrochloric

Gives off sulphuretted hydrogen.

Sulphite . .

Ditto

Gives off sulphurous acid (has neutral or feebly alkaline reaction).

Bisulphite . .

Ditto

Ditto (has acid reaction).

Tartrate *. .

Acetic acid in presence of potash

White crystalline precipitate of bitartrate.

" . .

Sulphuric acid and heat

Is charred and evolves the odour of burnt sugar.

" . .

Nitrate of silver

White precipitate, becoming black on boiling.

Bitartrate *

Nitrate of silver

Solution rendered neutral by potash, gives with the reagent a white precipitate becoming black on boiling (very sparingly soluble in water : is thus distinguished from neutral tartrate, which is readily soluble).

" . .

Sulphuric acid and heat

Same reaction as tartrate.

* In the preceding table the salts of organic acids marked * when ignited in a crucible or on a piece of platinum foil, become charred and oxidised, leaving a residue which consists of carbonate. This is black from the presence of carbon, if ignition has not been carried sufficiently far to convert all the carbon into carbonic acid. This residue gives the reaction of a carbonate, effervescing with acids, and it is frequently convenient to convert the carbonate into chloride, before applying tests for the base.