Group I. - Metals Of The Alkalis. Lithium, Sodium, Potassium, Rubidium, Caesium.

Group II. - Ammonium.

I have omitted silver and gold from this class, because both their physiological actions and physical properties appear to show that they do not belong to it. I have put ammonium into a group by itself and separated it from the other members of this class, because it differs from them in being a compound and not an element; in being volatile; and in having an entirely different physiological action.

General Characters. - They are all powerful bases and have a great affinity for oxygen. The oxides of the first group are non-volatile, and are sometimes termed fixed alkalis, while ammonia is volatile. They all have a strong alkaline reaction, neutralising acids readily, turning red litmus-paper blue, and turmeric paper brown.

General Reactions. - They are not precipitated from solutions by the successive addition of (1) hydrochloric acid, (2) hydrogen sulphide, (3) ammonium sulphide, (4) ammonium carbonate, and (5) sodium phosphate.

General Physiological Action. - The alkalis are of great physiological importance, and salts of potassium and sodium form a large proportion of the saline constituents of the body. These two elements are differently distributed, potassium being chiefly found in solid tissues, while sodium is more abundant in the fluids. They are found as carbonates, bicarbonates, chlorides, phosphates, and sulphates. The proportion of these salts in the body is, however, very different, as are also their uses in the economy. The chlorides are by far the most abundant, and sodium chloride may be looked upon as the most important constituent of the nutritive fluids in which all the tissues of the body are bathed. But while sodium chloride forms the saline basis of these fluids, the other constituents are indispensable for the continued life of the tissues. All the fluids of the body are alkaline, and death occurs whenever the alkalinity is diminished below a certain point, even though the fluids and tissues are far from having an acid reaction. Such a reaction is only observed in the tissues after death. The importance of the different saline constituents in nutrition has been most fully worked out in the case of the frog's heart (p. 305 et seq.).

In the case of the heavy metals, which are not normal constituents of the body, the action of their salts depends almost entirely on the base and only slightly on the acid with which it is combined. In the case of the alkalis, however, this is not so, the action of their salts depending much on the acid.

In consequence of this it is necessary in considering the physiological action of salts of the alkaline metals to divide them into at least three groups :1. Alkaline salts, hydrates, carbonates, and bicarbonates.

(Sub-groups - Salts of organic acids, acetates, citrates, tartrates).

2. Chlorides.

3. Sulphates and other salts which are slowly absorbed.

General Action of the Alkaline Group. - Alkaline salts have their activity diminished by combination with carbonic or organic acids. The hydrates have an intense local action on the tissues; and the carbonates have an action, the same in kind, but much less in degree. In the case of the bicarbonates it is still further diminished, and in the acetates, citrates, and tartrates it is absent. The hydrates of potassium and sodium dissolve horny tissues such as the epidermis. They combine with albumen and form a soluble alkali-albuminate.

When applied to the skin the hydrated alkalis, which have a great affinity for water, withdraw it from the tissues and form a solution which softens and partly dissolves the epidermis and then acts on the softer textures below, combining with and dissolving them. Round the part thus killed inflammation sets in, and a slough separates. The rapidity with which they absorb water and form a solution which flows readily over adjacent parts, where its action is injurious, is an objection to their application, and the part actually cauterised by them should always be less than the part we wish to destroy. From this very property of widely destroying the tissues over which they flow, or through which they soak, they are admirably adapted for application in cases where we desire this effect, as in cauterising poisoned wounds.

When applied as caustics to unhealthy sores, cancer, etc, their action is sometimes limited by adding lime and forming the so-called Vienna paste (p. 346). The water which they withdraw from the tissues is sucked up by the lime, forming a solid hydrate and preventing the caustic from becoming too fluid and running over other parts. When less concentrated they may only irritate the surface sufficiently to produce exudation, but they generally soften or dissolve the epidermis so much that vesicles do not form well. When still more diluted they may cause only congestion or redness of the skin. They are then said to act as rubefacients. This rubefacient action may be used for the purpose of relieving troublesome itching in skin-diseases, or to produce derivation from other parts.

Ammonia does not dissolve the epidermis, and so, unlike potash or soda, it does not act as an immediate caustic, but only passes through the epidermis and irritates the skin below, causing lymph to be effused between it and the epidermis, and thus acting as a vesicant. It may, however, act as a caustic if its evaporation is prevented and it is applied too long, the irritation then becoming so great as to lead to suppuration, or even to sloughing of the part.

From their great solvent power, and especially their power of dissolving greasy substances, alkalis are used for cleansing the skin, but when used alone they very frequently produce irritation, and we therefore generally employ them in the form of soap, or in the form of those salts which have only a very slight alkaline character, such as borax.