In the case of acids and bases, the physiological action of each is modified by their union, e.g. when caustic soda and hydrochloric acid unite, the caustic action of each is destroyed, and we obtain sodium chloride and water, which have different physiological actions, as well as different chemical characters, from either the acid or the base.

But if we examine a series of salts of the same base with different acids, or of the same acid with different bases, we find that both the acid and the base modify the physiological action of the compound.

Different Acids.

Sodium

hydrate

caustic.

,,

bicarbonate

antacid.

,,

sulphate

purgative.

,,

benzoate

antilithic.

,,

salicylate

antipyretic.

,,

cyanide

powerful poison.

Different Bases.

Sodium

chloride

neutral in action.

Potassium

,,

muscular poison.

Zinc

,,

caustic.

Barium

,,

muscular poison.

Silver

,,

inert.

Iron

,,

astringent, haematinic.

Mercuric

,,

corrosive, antiseptic.

This modification is in some cases due to a change in the physical conditions, and especially in the solubility of the compound. Thus the chloride of silver is inert so long as it remains in the form of a chloride, because it is insoluble. It thus differs much from the corrosive chloride of zinc, while if we were to compare the action of the nitrate of silver and zinc we should find considerable similarity.

1 Brit. Assoc. Reports, 1865, p. 280.

Another cause of difference is the different proportion of the acid to the base.

Thus the proportion of sodium (Na = 23) to the acid radical in the following sodium salts is as follows : in the hydrate as 23 to 18; in the bicarbonate as 23 to 61; in the sulphate as 23 to 96; in the benzoate as 23 to 121; in the salicylate as 23 to 137.

In this connection, too, the degree of saturation of the acid by the base must be considered. If, for example, the acid is not saturated, part of the action of the compound is due to its acid chemical properties; and if, on the other hand, a weak acid be combined with a strong base, this action is partly due to the alkaline chemical property.