It will be inferred from previous references to digestion and digestive troubles that acids similar to those normally secreted will prove valuable when Nature's laboratory fails to produce them in sufficient quality or quantity. In the use of acids as an aid to digestion two factors should be borne in mind, viz., while acids increase the dissolving or digesting power of the gastric juice when food is in the stomach, they have the effect upon an empty stomach of retarding the secretion of natural acid; they should always therefore be given immediately after food and not before a meal. The seeming contradiction of giving alkalis in some cases where acid secretion is defective, is explained by the fact that those remedies excite activity on the part of the acid- forming glands. The converse effect is observed upon glands which produce an alkaline fluid, they being stimulated to action by acids, and their secretion lessened by alkalis. From these considerations the reason for giving alkaline tonics before meals, and acids after them, will be obvious to the reader.

In the section on the physiology of digestion it has been explained how the fluids of the first portion of the duodenum act upon the partially digested food as it leaves the stomach, and here again the remedies under consideration continue their work. The continued employment of acids for a length of time is not desirable, as they may establish an artificial need for them, or result in catarrh of the stomach.

Besides the use of acids in various forms of indigestion, they are serviceable for their constringing action upon the lining membrane in cases of diarrhoea with a tendency to pass blood. Dilute sulphuric acid is generally preferred, or an old-fashioned aromatic acid in which ginger and cinnamon play a not unimportant part.

Sulphuric acid is given to arrest hsemorrhage in different organs, as the liver, kidneys, uterus, and lungs.

For all the purposes previously named it will be understood that the dilute acids of the Pharmacopoeia are implied, the strong preparations being destructive caustics, and on account of this property being sometimes employed to remove warts and check the progress of malignant growths. If used as escharotics, care should be exercised in not allowing them to come in contact with healthy parts, or to burn the hand of the person making use of them.