Sodium chloride, or common table salt, is by far the most important and valuable salt, and is used in the largest amount. It has long been a symbol of wisdom and hospitality in the East. It forms 60 per cent of the salts of the blood, and enters into the structural formation of all the tissues and secretions of the body in greater or less quantity, with the single exception of the enamel of the teeth. It is estimated that the quantity which may be daily appropriated from the food is about fifteen grammes. Salt slightly stimulates the renal secretion, which in turn leads to thirst and to drinking more fluid, which promotes interchange of the juices of the body. It also excites thirst more directly.

Common salt stimulates the appetite and influences beneficially the gastric secretion. It not only furnishes the chlorine for hydrochloric acid, but seems to act locally in the stomach by promoting this secretion as well as the conversion of pepsinogen into active pepsin. Cohn and Voit have proved that the absence of salt from the diet completely checks the production of hydrochloric acid in the stomach.

There are some few tribes of flesh-eating men who do not add salt to their food, relying for their needs upon what they derive from the food itself. This supply is therefore sufficient to maintain life. In fact, as a rule, man derives enough salts from the composition of his food to supply the tissues and juices of the body, and the additional quantity which he takes as table salt is mainly of service as a condiment, to give agreeable flavour to a mixed diet and to sharpen the appetite. The excess of salts in general is promptly eliminated in the urine.

In most men and many of the higher mammalia the craving for sodium chloride is instinctive. Stanley records in his book "In Darkest Africa " instances where savages are accustomed to travel many hundreds of miles under great difficulties to obtain a coveted supply of salt.

Herbivorous animals are even more dependent upon salt than are carnivores; cattle and sheep, for example, must be given salt in addition to that contained in their food to remain in good condition.

Overdoses of salt cause diarrhoea and even gastro-enteritis, and excite irritation of the nerves of the throat.

Large doses of salt have been given in pleurisy with the view of increasing the density of the blood and causing reabsorption of the pleuritic fluid by promoting osmosis towards the vessels. This treatment has not met with success. The popular use of salt to control pulmonary haemorrhage is of no practical value.

Almost all vegetables contain less sodium chloride than does milk, the food of the young growing animal, although many of them have more potassium.

Sodium chloride is of great service as a preservative of foods, either used as a brine in pickling, corning beef, preserving olives, etc., or in solid form to dry and keep meat and fish from decomposition.