The following mineral substances are utilised by the body along with the alimentary principles: -

Chlorides of sodium, potassium and ammonium, phosphates of sodium, potassium and magnesium, carbonates of sodium and calcium and bicarbonate of sodium, sulphates of sodium and potassium, iron and manganese in small quantities, and silicon and fluorine in still smaller quantities, are also to be found in the body.

Sodium, besides occurring in the combinations just mentioned, may also be found in association with vegetable acids used as food or food adjuncts, and possibly in loose combination with protein. Chloride of sodium, about which we shall have more to say in connection with the salt-free diet, controls osmosis and dissolves globulins. The total quantity contained in a normal body never exceeds 200 grams, and about 15 grams are excreted daily. The carbonates of sodium are to be discovered chiefly in the blood plasma, where they assist in carrying the carbon dioxide from the tissues to the lungs. They are partly formed from the malates, citrates and tartrates in fruits. Sodium salts are sparsely excreted in the faeces, so that most of what is ingested is absorbed, and apparently the capacity of the body for the absorption of sodium salts is unlimited.

Potassium salts have an affinity for the solid tissues, and thus chloride of potassium is to be found in fairly large quantities in the muscles, milk, and red blood corpuscles. The body supply is derived chiefly from vegetable foods, and as these are never all absorbed, potassium salts are always to be found in the faeces.

Calcium salts are to be found in food associated with albumins of acid character, and also in the inorganic forms of phosphate and carbonate. Vegetable foods are richer in calcium than flesh foods, although milk and eggs form an exception to this rule, and drinking water often contains a considerable proportion of lime. For the most part lime salts are excreted by the faeces, usually in the form of phosphates, carbonates, and compounds with the higher fatty acids. From .1 to .5 gram of calcium per day appears in the urine, so that a certain proportion is absorbed, chiefly as phosphates and carbonate, in the upper part of the intestinal canal. Their appearance in the faeces indicates either that they have never been absorbed or, having been so, have been eliminated again by the bowel wall. The greater the quantity of acid contained in the food or produced within the system, the greater will be the proportion of calcium excreted in the urine. Hence, the consumption of acid fruits in the diet causes an absorption of lime salts into the blood and tissues and their subsequent passage into the urine. Hence, also, the reason why the alkaline urine of vegetarians and herbivora contains so little calcium, whereas that of meat-eaters and the carnivora contains it in greater abundance. It will now be apparent why the urinary calcium is increased in acidosis. Both carbonate and phosphate of calcium are to be found in bone, teeth, and some of the body fluids.

Phosphorus in the body is always to be found as phosphoric acid, either in an organic or inorganic form. Lecithin and nucleo-protein are examples of the former variety, and are probably disintegrated by the digestive juices absorbed as salts of glycerophosphoric acid, 80 per cent. of the phosphoric acid subsequently appearing in the urine and the rest in the faeces. Most of the phosphorus, however, is ingested in the inorganic form, and the amount of the phosphates in the urine depends on the quantity of the calcium, acid, and alkali introduced into the body or already there. Eventually three-fifths of the phosphates are to be found in the urine and the rest in the faeces.

Phosphate of calcium forms one half of the bones and occurs in the dentine and enamel of teeth and in other solids and fluids, e.g., milk.

Phosphates of soda and potassium give the body their alkaline reaction to chemical test, and the sodium-di-hydrogen phosphate is chiefly responsible for the acid reaction of the urine.

Sulphur is absorbed chiefly in organic combination with protein, and is excreted in the urine partly as combined or oxidised sulphur and partly as unoxidised neutral sulphur.

Iron, of which about 3 grams are to be found in the blood, occurs chiefly in food in the inorganic form of haemoglobin or nucleo-proteins, e.g., the so-called "haemoglobin" of egg yolk and milk. Green vegetables, e.g., spinach, cabbage, fresh asparagus, wheat, strawberries, cherries, etc, are particularly rich in iron. It is absorbed from the duodenum in organic combination and excreted almost solely in the faeces, the body requiring from 16 to 30 milligrams per day.

The mineral salts yield no energy to the body, at least in a direct form. In contradistinction to the food-stuffs which are only absorbed as required by the body, i.e., in response to a definite demand, the saline substances are absorbed in greater quantities than are required. Little is known as to their definite effect, and speculation is rife as to the manner in which they are dissociated from the food-products in which they are incorporated, brought into solution, carried to the tissues requiring them, and finally broken down and expelled from the body. Bunge has a most novel conception as to the final disposal of the chlorides, which is treated at some length in his well-known book.