While dilute solutions of chloride of sodium are ready solvents of albuminous substances and are non-irritating, sodium chloride, in substance or in concentrated solutions, precipitates globulins, withdraws water from the tissues, and acts as an exceedingly powerful irritant to cut surfaces, mucous membranes, muscle, and nerve. Common salt taken in a large quantity at once will irritate the stomach and cause vomiting. It is absorbed with great rapidity, but it is also excreted so rapidly that it. produces no definite symptoms of irritation in any part of the body, excepting that part of the nervous system by which the sensation of thirst is perceived. This sensation becomes so urgent when much salt has been taken that any risk will be encountered in order to gratify it. Should it be impossible to obtain fresh water, other parts of the nervous system become involved, and travellers whose supply of water has failed in the desert, or shipwrecked sailors who have drunk sea-water, have become delirious. It is difficult to say, however, how far the delirium is due to the direct irritant action of sodium chloride on the brain, as many other factors may concur in its production. Under ordinary circumstances, the thirst occasioned by sodium chloride after its absorption, causes as much water to be drunk as will allow the salt to be excreted by the kidneys, leaving the proportion both of salt and water in the body nearly the same as before. During its stay in the body the salt does not appear to alter the composition of the tissues, and the chief alterations produced by it are probably due to its action on the solubility of albuminous substances and on the processes of osmosis between the intercellular fluid and blood, and the circulation of lymph in the tissues. In consequence of this, sodium chloride increases tissue-change, as is shown by an increase in the amount of urea excreted. A similar increase, however, occurs when the quantity usually taken is diminished, the amount of water daily consumed remaining the same. The alteration here is probably also due to increased rapidity of the circulation of fluid through the tissues (Voit), but it may also be due in part to the different solubilities of albuminous substances in solutions of sodium, chloride of different strengths. Certain albuminous tissues may thus be affected by one proportion of salt in the blood, others by another, so that increase and diminution of the normal proportion of sodium chloride may increase tissue-change in the body as a whole, though not in the same tissues. The proportion of chloride of sodium in the body' is not always the same. It depends on the quantity taken daily, and may be increased or diminished within certain limits. If a definite quantity be taken daily for some time, the same quantity will be found in the urine, so that the amount present in the body is constant. If the quantity consumed be now increased, no increase takes place in the excretion for about three days, a storage of salt taking place in the body. After about three days the quantity excreted daily in the urine will again be found equal to the quantity daily taken, the amount present in the body remaining constantly at the higher level. If the quantity daily taken be now diminished, no diminution takes place in the quantity excreted for about three days, and then the quantities daily taken and excreted again correspond. The amount stored up at first is now gone, and the proportion of salt in the body is now again reduced to its lower level.1

Increased consumption of sodium chloride not only increases the quantity of it and of urea in the urine but increases also the excretion of potassium salts.

On the other hand, potassium salts also increase the excretion of sodium. Between salts containing no chlorine, such as carbonate or phosphate, and the sodium chloride in the blood, a double decomposition takes place, potassium chloride, and sodium carbonate or phosphate, being formed. These newly-formed salts are unnecessary for the organism, and are excreted in the urine along with the unaltered remainder of the phosphate or carbonate administered. Considerable quantities both of chlorine and sodium may thus be removed from the organism. In consequence of this, herbivorous animals and people living chiefly on a vegetable diet, and who thus consume considerable quantities of potassium salts, feel the need of sodium chloride greatly, and on the American prairies the herds of buffaloes travel hundreds of miles to visit the salt licks. Beyond a certain point, however, the excretion of sodium chloride is not increased by potassium salts, and when the quantity of sodium salts in the body is low, excretion is not increased at all.

When an abnormal quantity of fluid is present in the tissues,

1 Ludwig, Manuscript Notes of Lectures, 1869-1870.

as in dropsies, an increase in the saline constituents of the blood may cause its absorption, especially if the quantity of water drunk by the patient be limited. It is probable that in addition to their diuretic action the alkaline salts affect the nutrition of the tissues themselves, and that salts of potassium are better than those of sodium in cases of dropsy, because of their action on the tissues.

General Action of the Sub-Gboup of Sulphates, etc.-This group contains salts which are sparingly absorbed, such as sulphates, phosphates, and bitartrates. That they are sparingly absorbed is shown by the fact that when administered internally they only appear to a small extent in the urine. They usually act as purgatives, but if from any cause their purgative action should be prevented, and they remain long in the intestine, absorption will occur, though slowly. In herbivorous animals, which have a much longer intestinal canal than carnivora, larger doses of these salts are required to produce a purgative action. The mode of action has already been discussed (p. 390 et seq.).

Comparative Action of the Alkaline Metals. - As the action of the base appears to be less modified by the acid radical in the case of the chlorides than of other salts of the alkaline metals, they are better adapted for experiments on the comparative action of the members of this class.

Group I. - The chlorides of lithium, sodium, potassium, rubidium, and caesium produce in frog's gradually increasing torpor, paralysis, and death. The chief action appears to be on the spinal cord, which is paralysed, a slight primary excitement occurring in the case of potassium and rubidium. Lithium and potassium paralyse also the ends of the motor nerves. Sodium does so also, though to a much less extent. Caesium and rubidium do not do so, excepting when given in very large doses.

The contractile power of muscle is almost always diminished by lithium, unaffected by sodium, and increased by the other members of this group in small or moderate doses. Large quantities of potassium diminish both the irritability and contractile power of muscle voluntary and involuntary.

In frogs the heart becomes weaker and finally stops in diastole.

Group II. - Ammonium differs entirely from the members of the first group in the symptoms it produces. While they paralyse the spinal cord with little or no previous excitement, causing torpor and death, ammonia at first stimulates the cord, producing tetanic convulsions. The action of ammonium is considerably modified by the acid radical with which it is combined. All the ammonium salts have an action on the spinal cord, motor nerves, and muscles, and, in advanced poisoning, paralyse these structures.

They do not, however, affect all these structures with equal readiness. The organ first affected, and consequently (p. 26) the symptoms of poisoning, vary with the salt employed. Some salts affect the spinal cord first, others the motor nerves. Ammonia and ammonium chloride produce tetanus. The bromide produces hyperesthesia with some clonic spasm, passing into tetanus, which, however, comes on very late.

The sulphate also produces hyperaesthesia and clonic spasms, but rarely tetanus. The phosphate produces paralysis without convulsions, either clonic or tonic, the only indication of any con-vulsant action being slight twitches accompanying movements in the hind limbs before reflex action has ceased. The iodide produces progressive paralysis and no tetanus. The brain appears to be affected before the spinal cord. This is shown by the frog croaking when stroked, as it does after removal of the cerebral hemispheres, and by the reflex from the conjunctiva failing before that from the limbs.

Ammonium salts appear to form a series, at one end of which the members stimulate the spinal cord and have no marked paralysing action on the motor nerves, while those at the other end have no marked stimulating action on the cord, but, on the contrary, have a marked paralysing action both on the cord and on motor nerves. At the stimulating end of this series are ammonia and ammonium chloride, and at the paralysing end ammonium iodide; whilst the bromide, phosphate, and sulphate lie between.