The obvious effect of diuretics is to diminish the quantity of liquid in the circulation. it is true that the portion of the blood evacuated by them is chiefly the water; but some of the saline matters, and no doubt also more or less of the organic constituents, escape; so that these medicines may be considered depletory. The fact is that, when they operate very profusely, they sometimes produce weakness, approaching even to prostration.

Another important effect, consequent upon the diminution of the watery portion of the blood, is the promotion of absorption. it has been fully established by experiment that the blood-vessels, when their contents are materially diminished, take up water with avidity, wherever they can find it, to supply the loss. The physical physiologist would say that, the density of the blood being increased, endosmosis is promoted; and the explanation may be admitted with some qualification. Thirst also results from the diuretic action, with the obvious purpose of securing a supply of liquid for absorption. But it is not taken up solely from the surfaces with external communication. The liquid in the tissues and cavities is also absorbed; and hence the use of this class of medicines in dropsy.

A third effect is to purify the blood of excrementitious or foreign matter, which may be serving as a source of injury.

A fourth is to impart to the urine the power of dissolving a larger quantity of the saline matter eliminated by the kidneys, while it is at the same time rendered less irritant itself by dilution.

A fifth effect, which is produced only by the stimulating diuretics, is to irritate the lining membrane of the pelvis and calices of the kidneys, the urinary passages, and the bladder, and thus not unfrequently either to alter the diseased condition of the surface, or to excite the parts when debilitated or paralyzed to greater activity.

In correspondence with the above indications, diuretics prove useful, and are more or less extensively employed in the following complaints.

Dropsy. it is in the treatment of dropsy that diuretics show their best powers; and in this disease there is no other remedy or combination of remedies that equals them in efficiency. The principle upon which they operate is obvious. Dropsy consists in an abnormal accumulation of effused fluid in the cellular tissue or serous cavities. Diuretics have the effect of promoting the absorption of that fluid, and its rejection by the kidneys. But if this were all they could do, their utmost advantage would be merely to remove the fluid while operating, and prevent its accumulation so long as they continued to operate. in cases of limited duration, in which the pathological condition on which the effusion depended, terminates, from its own essential character, in a certain period, the diuretics may be said to effect a cure; as stimulants which keep a typhus fever patient alive, until the time for the fever to cease spontaneously, may be said to cure that disease. it is probable that this sometimes happens. in many cases, the pathological condition is quite incurable, as often in organic disease of the heart, cirrhosis of the liver, the advanced stages of Bright's disease, etc. Here all that diuretics can do is to keep down the effusion, and thus protract life, and render the patient more comfortable. But there are also cases of dropsy in which diuretics appear of themselves to effect permanent cures; not to remove the effusion only, but to put the system into a state, in which there is no tendency to a return of the effusion. This I have very often seen, and every experienced practitioner must be familiar with the fact. Cases of dropsy come under the notice of the physician, which have been of long duration, which appear to be regularly advancing, and in which there seems to be no tendency to a favourable termination. Diuretics are prescribed. They act efficiently, and the enormous tumefaction disappears like a snowbank in the sun. Nor, when it has disappeared, does it return. The patient remains perfectly well, and may continue so for years. The diuretics then have done more than merely to remove the effused liquid. They have favourably modified, perhaps quite removed the pathological condition itself. How is this accomplished ? This question can be answered only by conjecture. Dropsy often depends on disease of the kidneys. The diuretics act specially on the kidneys, and may prove useful not merely by increasing their secretory activity, but by altering essentially their morbid condition. They may change the nutrition of the organ, and substitute a new and healthy tissue for the old and diseased. Or the kidney may be undergoing fatty degeneration from its enfeebled condition; and the diuretic may check the process by stimulating the organ into healthy action. Or, lastly, it may be inflamed, and the diuretic may cure the inflammation through its depletory and antiphlogistic properties. Not unfrequently the dropsy depends on a highly irritated or inflamed state of the tissue from which the fluid is exhaled, or perhaps on chronic inflammation of one of the large organs, as the liver or the heart, which may cause them to interfere with the course of the blood. in these cases, also, the diuretics may be supposed to operate by the antiphlogistic properties, which some of them possess in a high degree.

I know no variety of dropsy in which they may not be given. Upon theoretical grounds, their use has been condemned in the dropsy of Bright's disease; but I am quite sure, from a good deal of experience with them in that affection, that, if properly chosen, they are more effectual than any other medicines. it is true that the stimulating diuretics are not adapted to the acute forms of the affection; even squill has seemed to me occasionally to do harm; but from bitartrate of potassa or digitalis I have never seen mischief produced, but repeatedly cures in very unpromising cases.

Febrile and inflammatory Diseases. The refrigerant diuretics are beneficial in these affections by their sedative and depletory properties; but they are not frequently used, because their operation is in great degree incompatible with that of more efficient antiphlogistic remedies, as cathartics and diaphoretics. When given for the cure of dropsy, they are often useful incidentally, in accompanying febrile and inflammatory disorder; and sometimes, probably, through their eliminating property, in affections connected with impurities in the blood, as cur taneous eruptions.

Disease of the Kidneys and Urinary Organs. in acute inflammation of the kidneys, some of the saline diuretics are useful by their refrigerant and depletory effect, and, when the disease is seated in the pelvis of the kidney, by diluting the urine and rendering it less irritant to the inflamed membrane. in the same way, they act beneficially in inflammation of the bladder and urinary passages. The urine, in these complaints, is often loaded to saturation, and even beyond it, with the saline ingredients, in consequence of the small proportion of the liquid eliminated. By increasing this, the salts which might otherwise be deposited are held in solution, and those dissolved are rendered less irritant by the dilution.

In chronic inflammation of the pelvis of the kidney, with or without ulceration, the parts are frequently unable to resume healthy action, from their relaxed, debilitated, and indolent condition. These cases arc often attended with copious mucous discharges, and not unfrequently with the discharge of pus. Some of the stimulating diuretics, which pass out with the urine, and are thus brought into direct contact with the diseased surfaces, sometimes exercise a most happy influence over them.

Administration

The rules for the administration of diuretics, so far as they are applicable to the class in general, may be deduced from the observations already made. They should generally be attended with the use of drinks, which should always be cold, and may often be advantageously impregnated with mild diuretic medicines, as, for example, with the vegetable acids, cream of tartar, and sweet spirit of nitre. The milder diuretic infusions may often also be used, as those of juniper, parsley, the erigerons, etc., which may act beneficially as adjuvants to the more energetic diuretics employed. Sometimes benefit accrues from the gentle stimulant properties of these drinks, particularly the juniper-berry and wild carrot teas, in correcting the depressing effects of the saline diuretics on the digestive organs. in persons of intemperate habits, or requiring from debility the support of alcoholic stimulation, cold gin and water may be drank with advantage. in relation to the quantity of drink, the patient's own sensations are perhaps in general the best guide; though the advice should be given to take small draughts frequently, in preference to large quantities at once. Should the quantity of urine discharged equal the amount of drink taken, there can be no accumulation; as much is always discharged by cutaneous and pulmonary exhalation.

The diuretic should not, as a general rule, be given at the same time with cathartics or diaphoretics; and the patient should be kept cool, or at least should not be allowed to become heated, as by too much bed-covering at night, during their administration.

When the system is in a high state of excitation, with a plethoric condition, and a full strong pulse, it will generally be proper to reduce it somewhat, by depletion or antimonials, before beginning with the diuretics. They will be more apt to operate after this preliminary treatment, than if administered in the highest state of excitement.

As the diuretics are notoriously uncertain, failing often without apparent cause, and often also succeeding when success was hardly expected, it is necessary not to be too soon discouraged; but, after making a sufficient trial of one, to have recourse to another, or to conjoin two or more together, and, in reference to the more energetic articles, to return to them again and again. I have used certain diuretics long and freely with no apparent advantage, and, having abandoned them, have afterwards returned to them, and obtained the most satisfactory results, though no change was made in their mode of exhibition, and, so far as could be seen, no change had taken place in the patient which could account for the difference.