One of the most common diseases of the kidneys is the formation of gravelly matter in them, which is either voided in small particles, or becomes concreted into stones of various sizes. The four principal combinations forming stones found in the human bladder, are, first, the Fusible Calculus, consisting of phosphoric acid, magnesia, and volatile alkali, sometimes called the ammoniaco-magnesian phosphate, or Fusible Calculus; secondly,. the Mulberry Calculus, consisting chiefly of the oxalate of lime; thirdly, the Bone-earth Calculus,made of phosphate of lime or animal earth; and fourthly, the Uric Acid Calculus. The latter is by far the most frequent. Those stones that are formed of Uric Acid, are known by their red or dark yellow colour, being sometimes of a smooth, but generally rough surface. The ammoniaco-magnesian phosphates are of a pale or grey colour, smooth. The Mulberry stone (so called from a supposed slight resembrance to the fruit) is known by its dark colour, greater weight and compactness.

As the remedies to be used for the prevention of the formation of these substances must depend upon the kind of gravel or stone that is being formed, it is necessary to show how they may be distinguished one from another, when obtained from the urine,even in small quantities. The Uric Acid stone may be dissolved in a weak solution of Potash or Soda, and also in Lime Water; but is not acted upon by Muriatic Acid. The Fusible Calculus is partly soluble in water, and more so, when a few drops of Muriatic Acid are added to the water. The Bone-earth Calculus is soluble in Muriatic Acid. The Mulberry Calculus will not dissolve in alkalies, but will, in time, dissolve almost entirely, if soaked in Nitric Acid, (Aqua Fortis).

An excess of Uric Acid is generally supposed to be the immediate cause of the formation of sand and gravel, and this excess will much depend upon the mode of life and kind of diet used by the patient. Uric Acid is not an accidental or diseased production of the human system, but is one of the natural ingredients of the urine of a. person in perfect health. In the healthy state it is wholly dissolved in the urine; and when in excess, that is, when more is formed than the urine can hold in solution, it either remains in the bladder or is deposited by the urine after it is voided. A long use of fermented liquors, and of wines abounding in acid, may possibly, in some constitutions, prove occasional causes of Gravel and Stone. It has also been long supposed that water strongly impregnated with sulphate and carbonate of lime, constituting what is commonly known as hard water,predisposes those who drink it to Gravel and Stone. Some medical men do not believe in this; it is, however, a well known fact that the inhabitants of some districts of country are more afflicted in this way than others. The use of animal food tends to diminish the quantity of urine, at the same time that it increases the proportion of Uric Acid, whereas a vegetable diet has a contrary effect.

Persons in the decline of life, particularly those who have been much engaged in sedentary employments, as likewise those who are much afflicted with Dyspeptic complaints,or the Gout,are in general very subject to complaints of the kidneys; but it is well known that the period of life from infancy to about fifteen years of age, is most subject to the formation of stones in the bladder; and the children of the poor are more afflicted in this way than those of the wealthy. From the difference in the structure of the urinary passages in the sexes, men are much more liable to them than women. In warm climates we seldom meet with instances of stones forming of any size, either in the kidneys or bladder, as the particles of sand usually pass off before they can adhere together, owing to the relaxed state of the parts; but in cold climates they are frequently found of considerable size. In one case, where the stone was extracted after death,it entirely filled the bladder; it was sixteen inches in length, and weighed forty-four ounces; its extraction was attempted by the late Mr. Cline, the eminent surgeon, but of course unsuccessfully. The largest that was ever extracted entire, weighed sixteen ounces, but the patient died; but as long ago as the year 1746, Mr. Harmer, of Norwich, England, extracted one entire, which weighed nearly fifteen ounces,and the patient lived five years. Mr. C. Mayo extracted one weighing fourteen ounces and a half, but it was broken, and the patient lived several years. As many as one hundred and forty-two stones have been extracted from one person.

When the Uric Acid is in excess of its natural proportion, there is usually a frequent and almost irresistible desire to make water; but this does not arise from the bladder being full, for in general, only a small quantity is voided at a time, but from the irritation to the neck of the bladder, caused by the Acid. In cold weather the quantity is increased, and it is also increased by all causes producing mental agitation. There is often a sense of weight, or a dull pain in the back and the irritation in the neck of the bladder, sometimes extends along the urinary passage. The pulse, however, is not affected, and the tongue is clean; there is not much thirst, nor are the stomach and bowels usually out of order. •

A fit of the Gravel, as it is termed, is attended with a fixed pain in the loins, numbness of the thigh on the side affected, nausea and sometimes vomiting,and not unfrequently with a slight suppression of urine. As the irritating matter removes from the kidney down into the ureter, (the passage from the kidney to the bladder), it sometimes produces such acute pain as to occasion faintings and convulsive fits. The symptoms often resemble those of Inflammation of the Kidney; but the deposit of reddish-brown sand, or very fine powder of the same colour, from the urine, on becoming cold, will show the difference. One of the principal symptoms of gravel in the kidney is the dark appearance of the urine, as if it were mixed with coffee grounds, evidently depending on broken down particles of blood, proceeding from the obscure but continued irritation of the kidney. When this occurs, along with a dull, heavy pain in the loins, there can be very little doubt of the presence of gravel in the kidney. In mere inflammation of this organ, when no gravel is present, the urine does not put on the above appearance.