themselves, with their predominant axis, round a common centre, like the rays round the luminous body from which they emanate. Mostly one or two large crystals form the basis of, or are mixed up with, these globular masses. What I should like to call "sand," therefore, would be masses of uric acid, mostly globular, or irregularly roundish, or oblong, of very uniform size, from th to th of an inch in diameter, and answering to the above definition. Viewed by transmitted light under the microscope, the globules are perfectly impervious to light; and the uric acid crystals on the surface are faintly transparent, of a dark-brown colour. To the naked eye, the deposit is red, with a tinge of brown. When disturbed, the sediment mixes with the clear urine, but rapidly subsides to the bottom when the fluid comes to rest. Generally, no small or well-defined crystals of the ordinary kind are mixed with this sand.

This description of sand is sometimes met with in the pale and watery urine of early infancy.

Under the name of "gravel," I would comprise concretions varying in size from th of an inch diameter upwards, until, by their size, they become incapable of passing either the ureters or the urethra. These concretions are generally rough; and if many are discharged at one time are of variable size, from that of a pin's head to that of an almond (Prout). A variety of these crystalline concretions are the "pisiform," "the formation of which is attended by a remarkable feature, namely, the great number in which they are usually generated, a circumstance which may be said to be characteristic of them. Their great number occasions them to accumulate occasionally in the pelvis of the kidney; or in the lower portion of the ureters, where they terminate in the cavity of the bladder; and on such occasions severe nephritic attacks are sometimes the consequence. These concretions

vary in size, from that of a pin's head to that of a pea or marble. Their form is always more or less globular; though they sometimes present flattened or facetted surfaces, produced by their contact or attrition with each other. Their surface is usually smooth, sometimes even porcelainous or polished; and their internal texture is almost invariably crystallized, and usually lamellated. Their colour ranges through all the shades of yellow; and occasionally, though more rarely, they assume a dark-brown or reddish colour." (Prout.) These pisiform concretions are mostly deposited after the age of forty.

Loc. cit., p. 200.

2. Uric acid calculi.-The number of calculi, of which uric acid forms either the nucleus or the entire substance is very great, standing to the number of all other calculi in the proportion of two to three. They are found of all sizes, from the largest pisiform concretions to stones of five or six ounces, or more, in weight. If formed in the pelvis of the kidney, a uric acid stone may have a very irregular rough shape and surface. If, however, the body and crust of the stone have been formed in the bladder, as is mostly the case, the shape will, in general, be that of a flattened ovoid, the flattening, in many cases, being so trifling as to escape notice. The exterior of the uric acid calculus is slightly tuberculated; but in many cases the tubercles are so water-worn, that the surface is smooth. In colour, uric acid calculi vary from reddish-yellow or fawn-colour to brownish-red, or brown with an admixture of red, like old mahogany. On being divided by a saw, they are seen to be composed of concentric layers, which are of variable thickness when compared with each other. But every layer preserves its own thickness pretty regularly all round the calculus. The texture of the stone is best seen on the surface of a fracture. In hard and pure stones it is crystalline, fibrous, the fibres of each layer verging like radii towards the centre of the stone. On breaking a stone, the fracture will mostly be parallel with these crystalline fibres. Stones, however, which are less dense and less pure are earthy, and amorphous in fracture. Some few stones are so hard that they give a ringing noise on percussion, a sharp sound like a pebble, and on being chipped exhibit a conchoid fracture. These stones are very dangerous, when they become the subjects of the process of lithotrity, for their fragments are so sharp, that they wound the bladder, and cause infiltration of the urine into the adjacent cellular tissue; the consequences of which are mostly extensive mortification and death. It is, therefore, of high practical importance to ascertain the circumstances under which these hard stones may be formed.

The laminated structure of the uric acid calculi (and of all other laminated calculi) shows that they are formed by the gradual precipitation of uric acid from the urine, the precipitated substance being deposited in an equal layer all over the surface of the concretion, which forms the nucleus, and also over the surface of all subsequent layers; a circumstance which is the condition of the parallelism of the rings seen on section. The layers, however, show something more; namely, that the formation of the stone has been interrupted, or has taken place at different intervals. Of this circumstance there

could not be given so good a description as that of Dr. Prout: ' "Between the different intervals at which the different laminæ have been formed, periods have intervened during which no deposition has taken place. This remark not only applies to the different laminae of a heterogeneous calculus, but to the different laminæ of calculi composed of the same substance; as for instance to the different lamina of which lithic acid concretions usually consist. This explanation is in perfect accordance with the circumstances attending the formation of calculi, which often, as is well known, remain in the bladder for a great number of years, without attaining any remarkable size. Moreover, the constant state of change alone to which the urine in all individuals is liable, almost precludes the notion of homogeneity in a calculus. We may suppose, therefore, that certain changes take place in the urine, during which the law of continuity of deposition is suspended, and the surface of the concretion becomes, as it were, water-worn and less apt for future accretion; in short, assumes all the properties of a heterogeneous substance. Under these circumstances, when a tendency to deposition occurs, it will have to commence de novo, and, as it were, upon the surface of a foreign body. The consequence will be that the adhesion between the new and the old coats, or laminæ, will be less firm than in the intermediate parts, and that a calculus thus formed will be disposed, when broken, to separate into concentric lamina." To this we have only to add that one great cause of the formation of layers is the periodicity with which the bladder is emptied of its contents. If uric acid is really precipitated by acid fermentation, and if for this fermentation to produce a sufficient amount of acid a certain time is required, during which the ferment must be in contact with the substance to be fermented, then no uric acid can be deposited immediately after the bladder has discharged its contents; and no uric acid can be deposited if the bladder is so irritable as to discharge its contents at frequent and short intervals, a condition which, as I have already stated, I believe to be the main safeguard against the more frequent occurrence of stone in the bladder. If, on the other hand, the calculus does not irritate the bladder at ordinary times, or is the mechanical cause of a retention of part or the whole of the urine, so that the urine has time to be collected and to be fermented, a deposition upon the calculus will take place.

Chemical characters of uric acid concretions.-The chemical

Loc. cit., p. 361.

characters are those of uric acid. But as the concretions may be more or less pure, it is advisable to follow a method in analysing. The blowpipe decides whether the stone leaves any residue on combustion. A piece of the stone is then reduced to a powder, a weighed portion of which is extracted with ether, then with alcohol, and, at last, repeatedly with boiling water. The ether dissolves any fat; the alcohol takes up colouring matter; the boiling water removes urates and soluble inorganic salts, and a trace of the acid. If it is not necessary to be very accurate, the extraction with ether and alcohol may be omitted. The powder which is not soluble in water may now be dissolved in a dilute and warm solution of caustic potash or soda, and precipitated by carbonic acid, when snowywhite urate of soda will be mostly obtained. Or the solution in potash may be precipitated with acetic acid, when all the uric acid falls down in a very pure state, and is obtained by filtration, washing, and drying. The combined weights of the extracts and of the pure uric acid must be nearly equal to the weight of the powder taken for analysis.

Concretions of Urate of Ammonia, Soda, and Lime.

Calculi composed essentially of urates are of rather rare occurrence, and seem confined to children under puberty.1 They are of small size, and have a smooth or slightly tuberculated surface, and a pale slate or clay colour, sometimes with an admixture of red or brown. In rare instances their colour is fawn, and in such stones pink layers occur towards the centre. They are composed of concentric layers, but the layers are much less distinct and much thinner than those of uric acid calculi. They have a fine earthy fracture, and are easily broken.

The chemical diagnosis rests upon the solubility of the urates in boiling water, by which they are distinguished from all other calculi. After the urates have been obtained in a pure state by dissolving and filtering, the uric acid may be precipitated by acetic acid, collected on a filter, washed, dried, and weighed. The filtered fluid contains the acetates of the bases with which uric acid was combined. We evaporate to dryness, and expose the residue to a red heat, when soda and lime remain, the former combined with carbonic acid, the latter partly as carbonate, partly as caustic lime. The residue is now dissolved in a little acid, muriatic or acetic; the lime is precipitated as oxalate by the addition of oxalate

1 Vide Prout, loc. cit., p. 106, and Med.-Chir. Trans.,' vol. x, p. 389.

of ammonia, filtered, dried, and weighed. The filtered liquid contains the soda, which may be determined either as muriate or acetate, or better, after heating, as carbonate. If for every equivalent of base an equivalent of uric acid be taken, we find by the excess of uric acid the amount of ammonia which was present and combined with it.

The calculi composed of lithates frequently contain oxalate of lime and small quantities of the phosphates in intimate mixture with the lithates. They are then called mixed calculi.

Layers of Uric Acid and Urates in alternating and mixed Calculi.

These layers are due to the same causes as the massive concretions. But there seems to be one cause of the occurrence of urates to which it is necessary to advert, before concluding the chapter on uric acid. Ammonia at the temperature of the body very quickly changes uric acid into urate of ammonia. Let us apply this to a uric acid stone. The urine in calculous disorders very frequently becomes alkaline; nay, if the disorder last long enough, alkaline decomposition of the urine in the bladder is almost always present. În evidence of this, almost one half of all calculi possesses a cortical layer of mixed phosphates, the consequences exclusively of alkaline urine. Now there can be no doubt, that if a uric acid stone become the cause of such disorders of the urinary passages as will induce alkaline fermentation in the urine, the outer layers of this stone must be transformed into urate of ammonia, and this urate of ammonia (or soda) would undoubtedly be dissolved and carried away, unless the carbonic acid present diminished its solubility, and if the urine were dilute enough to act as a solvent, i. e., were not itself too much saturated with the urates. A stone with a uric acid nucleus, a body of urates, and a cortical portion of mixed phosphates, is to my understanding the type of this process, and suggests to me the following history: In the beginning there was a renal uric acid concretion, which increased by the acid fermentation in the bladder. Then the calculus caused disorder of the bladder, or of the urine (as by excessive treatment with alkaline remedies), which ended in the establishment of alkaline fermentation in the bladder, by which the outer layers of the uric acid concretion were first transformed into urate of ammonia, and afterwards encased in a crust of mixed phosphates.