We shall now briefly consider the physical and chemical characters of each calculus in par ticular, — appending in each instance an out line of the qualities of its component material, when occurring in the form of urinary sediment.
1. The uric acid calculus is generally of oval shape and somewhat flattened, ranges in weight from a grain to six or seven ounces and up wards, and varies in size within corresponding limits ; the calculus, from a section of which the subjoined cut is reduced, measures 2/ in ches in breadth and St inches in length. The external surface, commonly smooth, may he finely granular, and its colour brown of diffe rent tints and depths, unless it have received a thin coating of phosphates. On section it is generally found to be laminated, and when comparatively pure its fracture has a crystalline look ; when the contrary, the appearance is that of aggregated amorphous particles ; the general colour is that of the external surface (much impurity may, however, render it gray or otherwise alter it), but the different strata may vary very considerably in depth of hue from yellowish-brown to mahogany colour, ac cording to the amount of colouring matter pre sent in each ; its density is high in the direct ratio of its purity. Absolute purity never exists ; all uric acid calculi contain colouring, and, with rare exceptions, fatty matters, some mucus or albumen, and besides, minute quan tities of urate of ammonia, of soda, and of potash, with occasionally carbonate and phos phate of lime.
In 251 of the 763 alternating calculi, the composition of which is given by Dr. Prout, the nucleus consisted of' uric acid.
Uric acid occurs as an ingredient of urinary sediments, and although not, as Berzelius sup posed, their chief material in persons in health, (the amorphous urates vastly exceed it in abun dance,) it may in cases of gout form the entire of the deposit.
Under the microscope uric acid appears in the form of semi-transparent, thin rhomboidal scales, of slightly yellow tinge, generally, from impurity (the pure acid being brilliantly white), insoluble in cold and hot urine and in weak acids, soluble with effervescence (from equal volumes of carbonic acid and nitrogen) in con centrated nitric acid, the mixture acquiring a purple red hue (from mnrexide) at the close of evaporation, and very sparingly soluble in concentrated muriatic acid. (See fig. 78, a.) Urate of ammonia is the chief constituent of the pulverulent sediments of urine voided by healthy persons : it is insoluble in cold, soluble in hot, urine. It is commonly distinguished
microscopically as a pulverulent closely packed matter (fig. 79, a) ; in other instances it wears the form of globules of black colour * (fig. 79, b) ; when alkaline reaction is established (or sometimes, as we have seen, while the urine is still acid) these globules become stellate from the formation of minute silky needles apparently springing from their circumference ( fig. 79, c).
Sometirnes (and less rarely than has been sup posed) the rhomboidal prisms are thick, solid looking, and cuboid in appearance. We have lately seen a pelli,le(non-irridescent) of bloody urine after scarlatina formed of crystals of this kind (fig. 78, b). Sometimes, again, frag ments of, or entire, crystals unite so as to form lanceolate or stellate figures ; this form may be produced .(as shown by Rayer, fig. 78, c) by artificial precipitation.
2. The urate of ammonia. calculus (of which the existence was denied by Mr.Brande, on the plea that the ammonia evolved from certain calculi is in reality derived from associated triple phosphate, or from urea and the ammo niacal salts of the urine,) is said by Fourcroy and Prout to be rare, and more frequent in children than in adults, a statement which has been copied by various writers. The tables collected by Dr.Prout show that of 709 simple calculi 59 were composed of urate of animonia nearly pure, 159 mainly of this salt rnixed with variable proportions of the urate and oxalate of lime and phosphates. The shape of this cal culus is more irregular than that of uric acid masses, but still inclined to the ovoid ; it does not reach any large size, and has a smooth or granular surface. Internally it is laminated; of clay colour ; its fracture earthy, and its den sity considerably less than that of lithic acid. Of 163 alternating calculi grouped in Dr. Prout's tables we find that 239 had a nucleus of pure urate of ammonia. In the majority of cases, then, this salt or uric acid forms the ground work of calculous accumulation ; for they con stitute either the entire mass, or the nucleus, of 938 among 1473 calculi of ascertained com position in the following proportions, exclusive of many others in which they were mixed irregularly with other saline matters.