Fig. 14. Uric acid crystals from Urine. These crystals were given to me by my friend and pupil Mr. Atchley.

Fig. 15. Some of the same crystals as represented in fig. 14, more highly magnified, and seen in a different position.

Fig. 16. Crystals of uric acid from Urine. Preserved in glycerine.

Fig. 17. Small uric acid crystals connected together so as to form plates.

18. Curious forms of uric acid produced by rapid crystallization after the addition of nitric or hydrochloric acid to Urine.

uric acid which are quite colourless.

This substance

appears to have a very strong affinity for colouring matter, and colour may be regarded as one of the most constant characters of uric acid deposits.

Form of the crystals.-As so many different forms have been represented, it is hardly necessary to observe, that the form of the crystal is liable to great variation, although the drawings might have been multiplied to a much greater extent. I shall have occasion to give several other groups of uric acid crystals at a future time. The causes which determine these variations in form are not understood. The rapidity of crystallization may be shown to exert an important influence on the form of the crystal, and it is not improbable that some other constituents in the Urine may determine its character to some extent. The true form of uric acid is rhomboidal. It occasionally crystallizes in six-sided crystals, somewhat resembling cystine. (Plate X.) The latter is easily distinguished by its solubility in ammonia, and the formation of six-sided crystals as the ammonia is allowed to evaporate.

Chemical characters.—When any doubt exists as to the probable nature of a deposit suspected to consist of uric acid, it is desirable to apply chemical tests. Uric acid is insoluble in hot and cold water; it is very soluble in potash, from which solution crystals of uric acid are precipitated by the addition of excess of acid (nitric, hydrochloric, or acetic). Uric acid after being dissolved in nitric acid, and the solution carefully evaporated to dryness, upon the addition of a drop of ammonia, gives rise to the development of the bright purple colour characteristic of murexide. Tables, § 38.

B

PLATE VII.

URIC OR LITHIC ACID, C, H.NO

10 4 4 6'

Fig. 19. Uric acid in Glycerine.

Fig. 20. Halbert-shaped crystals of uric acid from Urine.

Fig. 21. Curious lamellar crystals of uric acid perfectly colourless, given me by my friend Mr. J. W. Lawrence.

Fig. 22. Acicular crystals of uric acid from Urine, sent me by Mr. Lawrence.

Figs. 23 and 24. Crystals from same specimens as figs. 15 and 14.