G4 ORGANIC CONSTITUENTS. 



obtaining uric oxide from uric acid by the action of deoxidising 

 agents, but be failed in bis attempt. 



16. Cystin. 



Cystin, cystic oxide. Cystin is an occasional constituent of 

 urinary calculi, and is sometimes found as a crystalline deposit 

 in the urine. It may be obtained by dissolving a portion of 

 one of these calculi in caustic potash, and adding acetic acid to 

 the boiling solution. As the fluid slowly cools, the cystin sepa- 

 rates in six-sided, colourless, transparent scales. It may also 

 be obtained in crystals from a solution in caustic ammonia, if 

 left to evaporate slowly. The scales are then thicker, and may 

 be considered as regular six-sided prisms. 



Cystin has an extraordinary composition. It contains 

 25-52 of sulphur. Its formula' is C, H^ N O^ S„. 



It has neither an acid nor alkaline reaction ; when heated, 

 it does not melt ; takes fire with a blueish flame, and gives ofl" 

 a very characteristic odour: is very slio-htlv soluble in water, 

 and quite insoluble in alcohol ; dissolves in dilute sulphuric, 

 nitric, hydrochloric, phosphoric, and oxalic acids, the saturated 

 solutions yielding, on gentle CA'aporation, salt-like compounds 

 of cystin and the acid ; these compounds separate in diverging 

 crystalline needles, which have an acid taste, and are not very 

 durable. Cystin dissolves readily in the fixed alkalies, and forms, 

 on evaporation, granular crystals. It dissolves in caustic 

 ammonia, but does not combine with it. Carbonate of ammonia, 

 is the best reagent for throwing it down from its acid solutions, 

 as it does not dissolve cystin. It may be removed from an 

 alkaline solution by acetic, citric, or tartaric acid, with none of 

 which it enters into combination : acetic acid is generally used. 



Diagnosis of cystin. Cystin may be recognized by the 

 peculiar crystalline form^ {six-sided plates) in which it separates 

 from its solutions ; by its insolubility in water and alcoliol ; by 

 its beha\ioiu' towards acids; and by its peculiar odoiu' on 

 burning. Its crystalline form and its behaviour towards acids 

 distinguish it clearly from uric acid : these tests, as w ell as its 

 solubiUty in hydrochloric and oxalic acids distinguish it from 

 uric oxide. 



' See Appendix I, Note 22. 



* I once observed an amorphous deposit of urate of ammonia yield, on the addition 

 of acetic acid, perfectly regular hexagons. This form is also depicted hy Rigby, in 

 his work on Dvsmenorrhoea. 



