210 CONJUGATED ACIDS. 



separates from urine, appears in most cases in the whet-stone 

 form, that is to say it forms flat tablets, which resemble sections 

 made with the double knife through strongly bi-convex lenses, or 

 rhombic tablets whose obtuse angles have been rounded. As the 

 urinary pigment adheres very tenaciously to the uric acid, it is only 

 rarely that these crystals are devoid of colour ; and if we see a 

 crystal presenting an extraordinary form and of a yellow colour, 

 the probability is that it is a crystal of uric acid. On artificially 

 separating uric acid from its salts it often appears in perfect 

 rhombic tablets, and even oftener in six-sided plates (resembling 

 those of cystine) ; when uric acid crystallises very slowly it forms 

 elongated rectangular tablets or parallelepipeds, or rectangular 

 four-sided prisms, with horizontal terminal planes ; the latter are 

 often grouped together in clusters ; we also have barrel-shaped or 

 cylindrical prisms, which are composed of the more rarely occurring 

 elliptic tablets ; and finally saw-like or toothed crystals, and many 

 derivatives of these forms. If we cannot decide with certainty 

 regarding the presence of uric acid from the form of a crystal, we 

 must dissolve it in potash, place it under the microscope, and add 

 a minute drop of acetic acid ; we shall then always obtain one of 

 the more common forms. 



A quantitative determination of the uric acid* in urine is 

 best made from the residue not taken up by alcohol ; by simply 

 treating it with dilute hydrochloric acid, the earths, &c., are got rid 

 of, and nothing but uric acid and mucus remains; their separation 

 may be effected by dissolving them in a dilute solution of potash, 

 from which the uric acid may be precipitated by acetic or hydro- 

 chloric acid. The pigment adhering to the uric acid exercises no 

 appreciable influence on the quantitative determination of this 

 substance (Heintz).t 



To institute a quantitative determination of the uric acid in the 

 blood or any other albuminous -riuid is a more difficult and far 

 more precarious operation. For this purpose we take the clear 

 serum and evaporate it to dryness, without previously removing the 

 coagulated albumen by filtration ; for if we filtered, the whole process 

 would be very prolonged, as the coagulated serum would become 

 little more than a solid mass of moist coagula, whose thorough wash- 

 ing, even by the addition of much water, would be impossible (see 

 the observations in a future page " on the quantitative determination 

 of albumen.") We now extract the solid residue of the serum 



* Journ. f. pr. Ch. Bd. 25, S. 17. 

 t M iiller's Archiv. 1846. S. 383-389. 



