116 THE SECRETIONS: 



been proved to give results on which no dependance can be 

 placed. 1 ] 



1. Urea. This constituent seldom occurs so abundantly 

 in the urine, as to be immediately detectible by the addition 

 of any reagent. A portion of urine is usually evaporated in 

 the water-bath to the consistence of a syrup, anhydrous al- 

 cohol is added, and the alcoholic solution is filtered, and eva- 

 porated on the water-bath nearly to dryness; some drops of water, 

 and subsequently of nitric acid are added, upon which crystals 

 of stellar and foliated shapes very speedily develop themselves. 



Upon leaving the alcoholic extract to spontaneous evapora- 

 tion, long acicular crystals of urea will be formed ; on examining 

 some of them under the microscope, they will be found to pre- 

 sent the appearance of four-sided prisms, as shown in figure 20, 

 If, (which however is not often the case,) the urea should be 

 present in very small quantity, and no crystals are formed for 

 some time after the addition of nitric acid, it only requires a 

 microscopic examination to ascertain whether the crystals are 

 those of nitrate of urea : if they are, they will occur in the 

 forms indicated in fig. 21. If, instead of nitric, oxalic acid has 

 been used for the detection of the urea, we obtain the forms 

 represented in fig. 22. 



2. Uric acid. It is but seldom that the uric acid exists in 

 such large amount, as to be precipitated in the form of a fine 

 crystalline red sediment when the urine cools. When, how- 

 ever, this is the case, the crystals, under the microscope, exhibit 

 the rhomboid form shown in fig. 23. Another method of 

 proving that the sediment consists of uric acid, is to place some 

 of it in a porcelain capsule moistened with nitric acid, and 

 to apply heat till the acid evaporates. A purple-red colour then 

 appears, which is characteristic of uric acid : this colour becomes 

 more intense on the approximation of a rod dipped in ammonia. 

 If no crystalline sediment is deposited as the urine cools, two 

 or three drachms of hydrochloric acid must be added to six or 

 eight ounces of urine, and the mixture must be allowed to 



1 [On the specific gravity of the urine in health and disease, especially in diabetes 

 and granular degeneration of the kidneys. By George E. Day. Lancet, June 15' 



1844.] 



