184 CIRCULATING FLUIDS: 



is very minute, the crystallized nitrate of urea may not be per- 

 ceptible for several hours, and even then probably not without 

 the aid of the microscope. In order to avoid any errors that 

 might arise through the crystalline form of other salts^ I first 

 made myself thoroughly acquainted with the appearance pre- 

 sented under the microscope by alcohol-extract of urine (con- 

 taining urea) when treated with nitric acid; then with the 

 appearance presented by alcohol-extract of blood to which a 

 little urine had been added, on the addition of nitric acid ; then 

 with that of alcohol -extract of blood devoid of urea ; and, lastly, 

 wdth blood which contains urea in the natural proportions. In 

 this manner I found that the salt which most commonly occurs 

 in the alcohol-extract of blood, the lactate of soda, may be 

 readily distinguished under the microscope from the nitrate of 

 urea, and that very minute quantities of urea may be detected 

 with certainty. 



Small quantities of urea may be recognized, by the pecuhar 

 and characteristic form of the nitrate, in fluids containing those 

 extractive matters and salts of mine or of blood that are soluble 

 in anhydrous alcohol. The forms which are principally and most 

 frequently observed are depicted in fig. 3 : a represents the 

 characteristic crystalline form of nitrate of urea ; b, c, d, e, 

 groups that are formed in a somewhat dilute solution of urea ; 

 f, groups that are formed in a very dilute solution, chiefly at 

 the edge of the fluid. Fig. 4 exhibits the crystalline form which 

 is produced by the addition of nitric acid to the alcohol-extract 

 of blood, containing no urea. These ciystals are not perceptible 

 until the fluid is evaporated nearly to dryness. Fig. 5 shoAvs 

 the form of the nitrate of mrea in blood containing a conside- 

 rable quantity of urea. I have several times observed these 

 appearances in Bright^s disease. With a little practice the 

 commencement of the crystallization of the nitrate may be 

 perceived ; it begins by exhibiting an appearance of numerous 

 fine parallel lines or streaks. 



Oxahc acid may likewise be used in microscopic researches 

 regarding the presence of urea in the blood. I have always, 

 however, preferred the use of nitric acid, because, in the first 

 place, it is not itself capable of crystallization as oxalic acid is ; 

 and, secondly, because the nitrates of potash and soda are much 

 more soluble than the corresponding oxalates. Fig. 6 shows 



