BLOOD. 183 



solution submitted to spontaneous evaporation. The microscope 

 then revealed the presence of nitrate of urea^ which was recog- 

 nized by its peculiar crystalline form. 



[Marchand got only slight microscopic indications of urea 

 from twenty pounds of the serum of the blood of a healthy cow ; 

 and as the urine of that animal contains a larger amount of 

 urea (4[j according to Sprengel) than that of man^ the blood 

 must likewise contain a larger proportion of this ingredient. 

 He calculates (assuming that there are twenty pounds of blood 

 in a man^s body, and that one ounce and a half of urea is eli- 

 minated in twenty-four hours) that the blood contains only the 

 15,360th part of its weight of urea, a quantity that could hardly 

 be determined analytically, if it were increased thirty-fold.'] 



After the extirpation of the kidneys, and in Bright^s disease, 

 it has been found in so large a proportion that its detection is 

 accomplished with comparative ease. My method, in looking 

 for ui'ea, is to treat a certain quantity of the blood with alcohol 

 for the purpose of throwing down the protein-compounds ; then 

 to filter ; and, subsequently, to wash the residue upon the filter 

 with alcohol. The alcoholic solution (including the washings 

 of the filter) must be evaporated to a small residue, and treated 

 with anhydrous alcohol. The solution is decanted from the 

 spirit-extract, which remains undissolved, is evaporated, and 

 again treated Avith anhydrous alcohol. This process must, if 

 necessary, be repeated until the residue is freely soluble in this 

 menstruum. 



The alcohol must then be evaporated, and the residue dis- 

 solved in water, which usually becomes slightly turbid in con- 

 sequence of the separation of traces of fat. This fat is not 

 easily separated by filtration ; if, however, this process is deter- 

 mined upon, a considerable quantity of water is added ; it is 

 heated, and allowed to stand for some time. The watery solu- 

 tion will then pass through the filter tolerably clear, but slowly. 

 It must be evaporated to a small residue, thoroughly cooled, 

 and nitric acid then added. If the quantity of m-ea is not too 

 minute, there are formed almost instantaneously an immense 

 number of glittering crystalline scales. If the quantity of urea 



' [That there is a peculiar difficulty iu the precise determination of this constituent 

 is shown hy an experiment iu which Marchand mixed one grain of urea with 200 of 

 serum. He could only recover -2 of a grain.] 



