480 



Sir A. Garrod. On the presence of [May 18, 



much richer in urea than the blood of the renal vein, in the propor- 

 tion, according to my experiments, of about three to one ; of about 

 two to one according to the experiments of M. Picard. 



As far as I am aware, urea has never hitherto been detected in the 

 blood of birds. Possibly it has never been looked for, as it would 

 scarcely have been imagined that urea could be contained in the 

 blood of an animal excreting only urate of ammonium. I have, how- 

 ever, recently found that the blood of birds contains urea in quantities 

 not less than those found in the blood of mammals. The views I 

 entertained on the formation of uric acid in the animal body led me 

 to investigate this subject, as they necessitated the presence of urea 

 in the blood of the uric- acid- excreting animals, as well as in the blood 

 of those which eliminate urea only. 



As the question of the existence of urea in the blood of dilferent 

 animals is of so much importance in connexion with the subject of 

 this communication, I have taken special pains to determine not only 

 that it is present, but also in what relative amounts, and the methods 

 I have made use of have been as follows : — 



In all my experiments I made use of dried blood, which form was 

 by far the most convenient for my purpose. It was thus obtained : — 

 Wide-mouthed bottles w T ere half filled with absolute alcohol, and the 

 blood as it flowed from the animal fell directly into the bottle, after 

 which the spirit and the blood were intimately mixed by shaking. 

 This mixture was then dried on a water-bath, reduced to a very fine 

 powder, and kept in air-tight bottles. In making experiments, I 

 assumed that one part of dried blood equalled five parts of liquid blood. 



To determine the amount of urea I adopt the following method. 

 The dried blood is treated with about six times its weight of distilled 

 water, very slightly acidulated with oxalic acid, and this is heated in 

 a water-bath to about 75° 0. for at least six hours. The next day the 

 residual blood is again treated in the same way with fresh acidulated 

 water and again heated. The fluids poured off are then filtered and 

 evaporated to dryness. Any urea abstracted from the blood is now 

 in the form of the sparingly soluble oxalate of urea. If any oily matter 

 is found in the residue of the evaporated fluids, this can easily be re- 

 moved by washing with potassium naphtha, in which the salt of urea 

 is totally insoluble. The residue is next treated with distilled water 

 and a small quantity of the carbonate of barium (sufficient to cause a 

 slight alkaline reaction) is stirred up with it. By this means the 

 urea is set free and the oxalic acid fully neutralised. 



After again evaporating to dryness, the residue is boiled with 

 absolute alcohol, by which means the urea (together with small traces 

 of other substances) is dissolved ; and when this has been filtered into 

 a large watch glass and evaporated spontaneously the urea is obtained 

 in a solid form. 



