CHAP. IV.] THE BLOOD. METHODS OF RESEARCH. 193 



employed in any series of researches. It may be described as 

 follows : 



Defibrinated blood, varying in quantity between 10 arid 20 c.c., is 

 placed in a dialyser, so as to form a layer on the parchment-paper not 

 deeper than 4 mm. The dialyser is placed in a vessel containing 

 a volume of absolute alcohol equal to twice that of the blood. 



In a period varying between one and four hours the fluid part of 

 the blood, holding the urea in solution, has passed into the alcohol, 

 leaving a solid mass behind. This is removed from the parchment- 

 paper, mixed with a little water and again placed in the dialyser. 

 The process is repeated three or four times, so as to make sure 

 that all the urea has been extracted. The alcohol is then poured 

 into a shallow porcelain dish, and after acidifying with oxalic 

 acid, so as to convert all urea into oxalate, the fluid is evaporated to 

 dryness. In the residue, crystals of oxalate of urea may be seen 

 with the naked eye, mixed with some fat, colouring matter, and 

 crystals of common salt. The fat and colouring matter are in 

 great part removed by the aid of petroleum naphtha, which readily 

 dissolves those substances, whilst it leaves the whole of the 

 oxalate of urea undi.ssolved. This is then dissolved in water and 

 mixed with a little barium carbonate, and the mixture evaporated to 

 dryness. On boiling with alcohol this fluid extracts the urea, leaving 

 traces of proteids and common salt. On evaporating the alcoholic 

 solution, almost pure crystals of urea are left ; the amount of this 

 substance is determined by weighing, or by Bunsen's method (see 

 'Determination of Urea in Urine'), or by means of the hypobromite 

 method. 



This method is so precise that Dr Hay craft has never failed, by 

 its aid, in preparing a naked-eye demonstration of urea from ten 

 cubic centimetres of normal blood. He has found that by this 

 method the maximum error does not exceed 6 p. c. of the total weight 

 of urea. 



Determination of the amount of Uric Acid in the Blood. 



The amount of uric acid in the healthy blood of man is so 

 small as to render its determination, and even its detection, impossible. 

 In the blood of Birds, whose urinary excretion is very rich in uric 

 acid, Meissner 1 succeeded, by a process for which we refer the 

 reader to his original memoir, in determining the amount of uric 

 acid. 



The following is the process employed by Dr Garrod in the 

 detection and estimation of uric acid in the blood of gouty patients. 

 " The serum of the blood is first dried over a water- bath, then 



1 Meissner, "Der Ursprung der Harnsaure des Harns der Vogel." Zeitschr. /. rat. 

 Med., Dritte Reihe. Vol. xxxi. p. 144 et seq. 



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