CHAP. IV.) THE BLOOD. METHODS OF RESEARCH. 191 



it is perfectly obvious that it must furnish results which must be 

 decidedly too low. 



Grehant's A carefully weighed quantity of defibrinated blood 



is mixed with twice its volume of strong alcohol 

 and set aside from one day to the next. The alcohol is then 

 filtered off, the insoluble coagulum is squeezed in a press and 

 washed with alcohol, the alcoholic liquids are concentrated in a water 

 bath. The residue is dissolved in water and is then introduced 

 into the vacuum of a mercurial pump, where it is subsequently mixed 

 with Millon's reagent (solution of mercurous nitrate in nitric acid). 

 The urea undergoes decomposition, yielding equal volumes of carbonic 

 acid and nitrogen, mixed with much nitric oxide. The gases are col- 

 lected and analyzed, and from the results the amount of urea is calcu- 

 lated. The author has found this method very troublesome of 

 execution and by no means as accurate as has been maintained 

 by its advocates. The decomposition takes place during a con- 

 siderable period of time, and in consequence of the disengage- 

 ment of nitric oxide continuing almost indefinitely the operator 

 is never sure when the process should be considered at an end. 

 Moreover the volumes of carbonic acid and nitrogen evolved are not 

 strictly equal ; in the author's experiments the volume of carbonic 

 acid was always somewhat below that of the nitrogen. 



By decom- This method of estimating urea, which will be 



position with described at length under the head of Urine, has been 

 sodium hypo- employed by the author in the determination of the 

 bromite. amount of urea in the blood. 



A weighed quantity of blood, usually about 50 grammes, is 

 mixed with twice its volume of absolute alcohol and set aside 

 in a stoppered bottle for about 24 hours. At the end of this time 

 the precipitate is collected on a linen filter, washed with absolute 

 alcohol and subjected to firm pressure in a screw press. The 

 alcoholic liquid is evaporated to dryness in the water-bath, and the 

 residue is taken up in absolute alcohol, filtered, evaporated to 

 dryness, dissolved in a little water, and the watery solution filtered. 

 The filtrate is now placed in the decomposing bottle of Dupre"s 

 apparatus (Fig. 38) and subjected to the action of sodium hypobromite. 

 The nitrogen evolved, instead of being collected in a wide tube 

 such as is shewn in the engraving, is, in the case of urea determina- 

 tions in blood, collected in a much narrower tube divided into tenths 

 of a cubic centimetre. 



From the volume of nitrogen obtained, the urea can readily be 

 calculated 1 . 



It may be objected to this process that in reality it only furnishes 

 us with the amount of nitrogen given off by the extractive matters 

 of blood when treated with alkaline hypobromites, and that as other 



1 The reader is referred for a full description of tlie process of estimating urea by 

 solutions of hypobromite to the section on Urine. 



