150 Frof. J. Emerson-Reynolds on the 



The weight of urea taken for each experiment is capable of 

 affording a volume of moist nitrogen gas at the temperature 

 and pressure above stated, which would expel 15 J drachms of 

 water. The maximum volume obtained from that weight of 

 pure dry urea was 15^ drachms. Thus there is a minimum 

 loss of 3'3 per cent. The maximum observed loss in the fore- 

 going experiments amounted to 14*9 per cent., and occurred 

 in the experiment in which the above-named weight of urea 

 was dissolved in ten drachms of water. The loss within the 

 above limits is tolerably regular, as the volume of nitrogen is 

 diminished by ^ of a drachm (nearly) for each drachm of water 

 added to the urea in the decomposition-vessel A. 



The loss of nitrogen referred to is, doubtless, due in part to 

 solution of the gas ; but it is chiefly attributable to the regular 

 diminution of the strength of the oxidizing agent used, 

 the hypobromite solution, and to a corresponding increase in 

 the extent of secondary changes which are known to occur in 

 the diluted liquids, and which involve a loss of gaseous ni- 

 trogen. Much of the error arising from the latter cause is 

 avoided by adopting the plan of employing a constant volume 

 of liquid ; hence the recommendations already made that two 

 drachms of urine should be measured into the bottle A, and 

 the measure rinsed out with not more than one drachm of 

 water. The total bulk of liquid in A ought then to measure 

 as nearly as possible three drachms. Even when the sample 

 to be tested is measured with a pipette, it is well to add one 

 drachm of water from an ordinary measure in order to bring 

 up the total volume of liquid to the amount recommended. 



When the simple precautions are taken which I have already 

 mentioned, the little apparatus described in this paper will en- 

 able a considerable number of estimations of urea to be made 

 with rapidity, and with sufficient accuracy for ordinary clinical 

 purposes. When very precise determinations are required, 

 Liebig's process must be resorted to, as all the methods in 

 which hypobromites or hypochlorites are employed are liable 

 to the errors pointed out above ; the accuracy of the results 

 is also affected by the action of the reagent used on uric and 

 hippuric acids, creatinine, and other nitrogenized compounds. 

 On the other hand, when we desire to ascertain the total 

 amount of nitrogen excreted by the kidneys, it is necessary to 

 resort to the precise method of estimation which I communi- 

 cated to the Surgical Society of Ireland*. 



* Vide Medical Press and Circular, May 13th, 1874, p. 402. 



