148 Prof. J. Emerson-Keynolds on the 



tightly-fitting india-rubber tube with /. While connecting 

 the generator and receiver a little water is necessarily expelled 

 from the tube beyond H ; but this water is thrown away, and 

 the dry two-ounce measure, I, then placed under the spout. 



Up to this point the hypobromite has not been allowed to 

 come into contact with the urine ; but now, on removing the 

 forceps D, the hypobromite flows out from c and rapidly mixes 

 with the urine, the urea of which yields up its nitrogen gas with 

 effervescence. As the gas evolved has no exit save through E, 

 it displaces from F its own volume of water, which falls into 

 the vessel I, and can then be measured when no more water 

 is expelled. The effervescence ceases after five or ten minutes, 

 according to the temperature. 



It is essential to good measurement that the pressure within 

 the apparatus should be the same at the end as at the beginning 

 of the experiment ; in order to secure this, the simple plan is 

 adopted of placing a wedge under the board S at the end in- 

 dicated, which is thus so tilted that the eye placed at a point 

 a little below D, and looking immediately above the surface of 

 the water in F, can just see the bend of the tube under H. 

 When the pressure within and without has been thus equalized, 

 the amount of water expelled is the measure of the nitrogen 

 evolved in A ; for we may in a test of this kind neglect the 

 extremely minute proportion of the nitrogen which has been 

 dissolved by the water. 



When it is desired to correct for temperature and pressure 

 by means of the usual formula, it is now necessary to disconnect 

 E and /, and to pass the bulb of a small thermometer through 

 f into the gas over the water in F ; after a minute or so the 

 temperature may be read off and recorded, and the barometric 

 reading made at the same time. In ordinary clinical ex- 

 periments, however, the correction for temperature may be 

 neglected when a thermometer in the room stands near to 52° 

 F. The neighbourhood of a fire or stove must be avoided in 

 making the estimations of urea. 



In measuring the water expelled we may either read off the 

 volume in drachms or sixths of a drachm ; but since ordinary 

 cylindrical two-ounce measures are rarely graduated to less 

 than half-drachms, the best plan is to pour the excess over a 

 definite number of drachms into a tall two-drachm measure, 

 bearing in mind that every ten-minim division represents the 

 sixth of a drachm. 



I find as the results of a large number of direct experiments 

 with a standard solution of pure urea, some of which will be 

 given further on, that one grain of urea produces sufficient gas 

 at a temperature of 52 u F. and a barometric pressure of 30*06 



