550 THE SECRETIONS. 



lution, and silver nitrate solution, contained 20 cubic centi- 

 metres of urine. On, therefore, taking \ 9 or 19.5 cubic centi- 

 metres of the nitrate, after the precipitation of the chloride of 

 silver, we obtained a quantity of fluid which contained all the 

 urea present in 10 cubic centimetres of the original urine. 



It may be well to state that when, as in many cases of acute 

 disease, the amount of chlorine present is very small, nearly 

 accurate results are obtained, if no correction for chlorine be 

 introduced. 



Other cnn-rctions must be introduced into Liebig's method 

 under peculiar circumstances: these will be stated dogmati- 

 cally, the student being referred to larger books for their ex- 

 planation. 



1st. When, in determining the amount of urea in 15 cubic 

 centimetres of mixture of urine and baryta solution, the num- 

 ber of cubic centimetres of mercury solution added exceeds 30, 

 we inii^t repeat the operation, adding to 15 cubic centimetres 

 of the fluid a quantity of distilled water equal to the difference 

 between :><) and the number required in the first operation. 



2d. When the amount of solution of nitrate of mercury 

 added to 15 cubic centimetres of the filtrate from the mixture 

 of urine and baryta mixture, is less than 30 cubic centimetres, 

 0.1 cubic centimetre must be subtracted from the amount of 

 mercury solution required, for every 5 cubic centimetres less 

 than 30 cubic centimetres. 



This correction is of little importance. 



II. Dni'if* im-(hinl for tin' (^'termination of Urea. 



This excellent method is based upon the fact already men- 

 tioned, that when a solution of urea (CH 4 N a O), such as urine, 

 is treated with a solution of hypochlorite, it splits up into car- 

 bonic acid, water, and nitrogen gas. If the mixture be effected 

 in a long graduated tube, and this be inverted and placed over 

 mercury, the whole of the N accumulates on the surface of the 

 fluid, the carbonic acid being absorbed by the solution of hy- 

 pochlorite use.l. From the volume of N evolved the quantity 

 of urea present may be calculated. (For details of this method 

 the reader is referred to a Treatise on the Pathology of the 

 Urine, by Dr. Thndichum, London: Churchill, 1858.) 



Daw's process is, like Liebig's, not absolutely correct. Uric 

 aci.l. and other nitrogenous substances present in urine, are de- 

 composed by hypochlorites ; as their quantity is, however, com- 

 paratively very .small, the error introduced is not large. The 

 writer can vouch, from personal observations, of the great accu- 

 \ racy of this method when applied to solutions of pure urea, and 

 believes that, if carried out with the apparatus devised by Dr. 



ufner for the determination of urea by solutions of alkaline 

 / hypobromites, it would prove the most useful and reliable 

 method for the determination of urea. 



