2 Dr. Marcet on a Colloid Acid of Urine. [1864. 



ration, for a period of about forty hours ; after which it is again concentrated, 

 filtered, and then a solution of basic acetate of lead is added to it, which 

 produces a precipitate : care should be taken not to use more of this so- 

 lution than is necessary to obtain a complete precipitation in the fluid. 

 The precipitate appears white, although containing a little colouring-mat- 

 ter ; it is to be collected on a filter and washed with distilled water until the 

 washings contain but a trace of lead. The insoluble substance on the 

 filter is a lead-compound of the organic colloid acid ; it still contains a 

 small quantity of colouring-matter, some hydrated oxide of lead, more or 

 less chloride of lead, and perhaps traces of sulphate of lead. In order to 

 effect the removal of these impurities, the insoluble compound is first de- 

 composed with sulphuretted hydrogen or sulphuric acid ; if sulphuretted 

 hydrogen is employed, the excess of this gas is afterwards expelled by boil- 

 ing, or better by blowing it out with a current of air. 



The acid fluid being now heated with animal charcoal and filtered, loses 

 the whole, or nearly the whole of its colouring-matter, and apparently much 

 of its urinous smell. Any hydrochloric acid present can be easily separated 

 from the free organic acid by treating the colourless fluid with carbonate of 

 silver and filtering ; the dissolved silver is afterwards eliminated by means of 

 sulphuretted hydrogen, and the excess of sulphuretted hydrogen again re- 

 moved by boiling the fluid, or passing through it a current of air. The 

 careful addition of baryta- water will precipitate any sulphuric acid present. 

 Finally the acid is precipitated afresh with basic acetate of lead, from which 

 it is separated by decomposing the thoroughly washed precipitate with 

 sulphuretted hydrogen. 



The object of the various operations thus described will be readily under- 

 stood. By evaporating urine with animal charcoal, it is first partly dis- 

 coloured ; the precipitation with baryta-water throws down the phosphoric 

 and sulphuric acids and the lime of the secretion, which would interfere 

 with further operations, and imparts to the fluid a strongly alkaline reac- 

 tion, this last condition being apparently necessary to avoid a loss of the 

 colloid acid during the dialysis. In order to prevent decomposition, which 

 would occur by concentrating with heat a strongly alkaline urine, the fluid 

 is dialyzed at once for about thirty-six hours, which operation removes 

 from it a considerable proportion of its crystalloid constituents ; among 

 these are chlorides, which it is advisable to get rid of, as much as possible, 

 before concentration and precipitation with basic acetate of lead, because 

 by so doing a great saving in the carbonate of silver, necessary to precipi- 

 tate the remaining hydrochloric acid, will be effected. 



If basic acetate of lead is used in excess, the precipitate begins to redis- 

 solve ; the precipitant should therefore be added very gradually, testing 

 the fluid now and then to ascertain whether the precipitation be complete. 

 I have also observed the precipitate to be soluble in a solution of caustic 

 potash. The lead-compound collected on a filter is to be thoroughly 

 washed, to remove any excess of basic acetate of lead, and a solution of the 

 lead-compound of the colloid acid, to which I shall refer hereafter. 



