ADDITIONS. 397 



manganate for 15 to 20 sees., washed with water, placed for 

 a few seconds to a few minutes in a mixture of 



1 gram pure oxalic acid 



1 gram potassium sulphite 



200 c.c. water, 



and again washed well with water. The parts of the tissue 

 other than medullated fibres are thus more completely de- 

 colourized than by Weigert's method. The sections may 

 finally be stained with alum-carmine. 



Solutions for Liebig's method of estimating urea. 



(a) Standard mercuric nitrate solution. Take 71*48 

 grams of pure mercury, add 5 vols. nitric acid Sp. 

 Gr. 1'425, and warm on water-bath until the mercury 

 is completely dissolved; evaporate the solution to 

 a syrupy consistence, until the addition of a few 

 drops of nitric no longer causes red nitrous fumes 

 to come off, evaporate further until the fluid acquires 

 a faint yellowish tinge. Stir and add about 1 vols. 

 of water ; if a precipitate is formed let stand, pour 

 the clear fluid into a litre flask, to the residue add 

 the minimal quantity of strong nitric acid necessary 

 to dissolve the precipitate, add this to the fluid in 

 the flask, wash the vessel out with water, add this 

 to the previous fluid and fill up to exactly a litre. 



One c. c. of this solution precipitates 10 mgs. of urea 

 and leaves just enough mercuric nitrate over to be 

 detected by sodium carbonate. 



The method given above can only be used when the 

 mercury is pure, if there is any doubt about this, the 

 mercuric nitrate solution diluted to somewhat less 

 than a litre should be titrated with a solution of pure 

 urea and then further diluted to the proper amount. 



(6) Baryta mixture. Add two volumes of barium 

 nitrate, saturated in the cold, to one volume of 

 barium hydrate, also saturated in the cold. 



