126 



PRACTICAL PHYSIOLOGY. 



[XIX. 



wash every trace of urine out of B. Place C in position, fill B with a freshly- 

 prepared solution of hypobromite, and place a concentrated solution of com- 

 mon salt in C to the depth of i cm. Fill D also with the salt solution, 

 avoiding the presence of air-bubbles. Insert D over B. Open the tap when 

 the hypobromite mixes with the urine and the gases are evolved. The quan- 

 tity of urea is calculated from the volume of N evolved. 



14. Gerrard's Apparatus (fig. 66). 



Method of Using. -Pour into the tube 5 cc. of the urine to be examined, and 

 in the bottle (a) 25 cc. or 6 fluid drachms of sodium hypobromite solution. 

 Place the tube carefully inside the bottle, as shown in 'the illustration, avoid- 

 ing spilling any of the contents. Fill the glass tubes (b, c] with water, so that 



the level reaches the zero-line, tak- 

 ing care that when this is r'one the 

 tube (c\ contains only a little water 

 by being placed high it having to 

 receive what is displaced from (b) by 

 the nitrogen evolved. Now connect 

 the india-rubber tubing to the 

 bottle, and noting lastly that the 

 water is exactly at zero, upset the 

 contents of the tube into the hypo- 

 bromite solution. Nitrogen is 

 evolved, and depresses the water in 

 (b). When this ceases, lower (c) 

 until the level of the water in both 

 tubes is equal. To be exact, dip 

 (a) into cold w r ater to cool the gas 

 before taking a reading, and note 

 the result, which shows percentage 

 of urea. 



The solution of hypobromite of 

 soda is . made by dissolving 100 

 grams of caustic soda in 250 cc. of 

 water, then adding 22 cc. of 

 bromine. 



To avoid the danger of the bro- 

 mine vapour, the bromine is sold in 

 hermetically sealed glass tubes, con- 

 taining 2.2 cc. ; one of these placed 

 FIG. 66. Gerrard's Urea Apparatus, as made i n the large bottle with 25 cc. of the 

 byGibb S) Cu X son,&Co.,Wednesbury. goda solu t ion giveSj when broken 



with a sharp shake, the exact 



quantity of hypobromite for one estimation of urea, and all bad odour is 

 avoided. 



15. Synthetic Preparation of Urea. Heat coarsely-powdered ferro- 

 cyanide ot potassium (FeOy.^KCy + 3H_,0, about 250 grams) over a fire in a 

 large porcelain vessel. Stir constantly, and heat until the whole assumes a 

 white colour, and the larger pieces when broken up show no trace of yellow. 

 If it be over-heated the powder becomes brown. The white mass is finely 

 powdered and mixed with half its volume of dry, finely-powdered, black oxide 

 of manganese. The whole is heated in a black metal pot in a draught 

 chamber until it begins to scintillate, and the mass becomes doughy. The 

 mass is heated until a small portion of it, when dissolved in water and after 

 acidulation with hydrochloric acid, is no longer rendered blue by ferric 



