5 o8 EXCRETION 



of cold, as a rabbit with a closely-clipped or shaven skin does; 

 suppression of the secretory function of the skin has nothing to do 

 with death in the first case any more than in the second (p. 293). 



PRACTICAL EXERCISES ON CHAPTER IX. 



Urine. 



For most of the experiments human urine is employed in the 

 quantitative work the mixed urine of the twenty-four hours. Urine may 

 also be obtained from animals. In rabbits pressure on the abdomen 

 will usually empty the bladder. Dogs may be taught to micturate at 

 a set time or place, or kept in a cage arranged for the collection of urine. 

 Or a catheter may be used (p. 690). 



i. Specific Gravity. Pour the urine into a glass cylinder, and remove 

 froth, if necessary, with filter-paper. Place a urinometer (Fig. 193) 

 in the urine, and see that it does not come in contact 

 with the side of the vessel. Read off on the graduated 

 9 stem the division which corresponds with the bottom 



, of the meniscus. This gives the specific gravity. 



2. Reaction. (a) Test with litmus-paper. Generally 



the litmus is reddened, but occasionally in health the 

 urine first passed in the morning is alkaline. Some- 

 times urine has an amphicroic reaction i.e., affects 

 both red and blue litmus-paper. This is the case when 

 there is such a relation between the bases and acids 

 that both acid and ' neutral ' (dibasic) phosphates are 

 present in certain proportions. The acid phosphate 

 reddens blue litmus, and the ' neutral ' phosphate 

 turns red litmus blue. 



(b) Titratable Acidity. To 25 c.c. of urine add 15 

 to 20 grammes of powdered potassium oxalate, and 

 one or two drops of a i per cent, solution of phenol- 

 phthalein. Shake the mixture rapidly for a minute 

 Fig. 193. Urin- or two, and then titrate with decinormal sodium hy- 

 ometer. droxide at once (while still cold from the solution of the 



oxalate) till a faint pink colour remains permanent on 

 shaking. The potassium oxalate is added to counteract the tendency 

 of the calcium present in urine to form basic phosphates, which would 

 be precipitated, and the acidity of the urine thus increased (Folin). 



3. Chlorides (a) Qualitative Test. Add a drop of nitric acid and a 

 drop or two of silver nitrate solution. The nitric acid is added to 

 prevent precipitation of silver phosphate. A white precipitate soluble 

 in ammonia shows the presence of chlorides. The precipitate appears 

 to be incompletely soluble in ammonia, since the ammonia brings down 

 a small precipitate of earthy phosphates. 



(b) Quantitative Estimation. The quantitative estimation of the 

 chlorine in urine without previous evaporation and incineration is best 

 made by one of the modifications of Volhard's method. It depends upon 

 the complete precipitation of the chlorine combined with the alkaline 

 metals, and also of sulphocyanic acid, by silver from a solution con- 

 taining nitric acid in excess ; and avoids the error introduced into simpler 

 methods, like Mohr's, by the union of some of the silver with other 

 substances than chlorine. A given quantity of a standard solution of 



