8 CHEMICAL PHYSIOLOGY. 



(e.) Add strong nitric acid = a precipitate which dis- 

 solves on heating, producing an intense yellow colour. 



(/) It also gives the biuret test, and that with Millon's 

 reagent (Lesson I., 1). 



8. IV. Fibrin is insoluble in water and in weak solutions of 

 common salt. When prepared from blood and washed, it is a 

 white, fibrous, soft, and very elastic substance, which exhibits 

 fibrillation under a high magnifying power (see " Blood "). 



(a.) Place some well-washed fibrin in a test-tube, add 

 some O'l per cent, hydrochloric acid, and observe that the 

 fibrin swells up and becomes clear in the cold, but does 

 not dissolve. 



(b.) Place a test-tube as in (a.) on a water-bath at 60C. 

 for several hours ; filter, and test the filtrate for acid-albumin 

 by neutralisation with very dilute potash. 



(c.) For the effect of a dilute acid and pepsin (see "Diges- 

 tion "). 



(d.~) It decomposes hydric peroxide, and turns freshly- 

 prepared tincture of guaiacum blue (see " Blood "). 



(e.) Place a very dilute solution of cupric sulphate in a test- 

 tube, add a flake of fibrin. The latter becomes greenish, 

 while the fluid is decolourised. On adding caustic potash, 

 the flake becomes violet. This is merely the biuret reaction 

 common to proteids generally. 



9. V. Coagulated Proteids are insoluble in water, dilute acids, 

 and alkalies, and are dissolved when digested at 35 to 40C. in 

 gastric juice (acid medium), or pancreatic juice (alkaline medium), 

 forming peptones. They give Millon's reaction. 



Preparation. Boil white of egg hard, and chop up the white. 



(a.) Test its insolubility in water, dilute acids, and 

 alkalies. 



(b.) It is partially soluble in acids and alkalies, when 

 boiled for some time. 



