ELEMENTAEY PHYSIOLOGICAL CHEMISTRY 169 



EXPERIMENT XVIII. Add some iodine solution; a port wine colour, 

 like that obtained with glycogen, results, which disappears on heating 

 and returns on cooling. It is only one form of dextrine erythro- 

 dextrine which gives the reaction, the other variety achroo-dextrine 

 does not. 



EXPERIMENT XIX. Try Trommer's test: no reduction is obtained, 

 but Cu(OH) 2 is held in solution. 



EXPERIMENT XX. Hydrolyse, and a reduction will be obtained. 



CHAPTER III. 

 PROTEIDS. 



OF all the chemical substances occurring in the animal organism, 

 proteids are the most important. 



With the exception of the urine, the sweat, and the tears, there is 

 no secretion or excretion which does not contain them, and they form 

 the chief constituent of the vital tissues. They are also indispensable 

 as foods, their absence from the diet being sooner or later followed by 

 death, for it is impossible to replace them by any other food-stuff. 



Chemical Nature. As to the chemical constitution of the proteid 

 molecule we know very little. By elementary analysis it is found to 

 contain the elements carbon, hydrogen, oxygen, nitrogen, and, nearly 

 always, sulphur. The percentage amount of each of these in the 

 various forms of proteid is so much alike that a determination of the 

 elementary composition helps us very little in distinguishing one 

 proteid from another. The following is the average percentage 

 composition for all proteids : 



C - - 50-6 - 54-5. 



H 6-5 7-3. 



N 15-0 17-6. 



8 0-3 2-2. 



21-50 - 23-50. 



The nitrogen and sulphur are each contained in the molecule in two 

 forms, the one loosely combined, the other firmly combined. The 

 loosely combined portion of each of these elements can be split off 

 from the rest of the proteid molecule by boiling with caustic alkali. 



EXPERIMENT I. The loosely combined Nitrogen. To about five 

 cubic centimetres of diluted egg-white add a few drops of 20 per cent, 

 caustic potash ; warm slowly, and hold a piece s of moistened red 

 litmus paper over the mouth of the test tube. The litmus turns 

 blue, showing that ammonia gas is being evolved. The ammonia may 



