664 PHYSIOLOGICAL CHEMISTRY. 



The nutritive value of gelatin is discussed under Metabolism 

 (page 646). 



As gelatin contains no tyrosiue and no tryptophane, it will be 

 found that the results obtained with the xanthoproteic and Mil Ion 

 tests below are only faintly positive, being due to impurities. 



Tests for gelatin. Pour upon a gramme of gelatin 25 c.c. of water and allow 

 to stand twenty-four hours. The gelatin now is swollen, but not dissolved. 

 Decant the water, add 8 c.c. of distilled water, heat over a water-bath until the 

 gelatin dissolves, then cool. To the gelatinous mass, thus obtained, add about 

 50 c.c. of water and heat again until dissolved. Use this solution for the fol- 

 lowing tests : 



1. Add tannin, or hydrochloric acid and phosphotungstic acid: voluminous 

 precipitates are formed. 



2. Boil a portion with one-third its volume of nitric acid : a faint-yellow 

 color is produced, showing the presence of an aromatic radical. 



3. Add caustic potash and a little cupric sulphate : a blue to violet color 

 appears without a trace of red. (Difference from albumoses and peptones.) 



4. Boil with Millon's reagent: a faint pink or red color. (Difference from 

 proteins in general.) 



5. Add bromine-water: a heavy yellow precipitate is formed, possessing 

 tough, adhesive properties. 



6. Add a few drops of acetic acid and boil ; add acetic acid and potassium 

 ferricyanide ; add mercuric chloride. Precipitation takes place in neither case. 

 (Difference from simple proteins.) 



Note that gelatin solidifies on cooling and becomes liquid again on 

 the application of heat (difference from proteins in general). 



Teeth consist of three distinct tissues, viz., dentine, forming the 

 chief mass, in its interior being the pulp cavity ; enamel, investing 

 the crown and extending some distance down the neck ; and cement, 

 covering the fangs. The composition of cement is almost the same as 

 that of bone, its organic and inorganic constituents having the rela- 

 tive proportions of 30 : 70. 



Dentine contains less water than bone and is also poorer in organic 

 matter. The following table gives the composition of the dentine of 

 an adult woman and man respectively : 



Woman. Man. 



Organic matter ossein and vessels . . . 27.61 20.42 



Calcium phosphate 66.72 67.54 



Calcium carbonate 3.36 7.97 



Magnesium phosphate 1.08 2.49 



Soluble salts, chiefly sodium chloride . . 0.83 1.00 



Fat 0.40 0.58 



Enamel is distinguished by the very small proportion of water and 



