Chemical Constituents of the Animal Body. 237 



sulphur, which may be detected by boiling the substance 

 with a strong solution of sodium hydroxide and then 

 adding a few drops of lead acetate solution, when a black 

 precipitate is produced. 



(2) Nearly all the proteins are laevo-rotatory. 



(3) The solutions of most (but not all) proteins when 

 heated in solutions made faintly acid with weak acids 

 (acetic or butyric) undergo a change the protein sepa- 

 rates out in an insoluble form known as a " coagulum," 

 and the protein is said to be coagulated. The best known 

 example is the coagulation of white of egg. The protein, 

 on coagulation, is now known to undergo a definite 

 chemical change, the exact nature of which has not yet 

 been elucidated. The degradation products of proteins 

 such as the albumoses and peptones (see p. 258) do not 

 undergo this change. By means of coagulation it is 

 possible to separate the proteins (although in a changed 

 form) from other substances in solution. The coagulation 

 of proteins is an irreversible process that is, the coagulum 

 does not redissolve when the heated solution is cooled. 

 The coagulative change does not readily take place in the 

 absence of water that is, when the dry protein is heated 

 alone. On account of the coagulative change it is in- 

 advisable to heat protein solutions above 40 when the 

 "natural" protein is required. A protein changed by 

 heat is stated to be " denaturated." 



(4) Proteins are precipitated from solutions by strong 

 acids. If strong nitric acid is allowed to flow into a test- 

 tube containing a protein solution, a white ring forms at 

 the junction of the liquids. (Heller's test.) 



(5) Proteins are nearly all precipitated by the follow- 

 ing reagents ferrocyanic acid (potassium ferrocyanide 

 and acetic acid), trichloracetic acid, sulphonyl salicylic 

 acid, and uranyl acetate. They are also precipitated by 



