84 PHYSIOLOGICAL CHEMISTRY 



acid with magnesium or sodium amalgam) is added to a pro- 

 tein solution, and concentrated sulphuric acid added so as to 

 form a layer at the bottom of the test tube, a violet ring will 

 form at the juncture of the two liquids. This test is due to the 

 tryptophane in the protein molecule, and only those proteins 

 containing this amino acid will respond to the test. When 

 glacial acetic acid is used, the test is believed to be due to the 

 presence as an impurity of glyoxylic acid, HCO COOH, or 

 other aldehydes. 



Sulphur Test. If concentrated sodium hydroxide and lead 

 acetate are added to a protein solution and the mixture boiled, 

 a brown or black color appears. The unoxidized sulphur of 

 the cystein or cystin is split off and combines with the lead to 

 form the dark brown or black lead sulphide. 



Precipitation Reactions. Colloids. Proteins may be pre- 

 cipitated by a variety of reagents. The behavior of protein so- 

 lutions with precipitation reagents, 'and in fact many other 

 properties of protein solutions indicate that the proteins do 

 not form true solutions such as that of sodium chloride in water. 

 They form what are known as colloidal solutions. Substances 

 of this nature are called colloids. The particles of a substance 

 in colloidal solution are so large that they will not pass through 

 the pores of a parchment membrane. The group was named 

 "colloids" by Graham to distinguish these compounds from 

 substances which form a "true solution," to which the name 

 crystalloids was applied. Crystalloids in solution are divided 

 into such minute particles that they will pass through the pores 

 of a parchment membrane. As a matter of fact, colloids often 

 crystallize, but as a rule less readily than crystalloids. 



Classification and Properties of Colloids. The colloids are 

 by no means a unified chemical group, for substances of the 

 most widely diverse chemical nature, such as metals, salts, acids, 

 bases, proteins, carbohydrates, etc., may form colloidal solu- 

 tions. The. term "colloidal" refers in fact -to a state of mat- 

 ter, and not to a class of compounds. Many substances of the 

 greatest biological importance form colloidal solutions, in fact 



