THE PROTEIN SUBSTANCES. 6/ 



the dry substance is heated with soda-lime. A positive result 

 with this test does not, of course, prove the presence of a pro- 

 tein compound, since all ammonium salts and amino com- 

 pounds in general respond to it; but with a negative result 

 proteins as well as these other compounds are certainly ex- 

 cluded. The reaction therefore serves as a preliminary test 

 in the examination of unknown substances for proteins. The 

 test may be easily carried out and is delicate. 



Ex. Mix some dried albumin or some wheat flour with an equal bulk 

 of soda-lime in a dry test-tube. Apply heat and note the escape of am- 

 moniacal vapors as shown by the odor or reaction on moist litmus paper. 

 The fixed alkali decomposes the protein matter very quickly, and ammonia 

 always results. 



COAGULATION TESTS. 



Many of the protein substances undergo a peculiar change 

 known as coagulation when heated, or treated with certain 

 reagents. The test is characteristic of most of the bodies 

 except some of the products of transformation. This coagu- 

 lation is usually accompanied by precipitation, that is, the body 

 is thrown out of solution and as a rule can not be restored to 

 its original condition. But there are cases of precipitation 

 without coagulation; the terms must not, therefore, be used 

 as synonymous. In coagulation proper the protein body 

 becomes permanently altered, so that it can not be brought 

 into its original condition again by addition of reagents or by 

 other means. On the other hand, it is in many cases possible 

 to throw a protein body out of solution by simply adding an 

 exces^ of some inorganic salt, without at the same time pro- 

 ducing any decided alteration in the character of the protein 

 precipitate. By largely diluting with water the precipitate 

 may be brought into the soluble condition again. This will 

 be illustrated later by the use of ammonium sulphate which 

 behaves in a characteristic manner with different proteins. 

 Many of these have definite precipitation limits with the sul- 

 phate. That is, they begin to separate when the amount of 

 the salt reaches a certain value, and precipitate completely 

 with a greater concentration. 



