PROTEINS. 95 



the mixture upon a microscopic slide, put a cover glass in position 

 and examine the mixture under the microscope. The formation 

 of canary yellow crystals indicates the presence of protein material 

 in the solution examined. The crystals are ordinarily needle-like 

 in appearance and show a tendency to assume rosette or bundle-like 

 formations but under certain conditions they may show knobbed 

 (nail-like) and branching variations. 



If a moderate excess of iodine is used in making the test a black 

 precipitate of iodonitro compounds is at once formed upon the ad- 

 dition of the ammonium hydroxide and yellow needles are sub- 

 sequently deposited upon it. In case just the proper amount of 

 iodine is used, the solution soon assumes a yellow color and the 

 black precipitate formed upon the addition of the ammonium hy- 

 droxide is gradually transformed more or less completely into the 

 yellow crystals. In either case the needles ordinarily form 

 within an hour, and frequently in a much shorter time. If 

 too great an excess of iodine is employed the heavy black pre- 

 cipitate may obscure or even prevent the reaction. The presence 

 of insufficient iodine or excess protein may likewise prevent 

 the reaction. In tests in which a concentrated protein solution 

 and an excess of iodine are used, the addition of ammonium 

 hydroxide immediately produces a grayish-green precipitate. In 

 such instances, if the proportions are favorable, and the mixture be 

 stirred with a glass rod for a few minutes, the precipitate is grad- 

 ually transformed into the crystals before mentioned. 



It is probable that all soluble proteins will respond to Bardach's 

 reaction, but the relative delicacy of the reaction as well as the value 

 of the test as compared with other protein tests remain to be deter- 

 mined. The only disturbing factor noted thus far is the presence of 

 earthy phosphates in the solution under examination. 



PRECIPITATION REACTIONS AND OTHER 

 PROTEIN TESTS. 



There are three forms in which proteins may be precipitated 

 i. e., unaltered, as an albuminate, and as an insoluble salt. A.n 

 instance of the precipitation in a native or unaltered condition is 

 seen in the so-called salting-out experiments. Various salts, notably 

 (NH 4 ) 2 SO 4 , ZnSO 4 , MgSO 4 ,Na 2 SO 4 and NaCl possess the power 

 when added in solid form to certain definite protein solutions, of 



