PROTEINS 103 



tated by this salt until a concentration is reached greater than that 

 secured by half -saturation. As an example of an albumin which does 

 not conform to the definition of an albumin as regards its precipitation 

 by ammonium sulphate may be mentioned the leucosin of the wheat 

 germ, which is precipitated from its solution upon ^//-saturation with 

 ammonium sulphate. The limits of precipitation by ammonium 

 sulphate, therefore, do not furnish a sufficiently accurate basis for the 

 .differentiation of globulins from albumins. It has further been deter- 

 mined that a given protein which is precipitable by ammonium sulphate 

 cannot be " sal ted-out" by the same concentration of the salt under all 

 conditions. 



EXPERIMENTS 



1. Influence of Concentrated Mineral Acids, Alkalis and Organic Acids. 

 Prepare five test-tubes each containing 5 c.c. of concentrated egg albumin solu- 

 tion. To the first add concentrated H 2 SO 4 , drop by drop, until an excess of the 

 acid has been added. Note any changes which may occur in the solution. Allow 

 the tube to stand for 24 hours and at the end of that period observe any altera- 

 tion which may have taken place. Heat the tube and note any further change 

 which may occur. Repeat the experiment in the four remaining tubes with 

 concentrated hydrochloric acid, concentrated nitric acid, concentrated potassium 

 hydroxide and acetic acid. How do strong mineral acids, strong alkalis, and 

 strong organic acids differ in their action toward protein solutions? 



2. Precipitation by Metallic Salts. Prepare four tubes each containing 2-3 

 c.c. of dilute egg albumin solution. To the first add mercuric chloride, drop by 

 drop slowly, until an excess of the reagent has been added, noting any changes 

 which may occur. If not added very gradually the formation of the precipitate 

 may not be noted, due to its solubility in excess of the reagent. Repeat the ex- 

 periment with lead acetate, silver nitrate, copper sulphate, ferric chloride, and 

 barium chloride, using very dilute solutions. 



Egg albumin is used as an antidote for lead or mercury poisoning. 

 Why? Is it an equally good antidote for the other metallic salts tested ? 



3. Precipitation by Alkaloidal Reagents. Prepare six tubes each containing 

 2-3 c.c. of dilute egg albumin solution. To the first add picric acid drop by drop 

 until an excess of the reagent has been added, noting any changes which may 

 occur. Repeat the experiment with trichloracetic acid, tannic acid, phospho- 

 tungstic acid, phosphomolybdic acid, and potassio -mercuric iodide. Are these 

 precipitates soluble in excess of the reagent? Acidify with hydrochloric acid 

 before testing with the last three reagents. 



4. Nitric Acid Test (Heller). Place 5 c.c. of concentrated nitric acid in a 

 test-tube, incline the tube, and by means of a pipette allow the dilute albumin 

 solution to flow slowly down the side. The liquids should stratify with the 

 formation of a white zone of precipitated albumin at the point of juncture. This 

 is a very delicate test and is further discussed on page 423. 



An apparatus called the albumoscope or horismascope has been devised for use 

 in the tests of this character and has met with considerable favor. The method of 

 using the albumoscope is described on p. 104. The instrument is shown in Fig. 

 130, p. 424. 



