PROTEINS. 97 



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, until an excess of the reagent 

 has been added, noting any changes which may occur. Repeat the 

 experiment with plumbic acetate, argentic nitrate, cupric sulphate, 

 ferric chloride and barium chloride. 



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

 Why? 



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 experi- 

 ment with trie hi or ace tic acid, tannic acid, phosphotungstic acid, 

 phospho-molybdic acid and potassio-mer curie iodide. Acidify with 

 hydrochloric acid before testing with the three last reagents. 



4. Heller's Ring Test. 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 p. 314. 



An apparatus called the albumoscope or horismacope has been 

 devised for use in the tests of this character and has met with con- 

 siderable favor. The method of using the albumoscope is described 

 below. 



Use of the Albumoscope. This instrument is intended to facili- 

 tate the making of " ring " tests such as Heller's and Roberts'. In 

 making a test about 5 c.c. of the solution under examination is first 

 introduced into the apparatus through the larger arm and the re- 

 agent used in the particular test is then introduced through the 

 capillary arm and allowed to flow down underneath the solution 

 under examination. If a reasonable amount of care is taken there 

 is no possibility of mixing the two solutions and a definitely defined 

 white " ring " is easily obtained at the zone of contact. 



5. Roberts' Ring Test. Place 5 c.c. of Roberts' reagent 1 in a 

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

 bumin solution to flow slowly down the side. The liquids should 



Roberts' reagent is composed of I volume of concentrated HNO 3 and 5 

 volumes of a saturated solution of MgSCX 



