THE PROTEIN SUBSTANCES. 19 



neutral liquid is first boiled and then the proper amount of acid 

 added carefully. A flocculent precipitate is formed, and if prop- 

 erly done the filtrate should be water-clear. If dilute acetic acid 

 be used for this test, the liquid must first be boiled and then 1, 2, 

 or 3 drops of acid added, depending on the amount of albumin 

 present, and boiled before the addition of each drop. If dilute 

 nitric acid be used, then to 10-15 c.c. of the previously-boiled 

 liquid 15-20 drops of the acid must be added. If too little nitric 

 acid be added a soluble combination of the acid and albumin is 

 formed which is precipitated by more acid. An albumin solution 

 containing a small amount of salts must first be treated with about 

 \% NaCl, since the heating test may fail, especially on using acetic 

 acid, in the presence of only a slight amount of albumin. 

 2. Behavior towards Mineral Acids at Ordinary Temperatures. 

 The albumins are precipitated by the three ordinary mineral acids 

 and by metaphosphoric acid, but not by orthophosphoric acid. 

 If nitric acid be placed in a reagent glass and the albumin solution 

 be allowed to flow gently thereon, a white, opaque ring of precipi- 

 tated albumin will form where the two liquids meet (HELLER'S 

 albumin test). 3. Precipitation by Metallic Salts. Copper sul- 

 phate, neutral and basic lead acetate (in small amounts), mercuric 

 chloride, and other salts precipitate albumin. On this is based the 

 use of albumins as antidotes in poisoning by metallic salts. 4. Pre- 

 cipitation by Ferro- or Ferricyanide of Potassium in Acetic Acid 

 Solution. In these tests the relative quantities of reagent, albumin, 

 or acid do not interfere with the delicacy of the test. 5. Precipi- 

 tation by Neutral Salts, such as Na 2 S0 4 or NaCl, when added to sat- 

 uration to the liquid acidified with acetic acid or hydrochloric acid. 

 6. Precipitation by Alcohol. The solution must not be alkaline, 

 but must be either neutral or faintly acid. It must, at the same 

 time, contain a sufficient quantity of neutral salts. 7. Precipita- 

 tion by Tannic Acid in acetic-acid solutions. The absence of neu- 

 tral salts or the presence of free mineral acids may not cause the 

 precipitate to appear, but after the addition of a sufficient quan- 

 tity of sodium acetate the precipitate will in both cases appear. 

 8. Precipitation by Phospho-tungstic or Phospho-molybdic Acids in 

 the presence of free mineral acids. Potassium-mercuric iodide and 

 potassium-bismuth iodide precipitate albumin solutions acidified 



