162 COMPOSITION OF THE BODY 



chloride is entirely removed. If a little of the solution is drawn 

 off from time to time and a drop of silver nitrate added with a 

 little dilute HNO 3 , the presence of chlorides can be detected by 

 a white precipitate. After the salt has entirely disappeared from 

 the dialyzing bag its contents can be removed and examined. As 

 globulins are insoluble in water any precipitate will probably be 

 a proteid of that class. Sometimes organic matter other than 

 proteids is separated out by dialysis from a saline solution. If a 

 precipitate is present it should be collected on a filter, and the 

 following tests performed : 



229, General Qualitative Tests for Proteids. i . Examine the 

 precipitate under the microscope. Globulins frequently separate 

 out in various crystalline forms. 



2. To a little of the precipitate in a test-tube add caustic potash 

 solution, and afterwards a trace of copper sulphate in very dilute 

 solution. A reddish to violet color more or less distinct depend- 

 ing upon the quantity of the proteid and copper present is to be 

 seen. This is called the biuret reaction. 



3. Boil a little of the material in concentrated nitric acid. Cool 

 the liquid by holding the test-tube under the water flowing from 

 the tap for a minute or two and add ammonia. If proteids are 

 present a yellow color will be imparted to the nitric acid on boil- 

 ing, which changes to orange, upon the admixture of ammonia. 

 This is the xanthoproteic reaction. 



4. Add Millon's reagent and heat gently at first, but if no re- 

 action is apparent, bring the liquid to the boiling point, when a 

 brick red color in the precipitate will indicate the presence of pro- 

 teids. If traces only are present the color will be produced in 

 the solution. Chloride tends to vitiate the test, by combining with 

 the mercury in the reagent. 



5. Dissolve some of the precipitate in a saline solution as di- 

 lute as may serve the purpose. Boil some of the solution in a 

 test-tube and then add a drop of dilute acetic acid. A precipitate 

 should occur if globulins are present. The acid must not be 

 added before the boiling point is reached. 



