14 SEPARATION OF PROTEIDS. [BOOK I. 



added to a solution containing even a trace of a proteid, and the 

 mixture heated, the liquid assumes a purple-red colour. This reaction 

 is common to all the proteids and to their immediate derivatives. 



Millon's reagent is made by dissolving 1 part by weight of mercury in 

 2 parts of nitric acid of specific gravity 1*42 and after complete solution 

 diluting each volume of liquid with two volumes of water. 



5. Xanthoproteic reaction. The liquid supposed to contain a 

 proteid is boiled for some time with concentrated nitric acid. If a 

 proteid be present the liquid assumes a yellow colour, which changes 

 to an amber-red when an excess of alkali is added to it. 



Methods of completely separating proteids from solutions containing 



them,. 



It is often of great importance to remove all the proteids which 

 a liquid contains, so as to proceed to the search for other substances. 

 The following methods are available : 



1. The liquid is treated with several times its volume of absolute 

 alcohol, and acetic acid added until the reaction is acid. After 24 

 hours the fluid is filtered ; the proteids are contained in the insoluble 

 matters on the filter. 



2. To the liquid rendered faintly acid and heated to boiling, and 

 from which all the proteids separable by mere boiling have been re- 

 moved, a solution of ferric acetate, made by saturating acetic acid with 

 recently precipitated ferric hydrate, is added. After boiling for a few 

 minutes a solution is obtained which contains neither proteids nor 

 iron. 



3. In some cases when soluble proteids precipitable by boiling 

 are present, by merely boiling the liquid they are entirely separated ; 

 such is usually the case with albuminous urine. If the liquid have 

 an alkaline reaction, a little acetic acid should be added, in quantity 

 just sufficient to neutralize the free alkali. If the quantity of acid be 

 either too scanty or too great the separation is incomplete; under 

 these circumstances the addition of a few drops of the solution of 

 ferric acetate mentioned in the last paragraph brings about the com- 

 plete precipitation and separation. 



Determination of the temperature at which the proteids coagulate. 



As will be shewn in the sequel, two groups of proteid bodies (the 

 albumins and the globulins) are precipitated from their solutions when 

 they are heated, and the temperature at which coagulation occurs 

 is in some cases an important characteristic. 



