102 THE PROTEIN SUBSTANCES. 



mine the quantity of acetic acid necessary to completely precipitate 

 the proteids in small measured portions of the neutralized liquid which 

 have previously been heated on the water-bath, so that the nitrate does 

 not respond to HELLER'T test. Now warm a larger weighed or meas- 

 ured quantity of the liquid on the water-bath, and add gradually the 

 required quantity of acetic acid, with constant stirring, and continue 

 heating for some time. Filter, wash with water, extract with alcohol 

 and then with ether, dry, weigh, incinerate, and weigh again. With 

 proper work the nitrate should not give HELLER'S test. This method 

 serves in most cases, and especially so in cases where other bodies are 

 to be quantitatively estimated in the nitrate. 



In many cases good results may be obtained by precipitating all the 

 proteid with tannic acid and determining the nitrogen in the washed 

 precipitate by means of KJELDAHL'S method. On multiplying the quan- 

 tity of nitrogen found by 6.25 we obtain the quantity of proteid. Many 

 other methods for the quantitative estimation of proteins have been 

 suggested. 



The removal of proteids from a solution may in most cases be per- 

 formed by boiling with acetic acid. Small amounts of proteid which 

 remain in the nitrates may be separated by boiling with freshly pre- 

 cipitated lead carbonate or with ferric acetate, as described by HOF- 

 MEiSTER. 1 If the liquid cannot be boiled, the proteid may be precipi- 

 tated by the very careful addition of lead acetate, or by the addition 

 of alcohol. If the liquid contains substances which are precipitated 

 by alcohol, such as glycogen, then the proteid may be removed by tri- 

 chloracetic acid as suggested by OBERMAYER and FRANKEL. 2 Recently 

 MICHAELIS and RONA have suggested a method for the removal of proteids 

 by using kaolin, colloidal ferric hydrate or a mastic emulsion. The 

 principle of these methods has already been given on page 97 and in 

 regard to the practical execution of the method we refer to the works 

 there cited. 



In the precipitation of proteid as well as the quantitative estimation by means 

 of heat, it must be borne in mind, as shown by SPIRO, S that several nitrogenous 

 substances, such as piperidine, pyridine, urea, etc., disturb the coagulation of the 

 proteids. 



Synopsis of the Most Important Properties of the Different Groups of 



Albuminous Bodies. 



As it is not possible to base the classification of the different proteid 

 groups according to their constitution, we are obliged to make use of 

 their different solubilities and precipitation properties in their general 

 characterization. As there exist no sharp differences between the various 

 groups in this regard it is impossible to draw a sharp line between them. 



Albumins. These bodies are soluble in water in neutral reaction and 

 are not precipitated by the addition of a little acid or alkali. They are 



1 Zeitschr. f. physiol. Chem., 2 and 4. 



2 Obermayer, Wien. med. Jahrb., 1888; Frankel, Pfluger's Arch., 52 and 55. 



3 Zeitschr. f. physiol. Chem., 30. 



