40 CHEMICAL CONSTITUENTS OF BODY AND FOOD. 



ever, decomposes as well as dissolves the proteid. Proteids are also soluble 

 in gastric and pancreatic juices, but here again they undergo a change, 

 being converted into the hydrated varieties of proteid known as proteoses 

 and peptones. Solutions of the proteids are precipitated by a large 

 number of reagents, but the proteoses and peptones furnish many ex- 

 ceptions to this statement. 



The principal precipitants of proteids are : 



1. Strong mineral acids, especially nitric, metaphosphoric, and 

 phosphotungstic acids. 



2. Acetic acid with potassium ferrocyanide. 



3. Acetic or oxalic acid, with excess of certain neutral salts, such 

 as sodium sulphate, sodium chloride, or magnesium sulphate. 



4. Salts of the heavy metals ; bf^ic lead acetate, mercuric chloride, 

 silver nitrate, copper sulphate, ferric chloride or acetate, potassio- 

 mercuric iodide, sodium tungstate, etc. The precipitates consist of 

 the proteid in combination with variable amounts of the metal, in 

 the form of albuminates. On the removal of the metal by a stream 

 of sulphuretted hydrogen, the proteid is recoverable in an unchanged 

 form. 



5. Tannin ; or tannin and sodium chloride together. 



6. Saturation with ammonium sulphate or sodiomagnesium sulphate, 

 or potassium acetate or carbonate. These precipitates are soluble on 

 diluting the solution of salt in which they are suspended. 



7. Picric acid. 



8. Salicylsulphonic acid. 



9. Trichloracetic acid. 



10. Alcohol, except in the presence of free alkali, when the proteids 

 are slightly soluble in hot alcohol. 



The precipitate given by the proteoses is in many cases (as with 

 nitric, trichloracetic, and salicylsulphonic acid, or with acetic acid and 

 potassium ferrocyanide) soluble on heating, but re-appears when the 

 solution cools. The greater number of the reagents mentioned do 

 not precipitate peptone. It is precipitated completely by alcohol, 

 tannin, and potassio-mercuric iodide, and incompletely by phospho- 

 tungstic and phosphomolybdic acids. 



The following are the methods used to remove all proteid from a 

 solution : 



1. Br Here's method 1 consists in the alternate addition of hydrochloric acid 

 and potassio-mercuric iodide. 



2. Gii'gensohn's method 2 consists in the addition of sodium chloride and 

 tannin. 



3. Devoto's method? This consists in boiling an acidulated solution 

 of the proteid with excess of ammonium sulphate crystals ; all proteids are 

 precipitated by this means except peptone. Proteoses, if present, are 

 precipitated but not coagulated, and can be extracted from the precipitate 

 by water. 



4. By trichloracetic acid. This method consists in adding to the solution an 

 equal volume of a 10 per cent, solution of trichloracetic acid, boiling and filtering 

 hot. The filtrate contains the proteoses and peptone, if these are present, and 

 the precipitate contains the other proteids. This is by far the most rapid and 



1 Sitzungsb. d. k. AJcad. d. Wissenscli., Wien, 1871. 



2 N. Repert.f. Pharm., Miinchen, Bd. xxii. S. 557. 



3 Zttchr. f. physiol. Chcm., Strassburg, Bd. xv. 



