4 o CHEMICAL CONSTITUENTS OF BOD Y 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 ; basic 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. Briicke's method 1 consists in the alternate addition of hydrochloric acid 
and potassio-mercuric iodide. 
2. Girgensolm's method - 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. h. Akad. d. Wissensch., Wien, 1871. 
- X. Report, f. Pharm., Miinchen, Bd. xxii. S. 557. 
3 Ztschr. f. physiol. Chcm., Strassburg, Bd. xv. 
