It is difficult to say how far these bodies exist as such in the living organism, but they can be obtained from nearly all parts, particularly those which contain active protoplasm, and after its death they can be detected in abundance. As may be seen, by testing for their presence in living protoplasm, the addition of any chemical reagent or treatment causes its death, so that, although albumins appear in the test tube, this cannot be accepted as proof that they would have answered to the tests before the protoplasm was changed by its death.
They do not occur normally in any secretion except those substances which tend to nourish the adult body, and to form and nourish the offspring, viz., the ovum, semen and milk. No satisfactory formula has been suggested to express their chemical composition, but the average percentage of the elements they contain is remarkably alike in all members of the group. This may be said to be in round numbers as follows: -
. . 22 per cent.
. 7 "
Nitrogen, . . .
. . 16 "
. . 53
. . 2 "
They are amorphous, of varying solubility, and, with one exception, indiffusible in distilled water.
As far as we know at present, albumins cannot be constructed de novo in the animal body, but must be supplied in one form or another as part of the food. Albumins are therefore always the outcome of the activity of vegetable life.
They can be recognized by the following tests: -
1. Strong nitric acid gives a pale yellow color to solutions or solid albumin, especially on heating, which turns to deep orange when ammonia is added (Xanthoproteic test).
2. Millon's Reagent (acid solution of proto-nitrate of mercury) gives a white precipitate which soon turns yellow, changing to rosy-red on boiling, or standing for some days.
3. Solution of caustic soda and a drop of copper sulphate solution give a violet color to the liquid.
4. Acetic acid and boiling give a white precipitate, except with derived albumins and peptones.
5. Acetic acid and potassium ferrocyanide give a flocculent white precipitate, except -with, peptones.
6. Acetic acid and equal volumes of sodium sulphate solution give a precipitate on boiling.
7. With sugar and sulphuric acid they become violet.
8. Crystals of picric acid added to solutions dissolve and cause bead-like local coagulations, except with peptones.