APPENDIX. 673 
transudations ; and, when injected into the blood, does not reappear in 
the urine, while the injection of egg-albumin is followed by its appear- 
ance in the urine apparently unaltered. In fact, this form of proteid 
constitutes a great part of the “albumin” of the urine of such signifi- 
cance in pathological conditions. However, increasing knowledge seems 
to point to the “albumin” of the urine, like many other forms of pro- 
teid, being more complex than was once supposed. 
II. Derived Albumins (Albuminates). 
1. Acid-Albumin,—This may be formed by the addition of a strong 
acid to egg-albumin, or, more gradually, by heating a weaker solution 
of egg-albumin with an extremely dilute acid. | 
Acid-albumin is characterized by non-precipitation on boiling, com- 
plete precipitation on the addition of a dilute alkali to the point of neu- 
tralization—that is, acid-albumin is insoluble in water or such like neu- 
tral liquids. It is soluble in an excess of acid or of alkali. : 
By treating finely minced muscle with a weak acid, a substance is 
obtained not readily distinguishable from acid-albumin, but known as 
syntonin. This is probably not identical with acid-albumin as formed 
_ by the method indicated above, though a distinguishing test of a wholly 
satisfactory character is not known. Neither this substance nor acid- 
albumin coagulates on boiling, in which it bears a resemblance to pep- 
tone. The parapeptone of digestion seems to be very similar to acid- 
albumin. A solution of acid-albumin in acid may be precipitated by 
the addition of an excess of common salt. 
2. Alkali-Albumin.—This corresponds to the foregoing, and may be 
formed in a similar way by the addition of an alkali instead of an acid. 
It is not coagulable on boiling, and is precipitated by dilute acid, in 
excess of which and of alkali it is soluble, but, like acid-albumin, is in- 
soluble in water and solution of neutral salts. The specific rotation 
varies with the mode of preparation, from which, as well as on other 
grounds, it is more than likely that there are different kinds of alkali- 
albumin. It is highly probable that acid-albumin and alkali-albumin 
are combinations of an acid or an alkali, as the case may be, with albu- 
min, and that the neutralization precipitate is not in itself either one or 
the other. 
3. Casein.—This substance is the proteid most characteristic of milk, 
from which it may be obtained by dilution ten to fifteen times with water, 
adding acetic acid till a precipitate begins to form, and then sending a 
current of COs through the fluid. After standing, the precipitate may 
be collected in a filter. It is freed from salts, sugar, fat, etc., by first 
washing with water and then with alcohol and ether.. , 
It is so like alkali-albumin that there is no agreement yet as to the 
differences between them. However, the presence in milk of potassium 
phosphate modifies the reactions of casein in this fluid. It may be precipi- 
tated also by adding magnesium sulphate to saturation to milk. This pre- 
cipitate is, however, easily soluble in water. The specific rotation of casein, 
when in solution in water is —80°, but in other solutions is different. 
43 
