APPENDIX. 675 
proteid residue of hemoglobin. It is not easily soluble in dilute acids 
or sodium, chloride, hence it is with hesitation ranked with the other 
globulins. 
2 IV. Fibrin. 
This body has peculiarities which warrant, in the present. state of our 
knowledge, its separation from the foregoing and placing it in a sepa- 
rate division. It is insoluble in water and dilute solutions of sodium 
chloride ; dissolved only with difficulty in concentrated neutral saline 
solutions, and in dilute acids and alkalies. ; 
Fibrin is highly elastic. It always swells under the action of weak 
(1 to 5 per cent) hydrochloric acid. But continued action of the acid 
changes the fibrin to syntonin. Heat hastens the process. By the ac- 
tion of alkalies, especially when aided by warming, fibrin is converted 
into alkali-albumin. By the prolonged action of solutions of sodium 
chloride (10 per-cent), sodium sulphate, etc., conversion into a substance 
very like myosin or globulin is effected. Myosin may be regarded as 
an intermediate product, lying between globulin and fibrin. This be- 
comes clear when comparing their respective solubilities in a ten-per- 
cent solution of sodium chloride. Fibrin and myosin, it will be re- 
membered, are both the products of coagulation processes. The highly 
filamentous character of fibrin distinguishes it physically from all other 
proteids. 
V. Coagulated Proteids. 
This class of bodies may be obtained from a variety of others by the 
use of heat, alcohol, acids, etc. By heating to about 70° C., solutions of 
egg-albumin, serum-albumin, and globulins are coagulated. Precipi- 
tated acid-albumin and alkali-albumin, and fibrin in solution in salines, 
are converted into coagulated proteids by boiling. The digestive juices 
act readily on them, converting them finally into peptones. 
VI. Peptones. 
Peptones are proteids which, though possessing little absolute diffu- 
sibility, as compared with solutions of ordinary salts, yet pass through 
animal membranes with much greater readiness than any other proteids. 
Also, unlike most other proteids, they are not coagulated by boiling. 
They are not precipitated by cupric sulphate, ferric chloride, nor usually 
by potassium ferrocyanide and acetic acid. Though precipitated by 
alcohol from solution in water, they do not undergo a true coagulation, 
even after standing long under this liquid. 
Peptones are coagulated by chlorine, iodine, tannin, the nitrates of 
mereury and silver, mercuric chloride, and the lead acetates. A mere 
trace of cupric sulphate to which a strong solution of caustic alkali has 
been added, introduced into a solution of peptones, gives rise to a red 
(pink) color. If more than a trace of the copper salt be added, the usual 
proteid reaction results. 
Peptones may be formed through the action of dilute or stronger 
acids at medium temperatures, or by the action of distilled water heated 
