Under the head of the albuminous bodies we find several classes which differ from each other in slight but very important points. The first class may be called -

(A) Albumins Proper, Or Native Albumins

They consist of -

1. .Egg Albumin, which does not occur in the ordinary tissues of the animal, can be procured by filtration from the white of an egg. It makes a clear or slightly opalescent solution in water, from which it is precipitated by mercuric chloride, silver nitrate, lead acetate, and alcohol. It is coagulated by heat, strong nitric and hydrochloric acids, or prolonged exposure to alcohol or ether.

2. Serum Albumin, on the other hand, is one of the chief forms of albumin found in the nutrient fluids.

It differs from egg albumin in -

(a) Not coagulating with ether.

(b) The precipitate obtained by strong hydrochloric acid being readily redissolved by excess of the acid. (V) Coagulum being more readily soluble in nitric acid. (d) Its specific rotary power being 56°,.while that of egg albumin is 35.5°. (e) If introduced into the circulation, it is not eliminated with urine, as is egg albumin.

(B) Globulins

Associated with the last during the life of the tissues we find another class of albumins, namely, the globulins, which do not dissolve in pure water, but are more or less soluble in a solution of common salt. These may be divided as follows: -

1. Globulin (crystalliii) occurs in many tissues, but is usually obtained from an extract of the crystalline lens made by triturating it with fine sand in a weak solution of common salt, and then passing a current of carbon dioxide through the solution. The globulin falls, being easily precipitable from its saline solution by very weak acid. This form of globulin does not cause coagulation when added to serous fluids, and in this respect differs from the next members of this division.

2. Paraglobulin {serum globulin) can be obtained by passing through diluted serum a brisk stream of carbon dioxide. It is also precipitated by adding sodic chloride to saturation. When a fluid containing paraglobulin is added to a serous transudation, it causes coagulation of the fluid, giving rise to fibrin.

3. Fibrinogen, a viscous precipitate got from serous fluids or blood plasma in the same way as the last, but with greater dilution and more prolonged use of carbon dioxide. It is similar in its characters to the last, but coagulates at a lower temperature (55° C.) (paraglobulin coagulating at 6o°-7o° C). On its addition to defibrinated blood, or a fluid containing paraglobulin, it forms a coagulum.

4. Myosin is obtained from dead muscle, being the soft, jellylike clot formed during rigor mortis from the dying muscle plasma. It is not so soluble as globulin, for it requires a stronger solution of salt (10 %) to dissolve it, and is precipitated from its saline solution by solid salt or by dilution. It is coagulated at 6o° C.

5. Vilellin, a white granular proteid obtained from the yelk of egg. It is very soluble in 10 per cent, saline solution, from which it can be precipitated by extreme dilution, but not by saturation with salt. It coagulates between 700 and 8o° C.

(C) Derived Albumins (Albuminates). 1. Acid Albumin (Syntonin)

Acid Albumin (Syntonin) can be made from any of the preceding by the slow action of a weak acid; or by adding strong acetic or hydrochloric acids to native albumin, such as exists in white of egg, and dissolving the jelly thus formed in water. It is only soluble in weak acids - exact neutralization precipitating it. With the least excess of alkali the precipitate redissolves, changing into alkali albumin.

If it be dissolved in weak acid it will not coagulate on boiling, but it coagulates and becomes incapable of re-solution if heated while precipitated by neutralization.

2. Alkali Albumin

Similar to the last, but produced by the action of either weak alkalies on dilute solutions, or strong solution of potash on white of egg. Its general behavior is the same as the above, but if prepared by strong solution of potash and allowed to stand some time it differs in composition, being deprived of its sulphur. It can then be distinguished by the absence of the brown coloration which appears on heating acid albumin with caustic potash and lead acetate.

3. Casein is the proteid existing in milk, and resembles alkali albumin in its reactions. It can be precipitated from milk by rennet, or acetic acid in excess, but not by exact neutralization, owing to the presence of neutral potassium phosphate, which must be converted into the acid salt before precipitation begins.

(D) Fibrin

A solid filamentous body, the result of chemical changes accompanying the death of the blood plasma, during which the so-called fibrin generators are set free. It swells in weak hydrochloric acid, but does not dissolve while cold. If heated to 6o° C. in acid, it changes to acid albumin and dissolves. By 10 per cent, neutral saline solutions, a substance like a globulin may be extracted from it. If heated, it assumes the characters of a coagulated proteid.

(E) Coagulated Albumin

If any of the above be heated over 700 C. (except acid and alkali albumin, which must first be precipitated by neutralization), they coagulate, and become extremely insoluble and lose their former characters. They are but very slightly acted on by weak acids, even when warmed. Strong acids dissolve them, but this solution is associated with a destructive change. They are, however, converted by the digestive ferments and juices into peptones, and thus dissolved.

(F) Peptone

This substance is formed by the action of the digestive ferments from any of the above albumins, in the stomach by pepsin in the presence of dilute acid, and in the small intestines by trypsin in the presence of dilute alkali. This change renders them more soluble and diffusible, and thus enables them to pass out of the alimentary canal into the system, and makes them more suited to take part in the nourishment of the body.

The leading characteristics of peptones may be thus enumerated: -

1. Very ready solubility in hot or cold water, acids or alkalies.

2. Not coagulable by heat.

3. They are precipitated by alcohol but not changed to the coagulated form.

4. They diffuse more readily through animal membrane than any other albumins.

5. They are not precipitated by copper sulphate, ferric chloride, or potassium ferrocyanide and acetic acid.

6. They are precipitated by iodine, chlorine, tannin, mercuric chloride, and the nitrates of silver and mercury.

7. Caustic potash and a trace of copper sulphate added to their solutions give a red color which deepens to violet if too much of the copper salt be used. The formation of peptones is a gradual process having many intermediate steps, in the earlier stages of which precipitates are formed by potassium ferrocyanide and acetic acid. {Vide Chaps. viii and ix, on Chemistry of Digestion).