action of diastatic ferments, maltose is the chief end product. In both

cases dextrin is an intermediate stage in the process.

FIG. 4.-Section of pea showing starch and aleurone grains embedded in the protoplasm of the cells. (Yeo, after Sachs): a, aleurone grains; st, starch grains; i, intercellular spaces.

Before the formation of dextrin the starch solution loses its opalescence, a substance called soluble starch being formed. This, like native starch, gives a blue colour with iodine. Although the molecular weight of starch is unknown, the formula for soluble starch is probably 5(C12H20010)20. Equations that represent the formation of sugars and dextrins from this are very complex, and are at present only theoretical.

Dextrin is the name given to the intermediate products in the hydration of starch, and two chief varieties are distinguished :erythro-dextrin, which gives a reddish-brown colour with iodine; and achroo-dextrin, which does not.

It is readily soluble in water, but insoluble in alcohol and ether. It is gummy and amorphous. It does not give Trommer's test, nor does it ferment with yeast. It is dextro-rotatory. By hydrating agencies it is converted into glucose.

Glycogen, or animal starch, is found in liver and muscle. It is also abundant in all embryonic tissues.

Glycogen is a white tasteless powder, soluble in hot water, forming, like starch, an opalescent solution. It is insoluble in alcohol and ether. It is dextro-rotatory. With Trommer's test it gives a blue solution, but no reduction occurs on boiling.

With iodine it gives a reddish or port-wine colour, very similar to that given by dextrin. Dextrin may be distinguished from glycogen by (1) the fact that it gives a clear, not an opalescent, solution with water; and (2) it is not precipitated by basic lead acetate as glycogen is. It is, however, precipitated by basic lead acetate and ammonia.

(3) Glycogen is precipitated by 60 per cent. of alcohol; the dextrins require 85 per cent. or more.

Cellulose. This is the colourless material of which the cell-walls and woody fibres of plants are composed. By treatment with strong mineral acids it is, like starch, converted into glucose, but with much greater difficulty. The various digestive ferments have little or no action on cellulose; hence the necessity of boiling starch before it is taken as food. Boiling bursts the cellulose envelopes of the starch grains, and so allows the digestive juice to get at the starch proper. Cellulose is found in a few animals, as in the test or outer investment of the Tunicates.

THE FATS

Fat is found in small quantities in many animal tissues. It is, however, found in large quantities in two situations, viz. adipose

FIG. 5.-A few cells from the margin of a fat lobule: fg, fat globule distending fat cell; n, nucleus; m, membranous envelope of the cell; cr, bunch of crystals within a fat cell; c, capillary vessel; venule; ct, connective tissue cell. The fibres of the connective tissue are not represented.

v,

tissue and milk. The consideration of the fat in milk is postponed to Lesson IV.

The contents of the fat cells of adipose tissue are fluid during life, the normal temperature of the body (36° C., or 99° F.) being considerably above the melting-point (25° C.) of the mixture of the fats found there. These fats are three in number, and are called palmitin, stearin, and olein. They differ from one another in chemical composition and in certain physical characters, such as melting-point and

solubilities. Olein melts at 0° C., palmitin at 45° C., and stearin at 53-66° C. It is thus olein which holds the other two dissolved at the body temperature. Fats are all soluble in hot alcohol, ether, and chloroform, but insoluble in water.

Chemical Constitution of the Fats.-The fats are compounds of fatty acids with glycerin, and may be termed glycerides. The térm hydrocarbon, applied to them by some authors, is wholly incorrect.

The fatty acids form a series of acids derived from the monatomic alcohols by oxidation. Thus, to take ordinary ethyl alcohol, C2HO, the first stage in oxidation is the removal of two atoms of hydrogen to form aldehyde, C2HO; on further oxidation an atom of oxygen is added to form acetic acid, C2H102.

A similar acid can be obtained from all the other alcohols, thus :acid H.COOH is obtained.

From methyl alcohol CH3.HO, formic

C2H.HO, acetic
C3H7.HO, propionic
CH,.HO, butyric

CH11.HO, valeric

and so on.

Or in general terms:

From the alcohol with formula CnH2n+1.HO the acid with formula Cn-1H2n-1.CO.OH is obtained. The sixteenth term of this series has the formula C15H31.CO.OH, and is called palmitic acid; the eighteenth has the formula C17H35.CO.OH, and is called stearic acid. Each acid, as will be seen, consists of a radicle, Cn-1H2n-1CO, united to hydroxyl (HO).

17

Oleic acid, however, is not a member of the fatty acid series proper, but belongs to a somewhat similar series of acids known as the acrylic series, of which the general formula is Cn-1H2n-3COOH. It is the eighteenth term of the series, and its formula is C17H33.CO.OH.

Glycerin or Glycerol is a triatomic alcohol, C2H5(HO),—i.e. three atoms of hydroxyl united to a radicle glyceryl (C2H5). The hydrogen in the hydroxyl atoms is replaceable by other organic radicles. As an example take the radicle of acetic acid called acetyl (CH3.CO). The following formulæ represent the derivatives that can be obtained by replacing one, two, or all three hydroxyl hydrogen atoms in this way :

Triacetin is a type of a neutral fat; stearin, palmitin, and olein

ought more properly to be called tristearin, tripalmitin, and triolein respectively. Each consists of glycerin in which the three atoms of hydrogen in the hydroxyls are replaced by radicles of the fatty acid. This is represented in the following formula :

Acid

Radicle

Fat

1.CO)

Palmitic acid C15H31.COOH Palmityl C15H31.CO Palmitin C,H,(OC15H31. Stearic acid CH.COOH Stearyl CH.CO Stearin CH(OCH.CO) Oleic acid C17H33.COOH Oleyl C17H33.CO Olein CH(OCH.CO)

Decomposition Products of the Fats.-The fats split up into the substances out of which they are built up.

Under the influence of superheated steam, mineral acids, and in the body by means of certain ferments (for instance, the fat-splitting ferment steapsin of the pancreatic juice), a fat combines with water and splits into glycerin and the fatty acid. The following equation represents what occurs in a fat, taking tripalmitin as an example :

C3H5(O.C15H31 CO)3+3H2O=C3H5(OH)3+3C15H31 CO.OH

[palmitin-a fat]

[glycerin] [palmitic acid-a fatty acid]

In the process of saponification much the same sort of reaction occurs, the final products being glycerin and the palmitate of the base employed, which is called a soap. Suppose, for instance, that potassium hydrate is used; we get

C3H5(O.C15H31CO)3+3KHO=C3H5(OH)3+3C15H31 CO.OK

[palmitin-a fat]

[glycerin] [potassium palmitate-a soap] Emulsification.-Another change that fats undergo in the body is very different from saponification. It is a physical rather than a chemical change; the fat is broken up into very small globules, such as is seen in the natural emulsion-milk.

1. Tests for Proteids. The following tests are to be tried with a mixture of one part of white of egg to ten of water. (Egg-white contains a mixture of albumin and globulin.)

(a) Heat Coagulation. Faintly acidulate with a few drops of 2-per-cent. acetic acid and boil. The proteid is rendered insoluble (coagulated proteid). (b) Precipitation with Nitric Acid. The addition of strong nitric acid to the original solution also produces a white precipitate.

(c) Xanthoproteic Reaction. On boiling the white precipitate produced by nitric acid it turns yellow; after cooling add ammonia; the yellow becomes orange.

(d) Millon's Test.-Millon's reagent (which is a mixture of the nitrates of mercury containing excess of nitric acid; see p. 4) gives a white precipitate, which turns brick-red on boiling.

(e) After the addition of acetic acid, potassium ferrocyanide gives a white precipitate.

(f) Add a drop of a 1-per-cent. solution of cupric sulphate to the original solution and then caustic potash, and a violet solution is obtained.

2. Repeat experiment (f) with a solution of commercial peptone, and note that a rose-red (biuret) reaction is obtained.

3. Action of Neutral Salts.-The following experiments may be performed with serum, which, like egg-white, contains albumin and globulin.

(a) Saturate it with magnesium sulphate by adding crystals of the salt to the serum and shaking vigorously in a flask. A white precipitate of serumglobulin is produced. Filter. The filtrate contains serum-albumin. (Serumglobulin is incompletely precipitated by diluting the serum with a large quantity of water and also by adding weak acetic acid to, or passing a stream of carbonic acid gas through diluted serum. See Lesson XXI.)

(b) Saturate similarly another portion with ammonium sulphate; a precipitate is produced of both the globulin and albumin. Filter. The filtrate contains no proteid.

4. Repeat the last experiment with a solution of commercial peptone. A precipitate is produced of the albumoses or proteoses it contains. Filter. The filtrate contains the true peptone. Ammonium sulphate precipitates all proteids except peptone.

The proteids are the most important substances that occur in animal and vegetable organisms; none of the phenomena of life. occur without their presence; and though it is impossible to state positively that they occur as such in living protoplasm, they are invariably obtained by subjecting living structures to analytical processes.