rial that will do so and some that are not much changed by rubbing, in order to compare the results.

Procedure: Weigh the pieces of material and then boil them in a 3% solution of hydrochloric acid. Rinse them in water, each one separately, and test the water for starch with iodin. (See page 289.) If starch is present, in any of the pieces of material, boil those in which it is again. When they are all starch free, dry and weigh them. The difference in weight will show the amount of dressing that was present.

Printing, Dyes, and Dyeing

The difference between the dyeing and printing of fabrics. By dyeing is meant the saturation of the fibers of a textile with a coloring substance; by printing, the impression of a colored design upon a fabric.

Printed materials can be usually distinguished from dyed by examining the wrong side of the fabric, if the pattern does not show through, it is printed.

Material may be dyed either before or after it is

woven.

Dyes. A few dyes are prepared from inorganic matter, notably, khaki, which is made by adding chrome alum to a solution of iron, but they are nearly all procured from organic matter. In the olden days, dyes were made principally from vegetable matter, and a few, as cochineal, from animal matter, but, at the present time, coal tar is their usual source. In fact, it has been estimated, that over fourteen thousand colors are secured directly or indirectly from coal tar.

Coal-tar dyes are often spoken of as the aniline dyes, because the first ones obtained from this source were made from aniline—a coal-tar derivative—now, however, a large number are obtained from carbolic acid, naphthalene, anthracene, and various other products of coal tar.

Coal-tar dyes vary greatly in their nature, some having acid and others basic properties as well as other different characteristics.

Mordants and lakes.-On account of the differences in the nature of dyes and those of textiles, all fabrics will not be equally well dyed with the same kind of dye; e. g., the protein matter in silk and wool will unite with substances in some dyes that will find nothing to combine with in the cellulose fibers of textiles of vegetable origin. In such case what is known as a mordant is used. By a mordant is meant any substance that will combine with some constituent of the textile or which can penetrate its fibers and which also has an affinity for the dyestuff so that it will unite with it and thus procure the saturation of the fibers with an insoluble colored substance. This insoluble compound formed by the use of mordants is called a lake.

Permanence of dyes. Even with the use of mordants, all dyes will not be equally permanent, and their permanence or fastness will vary in different materials.

The more common auxiliary causes of change of color in material are washing, the action of the acids of perspiration, and exposure to the sun's rays.

The following tests are ones that are frequently used to test the resistance of dyes to such things. Experiment 39. Object: To test the color fastness of materials.

Test for washing fastness.-Soak and rub a small piece of the material in boiling soap solution. Do this several times. If the color does not run and if comparison with an unwashed piece of material shows that it has not faded, the color is fast to washing.

Resistance to perspiration.-Soak a piece of the material to be tested in 30% acetic acid that has been heated to about 99° F. and then dry it, without rinsing, between parchment paper. When dry compare it with a piece of the material that has not been so treated.

Resistance to light. Place the pieces of material to be tested where they will be in the direct sunlight a considerable portion of the time for a month. Compare them daily with similar pieces that are not so exposed. Colors that will stand such exposure for a month are considered fast; those which have undergone appreciable though not very great change, are said to be fairly fast; those which show considerable change in two weeks are classed as moderately fast, and those which are much changed, as fleeting.

The comparative permanence of the fundamental colors in different materials is about as follows:

Brown lasts well in cotton materials, but is likely to fade in linen and woolens.

Dark blue is generally a fast color in all the textiles, but light blue generally fades quickly.

Black lasts well in wool and silk, but not in vegetable fibers.

Red is fairly permanent in woolen and silk materials, but only fairly so in others.

Gray is fairly permanent in all materials.

Pink fades soon, but usually does so uniformly and becomes a pretty shade.

Green cannot be depended upon except in highpriced woolens and silks.

Lavender is likely to be a fleeting color in all

materials.

CHAPTER XVIII

THE CHEMICAL CONSTITUENTS OF THE HUMAN BODY AND OF FOOD

Classification, Nature, and Uses of the Substances Composing Animal Bodies and Plants-Tests for Proteins, Starches, Sugars, Salts-Origin of Food Material.

In order that the machinery of the human body (i. e., the heart, lungs, muscles, etc.) may be kept at work, it must be provided with fuel and with matter containing the same elements as itself which its cells can utilize for their building and repair. The elements that the body needs for these purposes are contained in many compounds that are very common in nature, but almost the only combinations that the human body can utilize are those formed in certain plants and in some of the lower animals. Though the tissues of plants and of animals are so unlike in appearance, the compounds of which they are composed are so similar in their chemical composition that they, including those constituting the human body, are classified under the same headings.

Classification of the substances composing plants and animals.-The various substances that enter into the composition of both plants and animals are classi