Dilute sulphuric acid should completely dissolve Zinc White without effervescence, a property not possessed by any other white pigment. Effervescence would indicate the presence of White Lead, whiting or magnesite. Most other adulterants would remain as insoluble residue (barytes, clay, silica, etc.). The acid solution, when treated with an excess of ammonium hydroxide, upon the addition of ammonium sulphide should yield a white precipitate of zinc sulphide. A discolored precipitate would indicate the presence of other metals.

Zinc White is, also, soluble in ammonium hydroxide and alkaline solutions.

Zinc Oxides are classified as follows:

French Process Zinc Oxide-White Seal, Green Seal and Red Seal-used chiefly for artists' colors and enamels.

American Process Zinc Oxide, lead free-used chiefly for paint and rubber goods.

U. S. P. Zinc Oxide-used for pharmaceutical prepara tions.

Leaded Zinc Oxides, 5 per cent. to 35 per cent. basic lead sulphate, are used chiefly for heavy paints.

ZINC YELLOW

Citron Yellow, Zinc Chrome, Jaune de zinc,
Giallo di zinco, Amarillo de zinc, Zinkgelb

Zinc Chromate-ZnCrO4.

Commercial Zinc Yellows vary considerably in composition, the carefully prepared chemically pure variety showing exceptional permanence to light and air, while the impure products lack dependable stability in this respect.

Pure Zinc Yellow is very stable in mixtures with other durable pigments, mixtures with lakes and certain organic colors are best regarded as of uncertain stability. It is employed only as an oil and water color.

Most dilute acids readily dissolve Zinc Yellow. Hydrogen sulphide does not discolor or blacken this pigment (contrast with Chrome Yellow). The same chromic acid reaction with alcohol and hydrochloric acid is obtained as with Chrome Yellows, Strontium and Barium Chromate (see page 187, test for chromate radicle) and no precipitate is formed upon addition of sulphuric acid to the solution. Alkalies in excess readily decompose Zinc Yellows, yielding yellow solutions. Zinc Yellow is also dissolved by ammonia. Heat destroys this pigment.

Zinc Yellow has been used as an artists' pigment only in comparatively recent years.

COLOR THEORY

Color is not a substance, but a product of partial or selective absorption, or transmission of irregular or scattered reflection, or of dispersion in refraction of white light. Sunlight, which we recognize as white light, is a compound of many rays of varying vibrations, that is of many colors all blended together so that the eye distinguishes but one kind. If however this light is made to fall on a glass prism, it is bent or refracted and the mixture is broken up. We obtain instead of the white light, which enters the prism, an elongated band of different colors. This dispersion, when of a sunbeam, produces what is termed the solar spectrum.

The prismatic spectrum is composed of a series of colors, arranged in similar order to those of the rainbow. Each color vibration of different wave length, is refracted to a different degree; thus the rays of shorter wave length, such as that of a certain tint of blue measuring one fifty-five thousandth of an inch is more refracted than that of a certain tint of red having a wave length of one thirty-thousandth part of an inch.

The colors of the spectrum blend one into the other and although we might name a hundred different colors in the spectrum, each a different color sensation absolutely, we cannot however accurately distinguish such fine divisions with the eye and ordinarily for convenience, divide the spectrum into seven regions, as follows; Red, Orange, Yellow, Green, Blue, Indigo, and Violet.

Beyond the visible spectrum at the violet end we have another set of rays which are more highly refracted, of very small wave length and are known as the ultraviolet or chemical rays.

Beyond the red end, there are also invisible less re

fracted rays of greater wave length, known as the dark, heat, or ultra-red rays.

Upon careful study it was found that the solar spectrum is crossed by a vast number of dark lines, called Fraunhofer's lines after Fraunhofer, who early in the nineteenth century showed that when a solar spectrum is produced in such a manner that the colors are sharply separated, these lines are visible. Through the use of the spectroscope, an instrument invented by Bunsen and Kirchoff, to produce and view the spectrum, it was found that white light from an incandescent solid gives a continous spectrum. Light however coming from incandescent vapors, yields a spectrum consisting of a series of bright lines; for each element the color and position of these lines will differ. By analysis of the spectrum produced by the vapor of a substance it is possible to discover of what element it is composed, such minute quanities as 1/3,000,000 milligram of sodium being detectable by means of the spectroscope.

Artists finding that by skillful use of three colored pigments, red, yellow and blue, nearly all the hues or tones of colors can be obtained, accept these three colors as the primary colors.

By a primary color we understand a color which cannot be produced by the admixture of other colors.

A secondary color is one produced by admixture of any two primary colors.

A tertiary color, or better termed a triad, is one which is produced by the admixture of any two secondary colors.

The above terms are exceedingly arbitrary and consequently not very scientific, as will be shown later on. However, at present we will only consider the primary color sensations red, yellow and blue, as pertaining to the admixture of colored pigments.

Any two colors are said to be complementary, to each

other, when the product of their combinations in proper proportions produce the sensation of white light on the retina of the eye. We can very easily illustrate this by the use of Maxwell's rotating color disc, which is a round disc on which two complementary colors are placed in proper proportion and rapidly revolved about an axis through its center. The two complementary color sensations will fall in rapid succession on the same parts of the retina of the eye and will become blended into one, which will appear as a gray. This gray must be accepted as white, due to the loss of considerable white light in reflection. When using colored beams of light the resultant sensation will be white. In like manner, if on a circular cardboard we paint the seven principal spectral colors and rotate the same very rapidly round its centre axis, the result will be the same as mentioned above when complementary colors were taken.

The study of the contrast of color is a very important subject and reference should be made to any authoritative work on color in which this subject is given in detail. The three principal influences which affect the contrast of color, are the difference of purity, brightness and hue.

Two complementary colors of equal purity and brightness when placed in juxtaposition gain in brilliancy without suffering alteration in hue. When one of these colors is not equal in hue, purity, brightness or truly complementary, this difference will be accentuated. If the two adjacent colors are of different hue, this difference is increased, each appearing to have been mixed with the complementary of the other.

The background upon which the colors are examined, if black, gray, white or colored, will also considerably influence the resultant contrast. A simple instance of simultaneous contrast is illustrated by examining a strip of color on a black, gray or white ground. On black,