Lithopone whites, showing by analysis less than 30 per cent zinc sulphide, are inferior grades made by adding barytes and possibly other extenders, such as carbonate of lime, china clay or sulphate of lime in varying proportions. To find the contents of zinc sulphide, which imparts body and resisting power to color in the pigment, it is not always necessary to submit samples to analysis, but comparative value may be determined by physical test by comparing various samples with an adapted standard, making rubouts with color. The one showing most resisting power to color will be the one containing greatest percentage of zinc sulphide. Still the lithopone may have the normal percentage of zinc sulphide and yet be rather weak in resistance, and this is the case when the material has not been furnaced long enough, retaining some water of combination. When this is the case, it may be determined by the excessive quantity of vehicle required in grinding and by the rather slimy feel under the brush when worked out as a paint. That sort of lithopone is unfit for use in paste whites, that are afterwards thinned with oil or varnish, nor is it fit as a base for the modern flat wall finishes, and it is really preferred only by shade makers for painting shade cloth, because of the greater volume of oil and volatile matter it carries and the smaller quantity of color required to produce tints. Lithopone is readily recognized from zinc oxide by its smaller bulk and when the powders are treated by wetting up with dilute hydro-chloric acid. In the case of lithopone, sulphuretted hydrogen is evolved immediately under slight effervescence, while zinc oxide remains dormant and emits no odor, excepting that of the acid. As is now well known, lithopone or any other compounds of zinc sulphide are sensitive to direct sunlight, and while some makers claim that they have been able to produce a sunproof article, practical men are still very skeptical on that point. The writer has found several samples that were actually sunproof, yet in one instance the second exposure trial was a disappointment, while in several other cases the price was prohibitive in comparison with zinc oxide. There is no question but that vast improvements have been made during the past ten years in making the material more stable. The paint made from lithopone is unaffected in its whiteness by sulphuretted hydrogen gas or sulphuric acid vapors, but must be kept free of lead or copper salts, as these will invariably discolor it. A quick test for its resistance to strong light or direct sunlight may be made by rubbing up each sample with white damar varnish in pestle and mortar and applying the several samples on a board previously grounded with zinc white, side by side, and exposing the board to the direct rays of the sun, shading one-half of the strips of paint in the most convenient way, so that whatever discoloration takes place during the setting or drying process may be readily observed.