The most valuable white for the colour maker is evidently that which reflects, unaltered, all the rays of light; it is the most brilliant, and free from every tinge of colour. The physical condition of the substance is most important. Solid substances are either crystalline, that is, possess definite shapes formed according to a regular law, or they are amorphous, that is, are composed of irregularly formed particles. Snow and white lead may serve as representatives of these two classes. Snow is composed of small colourless crystals of ice, the flat surfaces of which reflect, undecomposed, the light falling on them. The smaller are the crystals, the more pure appears to us the whiteness of the snow, and the thinner is the layer of snow required to produce the sensation of whiteness. But if the snow crystals are larger, the white appears to have a bluish tinge, and only a thick layer of snow is opaque. White lead, being an amorphous substance in a condition of very fine division, reflects the light very regularly, so that a thin layer of white lead appears quite opaque. Among the artificial pigments, crystalline or amorphous, exactly the same conditions hold good as between snow and white lead. Of amorphous pigments a very thin layer is in most cases sufficient to make the surface upon which they are spread invisible, or, as the technical expression runs, “to cover,” whilst crystalline substances possess a smaller covering power. A striking example of this is seen by a comparison of two white pigments, white lead and “patent white” (lead oxychloride). The former is amorphous, the latter crystalline. Both are completely colourless and reflect white light, but in consequence of its amorphous condition and finer particles, white lead possesses far greater covering power than “patent white”. Among all other colours the same rule holds. Amorphous pigments have always a greater covering power than crystalline. The smaller the crystals of the latter the greater is their covering power, so that in preparing pigments of a crystalline character care must be taken to make the crystals as small as possible. From the above definition of white pigments it follows that an immense number must exist, since every colourless substance in a state of fine division appears white. Generally only those bodies are used which are insoluble in water, or almost insoluble, and which possess great covering power. The following may be mentioned as white pigments, only a few of which are in use: white lead, white zinc, permanent white, lead oxychloride, lead sulphate and sulphite, zinc oxychloride, lead antimoniate, antimony white, tin white, tungsten white, and in addition certain earths, pipe clay, china clay, etc. Several of these pigments are far too expensive for ordinary use, and have no advantage over much cheaper pigments except for very special purposes, such, for example, as bismuth white for cosmetics.