To justify its name, should be a chromate of strontia, a compound very slightly soluble in water, and not more stable than the zinc chromate. The pigment, however, now sold as strontian yellow is usually formed by admixture, and contains no strontia whatever. Its absence cannot be considered a disadvantage, for the substitute possesses a durability to which the original could lay no claim. Other things being equal, we prefer an original pigment to one compounded, but a good mixture is decidedly better than a bad original. A light primrose, clear and delicate. * * * * * The foregoing comprise those yellows more generally employed, advisedly or not, as the case may be. The following are for the most part not commercially obtainable, a remark that will apply in ensuing chapters to all numbered colours printed in italics. As a rule, these have become obsolete as pigments, or have never been introduced as such. The former could not well be omitted in a work of this kind, and the latter deserve notice as being at least suggestive. At present, many of them must be regarded as mere curiosities, being obtainable only from materials of excessive rarity. In time, however, the sources whence they are derived may possibly be found in greater abundance, and these now fancy products prove of value to the palette. The new metal indium, for instance, furnishes a bright yellow sulphide, like that of cadmium. The colour could not be affected by foul air, and might possess other advantages which would render indium yellow a desirable pigment. With regard to those compounds available for artistic use, but which have not to our knowledge been adopted, several are quite ineligible. It may be thought that they are needlessly referred to, but they are mentioned as a warning and a guide. Strange preparations have been offered as pigments, and sometimes accepted, witness turbith mineral, iodine yellow, &c. In these days of chemistry there is less chance for them, but they are continually submitted to one's notice, their merits being enlarged upon in proportion to their worthlessness. Through an exceptional ignorance they may still gain a place, and it has been deemed, therefore, not superfluous to allude to them. At the same time we do not pretend to exhaust the list, any more than we claim to note all substances possessing colour, but yet not admissible as pigments. Some there are which do not retain that colour on drying; others, whose preparation involves processes too nice, complicated, or expensive, for manufacturing purposes. There are many colours, again, which exist only on paper. We have too often found the imaginations of chemical writers far more vivid than the colours they describe. Gorgeous yellows turn out dingy drabs; dazzling scarlets dirty reds; and brilliant blues dusky slates. As respects colours, most books of science need revising. 52. _Arsenic Yellow_,