As an example of this, mention may be made of iron oxide, which occurs in Nature in the form of various minerals which can be made into pigments by mechanical treatment. In many cases, this treatment has already been carried out by Nature, and deposits of iron oxide are found in which the material has only to be incorporated with a vehicle to make it fit for immediate use as a painters’ colour. Moreover, the same oxide is obtained, in large quantities, as a by-product of the treatment of other minerals. From the point of view of chemical composition, this by-product is of very low value, by reason of the large supplies of native oxide available. By means of a very simple chemical treatment, however, this by-product oxide can be considerably improved in commercial value, being, in many cases, convertible, by merely heating it to certain temperatures, into a variety of colours which sell at remunerative prices. Consequently, in view of the present condition of the chemical industry, the term “earth colours” can be enlarged to include a number of waste products which fetch good prices as colours, though otherwise practically valueless in themselves. The number of earth pigments is very large, and comprises representatives of all the principal colours. For painting purposes, few pigments beyond the earth colours were known to the ancients; and most of the colours in the paintings which have come down to us from antiquity are pure earth pigments, thus affording proof of their great durability, having retained their freshness unimpaired for hundreds--and some for thousands--of years. The earth colours might be divided into such as occur ready-formed in Nature, and require only mechanical preparation, and which either require special treatment (_e. g._ calcining), or are artificial products (like the iron oxide mentioned above). Since, however, such a classification would not advantage our knowledge of the nature of this class of colours, it appears useless and superfluous, and we will therefore simply confine ourselves to arranging the earth pigments according to their colour--white, yellow, red, etc. Adopting this classification, the following minerals and chemical products may be considered as earth colours:-- _White._--These include the varieties of calcium carbonate, such as chalk, marble, precipitated chalk, calcium phosphate, calcium sulphate (in the form of gypsum, alabaster, muriacite and the precipitated gypsum produced as a by-product in many chemical works), heavy spar, the different varieties of clay, and magnesia. _Yellow._--This group comprises ferric hydroxide (hydrated oxide of iron) in the form of the various minerals known as ochre; all the preparations chiefly composed of this hydroxide, and all those prepared by artificial means. A very important member of this group is orpiment; the other arsenical compounds frequently met with native, being however, on account of their poisonous properties, no longer used as pigments.