Moreover, the apparatus is only suitable for calcining at medium temperatures; and when highly calcined products are in question, the operation is best performed in fire-clay cylinders, or in thick cast-iron drums, similar to gas retorts, built into a furnace. Other devices for calcining ochre will be described later. OCHRES FROM VARIOUS DEPOSITS As previously stated, ochres are frequently met with in Nature, both in the immediate vicinity of iron ore, and also at considerable distances from such deposits. In the latter case, the ochre must be assumed to be the decomposition products of ferruginous minerals and to have been carried off by water until the latter became stagnant and allowed the ochre to settle down. In their method of deposition these ochres are therefore analogous to clay, and they, too, often contain large quantities of extraneous minerals, which have given rise to the diversified substances grouped under the name of ochre. Although ochres are so widespread in Nature, only certain kinds, found in certain localities, have acquired a high reputation. For the most part, these ochres are such as have already been prepared in a high degree, by Nature, for the purpose for which they are employed. Thus, we find that all the ochres which have acquired a high repute among painters for particular beauty of tone and permanence, are distinguished by two properties: a high content of ferric hydroxide and great purity. The former of these properties imparts brightness of colour; and such products will furnish, on calcination, a wide range of colour shades. When, as is the case with the finer qualities of ochre, the mineral contains only a very small proportion of impurities, there is no difficulty in bringing it, by simple grinding or levigation, into a condition in which it is at once fit for use as a pigment. The Italian ochres have, for long ages, enjoyed a high reputation for their beauty of colour and permanence. This category includes, for example, the renowned Siena earth, Roman earth, Italian umber, and other ochre colours. This high renown is probably due less to the inherent properties of the mineral than to the circumstance that the art of painting attained a high state of development at an early period, and that the artists paid special attention to the use of bright and permanent colours for their work. Although, at present, many deposits of ochre are known that are quite able to compete, on the score of beauty, with the best Italian products, the good name of these latter has nevertheless been maintained. It is true that the name of Italian ochre is often merely borrowed, for application to a product originating in some other country, varieties of terra di Siena, for instance, being put on the market that have actually been derived from deposits in Germany.