The amount of mechanically mixed water will naturally vary with the conditions to which the clay has been subjected; it will be greatest in wet situations and will diminish as the clay is allowed to dry. The 'combined water,' on the contrary, appears to be a function of the true clay present in the material, and reaches its highest proportions in the china clays and kaolins, which contain approximately 13 per cent. On heating a clay to 105° C. the moisture or mechanically mixed water is evaporated, but the combined water remains unaffected[4] until the temperature is raised to more than 600° C., when it is driven off and the clay is converted into a hard stone-like mass with properties entirely different from those it previously possessed (see Chapter VI). [Footnote 4: Strictly, there is a slight loss at lower temperatures, but it is too small to be important.] The Physical Characters of Clays. The physical characters of clays are of far more interest and importance than their chemical ones, though the two are naturally connected in many ways, and just as the chemical composition of clays is a subject of extreme complexity so is a study of many of their physical properties. Hence only a few of the more important characteristics can be mentioned here: for further details the reader must consult a larger treatise (2). Clays are moderately soft, solid bodies, particularly when moistened, and can usually be cut with a knife, though some indurated clays and shales are almost as hard as felspar. Their apparent specific gravity varies greatly, some clays being much more porous than others, but the true specific gravity is usually between 2·5 and 2·65; it is similar to that of quartz and slightly lower than that of felspar and mica. Many clays appear to be devoid of structure, but those obtained from a considerable depth below the surface are frequently laminated and have a structure not unlike that of mica. This will be discussed later. Examined under a microscope, clays are seen to consist of grains of a variety of sizes, the largest of which will usually be found to be composed of adventitious materials such as sand, quartz, felspar, mica, chalk and limestone. The smallest particles--to which clays owe their chief characteristics--are so minute as to make any examination of their shape very difficult, but they are usually composed of minute crystalline plates together with a much larger proportion of apparently amorphous material. The exact nature of both the crystals and the amorphous material is still unknown in spite of many investigations; in the purer clays both forms of substance appear to have the same chemical composition, viz. that of _kaolinite_ (H4Al2Si2O9), which the crystalline portion closely resembles. Clays emit a characteristic yet indefinable odour when moist; the cause of this is very imperfectly understood, though it is not improbably due to decomposing organic matter, as this occurs in most clays.