In the description of the raw materials, if we were to overstep the line drawn here, we could include a great variety of compounds, those, for example, used in the manufacture of the so-called aniline dyes. These substances form, however, as we have stated, the object of a particular branch of manufacture, which forms a separate division of colour chemistry, but with which is not to be confounded what has been hitherto designated the manufacture of colours. CHAPTER IV. ASSISTANT MATERIALS. =Water=, H₂O = 18.[1]—This substance plays a tremendous part in colour making; almost all the substances which are used in solution are dissolved in water; the removal from precipitates of admixed foreign bodies, the so-called washing, is always accomplished with water. The chemist does not understand by water quite that liquid which in general speech is so designated. We must consider the water which is at the disposal of the colour maker. [1] We append the chemical formula and the molecular weight to the description of each compound. Water, in the chemical meaning of the word, is a liquid composed only of hydrogen and oxygen, and leaving no residue when evaporated. Such water is not found in nature; it can only be obtained by distillation of well or river water. The water which falls in long continued rain, or is obtained by melting snow, is most nearly like distilled water; it contains only small quantities of dissolved substances, and generally such as would be without influence in colour making. Water of this description is available for but a limited use; the large quantities of water required in a colour works must be taken from springs or streams. These waters contain, however, more or less large amounts of dissolved salts, which act in a marked manner upon the substances dissolved in them. In almost all spring and well waters is found carbonate of lime; such waters are called “hard”. River water contains generally little carbonate of lime; it is then called “soft water”. The influence of the carbonate of lime is especially evident when salts of lead, copper, iron and other heavy metals are dissolved in water; the carbonate of the particular metal gradually separates from solution, and the liquid becomes very turbid.