CaO, lime, instead of calcium oxide. Al2O3, alumina, instead of aluminium trioxide. SiO2, silica, instead of silicon dioxide. Na2O, soda, instead of sodium oxide. K2O, potash, instead of potassium oxide. MgO, magnesia, instead of magnesium oxide. In _analysing_ a body, the first step consists in determining the nature of the elementary substances contained therein. That may be accomplished in the _dry way_ by means of the blowpipe and accessories, as explained in the last chapter. Such an examination, as previously remarked, is known as a _qualitative_ analysis. Or, it may be accomplished in the _wet way_ by ordinary chemical examination. The next step is to determine the amount of the constituents present, and that is known as a _quantitative_ analysis. In making a qualitative analysis, the chemist is assisted by the knowledge that certain basic substances and certain acids produce peculiar phenomena in the presence of known substances or preparations termed _reagents_. There is a great difference between a _chemical compound_ and a simple _mixture_ of elements; and it is not always easy (_e.g._, some alloys) to say whether a substance is in the one state or the other. This distinction is well exemplified by the air we breathe. The chemist finds by analysis that the air is nearly constant in composition, containing essentially in 100 parts 76.8 by weight of nitrogen (including about 1 per cent. of the recently-discovered element, argon), and 23.2 of oxygen. Small proportions of water vapour, carbon dioxide, etc., may be ignored for our present purposes. In view of this comparatively uniform composition, the question at first arises as to whether the air is, or is not, a chemical compound? The answer is in the negative, for, amongst other things, it can be shown that the ratio of 76.8 to 23.2 is not that of the atomic weights of the two elements present, viz., 14 : 16, nor of any simple multiples of these. We will now quote a few analyses of well-known earths, and explain each in turn: _Chemical Composition of China-clays._[8] -----------+---------+---------------+------------- | Kaolin. |Kaolin average.|Sandy Kaolin. +---------+---------------+------------- Silica | 46.32 | 44.60 | 66.68 Alumina | 39.74 | 44.30 | 26.08 Iron oxide | .27 | .20 | 1.26 Lime | .36 } | 1.60 | .84 Magnesia | .44 } | | trace Water | 12.67 | 8.74 | 5.14 -----------+---------+---------------+-------------