ther hand, the salt can be
separated from the iron quite easily. Thus, if several grams of the
mixture are placed in a test tube, and the tube half filled with water
and thoroughly shaken, the salt dissolves in the water. The iron
particles can then be filtered from the liquid by pouring the entire
mixture upon a piece of filter paper folded so as to fit into the
interior of a funnel (Fig. 1). The paper retains the solid but allows
the clear liquid, known as the _filtrate_, to drain through. The iron
particles left upon the filter paper will be found to be identical with
the original iron. The salt can be recovered from the filtrate by
evaporation of the water. To accomplish this the filtrate is poured into
a small evaporating dish and gently heated (Fig. 2) until the water has
disappeared, or _evaporated_. The solid left in the dish is identical in
every way with the original salt. Both the iron and the salt have thus
been recovered in their original condition. It is evident that no new
substance has been formed by rubbing the salt and iron together. The
product is called a _mechanical mixture_. Such mixtures are very common
in nature, almost all minerals, sands, and soils being examples of this
class of substances. It is at once apparent that there is no law
regulating the composition of a mechanical mixture, and no two mixtures
are likely to have exactly the same composition. The ingredients of a
mechanical mixture can usually be separated by mechanical means, such as
sifting, sorting, magnetic attraction, or by dissolving one constituent
and leaving the other unchanged.

[Illustration: Fig. 2]

DEFINITION: _A mechanical mixture is one in which the constituents
retain their original properties, no chemical action having taken place
when they were brought together._

~Chemical compounds.~ If iron filings and powdered sulphur are thoroughly
ground together in a mortar, a yellowish-green substance results. It
might easily be taken to be a new body; but as in the case of the iron
and salt, the ingredients can readily be separated. A magnet draws out
the iron. Water does not dissolve the sulphur, but other liquids do, as,
for example, the liquid called carbon disulphide. When the mixture is
treated with carbon disulphide the iron is left unchanged, and the
sulphur can be obtained again, after filtering off the iron, by
evaporating the liquid. The substance is, therefore, a mechanical
mixture.

If now a new portion of