two previous ones, dissolving a small piece of Pb, and using a
strip of Zn to precipitate the Pb. 3 Pb + 8 HNO3 - 3 Pb (NO4)2 +
4 Ha0 + 2 NO. Pb (NO3) 2 + Zn = ? h.

62. Explanation. -These experiments show that Cu will replace Ag
in a solution of AgNO3, that Pb will replace and deposit Cu from
a similar compound, and that Zn will deposit Pb in the same way.
They show that the affinity of Zn for (NO3) is stronger than
either Ag, Cu, or Pb. We. express this affinity by saying that Zn
is the most positive of the four metals, while Ag is the most
nega- tive. Cu is positive to Ag, but negative to Pb and Zn.
Which of the four elements are positive to Pb, and which
negative? Mg would withdraw Zn from a similar solution, and be in
its turn withdrawn by Na. The table on page 43 is founded on this
relation. A given element is positive to every element above it
in the list, and negative to all below it.

Metals are usually classed as positive, non-metals as negative.
Each in union with O and 1=I gives rise to a very important class
of compounds,=--the negative to acids, the positive to bases.

In the following, note whether the positive or the negative
element is written first:--HCl, Na20,-As2S3, -MgBr2, Ag2S. Na2SO4
is made up of two parts, Na2 being positive, the radical SO4
negative. Like elements, radicals are either positive or
negative. In the following, separate the positive element from
the negative radical by a vertical line: Na2CO3, NaNO3, ZnSO4,
KClO3.

The most common positive radical is NH4, ammonium, as in NH4Cl.