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 Mg would withdraw Zn from a similar solution, and be in its turn withdrawn by Na.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. It always deports itself as a metal. The commonest radical is the negative OH, called hydroxyl, from hydrogen- oxygen. Take away H from the symbol of water, H-O-H, and hydroxyl --(OH) with one free bond is left. If an element takes the place of H, i.e. unites with OH, the compound is called a hydrate. KOH is potassium hydrate. Name NaOH, Ca(OH)2, NH4OH, Zn(OH)2, Al2(OH)6. Is the first part of each symbol above positive or negative? H has an intermediate place in the list. It is a constituent of both acids and bases, and of the neutral substance, water. ORDER. -- Negative or Non-Metallic Elements. Acid-forming with H(usually OH). Oxygen Sulphur Nitrogen Fluorine Chlorine Bromine Iodine Phosphorus Arsenic Carbon Silicon Hydrogen Positive or Metallic Elements. Base-forming with OH. Gold Platinum Mercury Silver Copper Tin Lead Iron Zinc Aluminium Magnesium Calcium Sodium Potassium CHAPTER XIII. ELECTROLYSIS. The following experiment is to be performed only by the teacher, but pupils should make drawings and explain. 63. Decomposition of Water.