ntrated
sulphuric acid. This dissolves the silver, while the gold is not
attacked.

~Physical properties.~ Gold is a very heavy bright yellow metal,
exceedingly malleable and ductile, and a good conductor of electricity.
It is quite soft and is usually alloyed with copper or silver to give it
the hardness required for most practical uses. The degree of fineness is
expressed in terms of carats, pure gold being twenty-four carats; the
gold used for jewelry is usually eighteen carats, eighteen parts being
gold and six parts copper or silver. Gold coinage is 90% gold and 10%
copper.

~Chemical properties.~ Gold is not attacked by any one of the common
acids; aqua regia easily dissolves it, forming gold chloride (AuCl_{3}),
which in turn combines with hydrochloric acid to form chlorauric acid
(HAuCl_{4}). Fused alkalis also attack it. Most oxidizing agents are
without action upon it, and in general it is not an active element.

~Compounds.~ The compounds of gold, though numerous and varied in
character, are of comparatively little importance and need not
be described in detail. The element forms two series of salts
in which it acts as a metal: in the aurous series the gold is
univalent, the chloride having the formula AuCl; in the auric
series it is trivalent, auric chloride having the formula
AuCl_{3}. Gold also acts as an acid-forming element, forming
such compounds as potassium aurate (KAuO_{2}). Its compounds
are very easily decomposed, however, metallic gold separating
from them.

EXERCISES

1. From the method of preparation of platinum, what metal is likely to
be alloyed with it?

2. The "platinum chloride" of the laboratory is made by dissolving
platinum in aqua regia. What is the compound?

3. How would you expect potassium aurate and platinate to be formed?
What precautions would this suggest in the use of platinum vessels?

4. Why must gold ores be roasted in the chlorination process?

CHAPTER XXXII

SOME SIMPLE ORGANIC COMPOUNDS

~Division of chemistry into organic and inorganic.~ Chemistry is usually
divided into two great divisions,--organic and inorganic. The original
significance of these terms was entirely different from the meaning
which they have at the present time.

1. _Original significance._ The division into organic and inorganic was
originally made because it was believed that those substances which
constitute the essential p