cast-iron moulds. These moulds should be dry and smooth. They act best
when warmed and oiled or black-leaded.

Air entering through the fire-bars of a furnace and coming in contact
with hot coke combines with it, forming a very hot mixture of carbonic
acid and nitrogen; this ascending, comes in contact with more coke, and
the carbonic acid is reduced to carbonic oxide; at the top of the
furnace, or in the flue, the carbonic oxide meeting fresh air, combines
with the oxygen therein and re-forms carbonic acid. In the first and
third of these reactions, much heat is evolved; in the second, the
furnace is cooled a little. It must always be remembered, that the
carbonic oxide of the furnace gases is a reducing agent. When these
gases are likely to exert a prejudicial effect, and a strongly oxidising
atmosphere is required, the work is best done in a _muffle_.

FOOTNOTES:

[3] There is an exception to this, as when the slag is liable to be
acted on when exposed to the air and to the gases of the furnace. In
this case a layer of fused common salt floating on the slag, so as to
protect it from the air and furnace gases, is a distinct advantage.




CHAPTER III.

WET GRAVIMETRIC METHODS.