Chromic acid is employed in the form of a solution acidified with
acetic or hydrochloric acid, which, in order to obtain the advantages
(_see_ below) attendant upon the use of a solid purifying material,
is absorbed in that highly porous and inert description of silica known
as infusorial earth or "kieselguhr." This substance was first recommended
by Ullmann, and is termed commercially "heratol" As sold it contains
somewhere about 136 grammes of chromic acid per kilo. Cuprous chloride is
used as a solution in strong hydrochloric acid mixed with ferric
chloride, and similarly absorbed in kieselguhr. From the name of its
proposer, this composition is called "frankoline." It will be shown in
Chapter VI. that the use of metallic copper in the construction of
acetylene apparatus is not permissible or judicious, because the gas is
liable to form therewith an explosive compound known as copper acetylide;
it might seem, therefore, that the employment of a copper salt for
purification courts accident. The objection is not sound, because the
acetylide is not likely to be produced except in the presence of ammonia;
and since frankoline is a highly acid product, the ammonia is converted
into its chloride before any copper acetylide can be produced.
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