162 PHYSICS. 
of the receiver, thence to escape mixed together through the bent tube. 
It is to be observed that oxygen can only be procured in the gaseous 
state when the positive pole consists of one of the noble metals (gold or 
platinum best of all): under other circumstances the oxygen unites with 
the substance of the positive metallic pole, forming an oxyde. 
All oxydes and combinations of oxygen, likewise alkalies and salts, are 
decomposable by the galvanic current in the same manner as water. The 
decomposition of salts in which the acid appears at the positive pole, and 
the base at the negative, may be exhibited by the following experiment : 
Fill a U-formed bent tube (pl. 20, fig. 66) with a solution of salt, colored 
violet by litmus, and immerse in one leg the positive, and in the other leg 
the negative pole of a battery. On establishing a current, the fluid at the 
positive pole will become red, that at the negative blue, showing that free 
acid has passed to the former, and alkali to the latter. 
One of the most important applications of the chemical action of 
galvanism is to be found in the recently discovered art of galvanoplastics 
or electrotype. In this a constant battery with porous partitions is 
required, that of Becquerel or Daniel answering very well, with slight 
modifications. The theory of the electrotype rests upon the decomposition 
of certain salts, as sulphate of copper, in which the sulphuric acid and 
oxygen form new combinations, and the copper is precipitated in the 
metallic state upon the negative element. If this latter have a definite 
surface, a perfect cast of it will be made by the copper deposited. In this 
way copies of coins, medals, engraved plates, é&c., may readily be taken. 
Pl. 20, fig. 67, represents a convenient form of battery for the electrotype. 
In a large glass cylinder of six or eight inches in diameter, a second and 
narrower one is suspended, open above, but closed below by a piece of 
bladder. To sustain the inner cylinder a wire is twisted tightly about it, 
and from this ring of wire proceed three arms which rest on the edge of the 
outer cylinder, as seen in the figure. The inner vessel is filled with very 
dilute sulphuric acid, and the outer with a solution of sulphate of copper ; 
cross-pieces of wood in the inner cylinder support a block of zine, to which 
is soldered the copper wire, c, thus forming a connexion with the mercury 
cup on the outside. A second wire, dipping in the same mercury cup, is 
soldered to the metallic substance of the mould immersed in the sulphate 
of copper. This substance must be something more electro-negative than 
zinc, and may consist of Rose’s fusible metal (composed of copper, bisrauth, 
and lead), or tin foil, as also of gypsum, wax, stearine, or a mixture of the 
two latter; these being non-conductors, must be coated with graphite or 
silver bronze. One of these substances being selected, a cast of the object 
to be copied is taken in it, and after coating all those parts of the matrix 
of which no copy is desired, with some resinous solution, it is to be placed 
in the battery as above mentioned. A due connexion between the poles 
being established by the mercury in the cup, a slow deposit of copper will 
take place on the matrix, which may amount to a considerable thickness in 
the course of some hours or days. 
It is not copper alone that may be deposited from its solutions in a 
336 
