it. A "stopping"
of ordinary copal varnish seems to be the usual thing, but a thin coat
of wax or paraffin or photographic (black) varnish does practically as
well.

I do not propose to deal with the subject of electrotyping to any
extent, for if practised as an art, a good many little precautions are
required, as the student may read in Gore's Electro-metallurgy. The
above instructions will be found sufficient for the occasional use of
the process in the construction of apparatus, etc. There is no
advantage in attempting to hurry the process, a current density of
about ten amperes per square foot being quite suitable and
sufficiently low to ensure a solid deposit.

Sec. 143. Blacking Brass Surfaces.

A really uniform dead-black surface is difficult to produce on brass
by chemical means. A paste of nitrate of copper and nitrate of silver
heated on the brass is said to give a dead-black surface, but I have
not succeeded in making it act uniformly. For optical purposes the
best plan is to use a paint made up of "drop" black, ground very fine
with a little shellac varnish, and diluted for use with alcohol. No
more varnish than is necessary to cause the black to hold together
should be employed.

In general, if the paint be ground to the consistency of very thick
cream with ordinary shellac varnish it will be found to work well when
reduced by alcohol to a free painting consistency.

A very fine gray and black finish, with a rather metallic lustre, may
be easily given to brass work. For this purpose a dilute solution of
platinum tetrachloride (not stronger than 1 per cent) is prepared by
dissolving the salt in distilled water. The polished brass work is
cleaned by rubbing with a cork and strong potash till all grease has
disappeared, as shown by water standing uniformly on the metal and
draining away without gathering into drops.

After copious washing the work is wholly immersed in a considerable
volume of the platinum tetrachloride solution at the ordinary
temperature. After about a quarter of an hour the brass may be taken
out and washed. The surface will be found to be nicely and uniformly
coated if the above instructions have been carried out, but any
finger-marks or otherwise dirty places will cause irregularity of
deposit. If the process has been successful it will be found that the
deposit adheres perfectly, hardly any of it being removed by vigorous
rubbing with a cloth. If the deposit is allo