Pyrogallic acid 25 "
Citric acid 20 "
Alcohol of 36 deg. 50 cub. cents.

The process is carried on exactly as if developing an ordinary negative;
but the action of the developer is stopped at the precise moment when
the positive has acquired intensity sufficient for the purpose for which
it is to be used. Fixing, varnishing, etc., are then carried on the
usual way. The great advantage of this process consists in the fact of
its rendering positives of much greater delicacy than those that are
taken by contact; and, on the other hand, by means of it we are able to
avoid two distinct operations, when for certain kinds of work we require
positive plates where a negative would be of no service. M. V. Rau,
the assistant who has carried out this process under the direction of
Captain Bing, has described it in a work which has just been published
by M. Gauthier-Villars.

_Experiments of Captain Bing on the Sensitiveness of Coal Oil_.--The
same Captain of Engineers has undertaken a series of very interesting
experiments on the sensitiveness to light of one or two substances to
which bitumen probably owes its sensitiveness, but which, contrary to
what takes place with bitumen, are capable of rendering very beautiful
half tones, both on polished zinc and on albumenized paper. These
sensitive substances are extracted by dissolving marine glue or coal-tar
in benzine. By exposure to light, both marine-glue and coal-tar turn of
a sepia color, and, in a printing-frame, they render a visible image,
which is not the case with bitumen; their solvents are in the order of
their energy; chloroform, ether, benzine, turpentine, petroleum spirit,
and alcohol. Of these solvents, benzine is the best adapted for reducing
the substances to a fluid state, so as to enable them to flow over the