discoveries. The question was in the air, and was taken up almost
simultaneously by three able experimenters--a Russian physicist, Prof.
Latchinof, of St. Petersburg, Dr. D'Arsonval, the learned professor of
the College of France, and Commandant Renard, director of the military
establishment of aerostation at Chalais. Mr. D'Arsonval collected
oxygen for experiments in physiology, while Commandant Renard
naturally directed his attention to the production of pure hydrogen.
The solutions of the question are, in fact, alike in principle, and
yet they have been developed in a very different manner, and we
believe that Commandant Renard's process is the completest from an
industrial standpoint. We shall give an account of it from a
communication made by this eminent military engineer, some time ago,
to the French Society of Physics.

_Transformations of the Voltameter._--In a laboratory, it is of no
consequence whether a liter of hydrogen costs a centime or a franc. So
long as it is a question of a few liters, one may, at his ease, waste
his energy and employ costly substances.

The internal resistance of a voltameter and the cost of platinum
electrodes of a few grammes should not arrest the physicist in an
experiment; but, in a production on a large scale, it is necessary to
decrease the resistance of the liquid column to as great a degree as
possible--that is to say, to increase its section and diminish its
thickness. The first condition leads to a suppression of the platinum,
and the second necessitates the use of new principles in the
construction of the voltameter. A laboratory voltameter consists
either of a U-shaped tube or of a trough in which the electrodes are
covered by bell glasses (Fig. 1, A and B). In either case, the
electric current must follow a tortuous and narrow path, in order to