pile, as is well known, presents a feeble internal resistance, and a
greater electro-motive power than the Bunsen element. Unfortunately, its
energy rapidly decreases, and the alteration of the liquid, as well as the
large deposit of oxide of chromium that occurs on the positive electrode,
prevents its being employed in experiments of quite long duration. Mr.
Grenet, it is true, obviated these two defects by first renewing the
liquid slowly and continuously, and causing a current of air to bubble up
in the pile so as to detach the oxide of chromium in measure as the
deposit formed. Thus improved, the bichromate pile was employed on a large
scale in the lighting of the Comptoir d'Escompte. In an extensive
application like this latter, the use of compressed air for renewing the
liquid can be easily adapted to the bichromate pile, as the number of
elements is great enough to allow of the putting in of all the piping
necessary; but when it is only desired to use this pile for laboratory
purposes, and when there is need of but a small number of elements, it is
impossible to adopt Mr. Grenet's elements in the form required by an
electric lighting installation. It becomes absolutely necessary, then, to
come back to a simpler form, and attempt at the same time to obviate the
defects which are inherent to its very principle. In accordance with this
idea, it will be well to point out the arrangement adopted by Mr. Courtot
for his bichromate of potash piles--an arrangement that is very simple,
but, sufficiently well worked out to render the use of it convenient in a
laboratory.

[Illustration: Fig. 1.--COURTOT'S ARRANGEMENT OF THE BICHROMATE PILE.]

Fig. 1 gives the most elementary form. It consists of an earthen vessel
into which dip four carbon plates connected with each other by a copper
ring which carries one of the terminals. In the center there is a
cylindrical porous vessel that contains a very dilute and feebly