Since Poggendorff in 1842 thought of substituting in the Bunsen battery a solution of bichromate of potash and sulphuric acid for nitric acid, and of thus making a single liquid pile of it, in suppressing the porous vessel, his idea has been taken up a considerable number of times.  Some rediscovered it simply, while others, who were better posted in regard to the work of their predecessors, took Poggendorff’s pile as he conceived it, and, considering the future that was in store for it, thought only of modifying it in order to render it better.  Among these, Mr. Grenet was one of the first to present the bichromate of potash pile under a truly practical form.  As long ago as 1856, in fact, he gave it the form that is still in use, and that is known as the bottle pile.  Thus constructed, this 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 acidulated solution of bichromate of potash into which dips a prism of zinc, which may be lifted by means of a rod when the pile ceases to operate.  It is true that the presence of the porous vessel in the bichromate of potash element increases the internal resistance, but, as an offset, although it decreases the discharge, it secures constancy and quite a long duration for it.