709.  The third form of apparatus contains both electrodes in the same tube; the transmission, therefore, of the electricity, and the consequent decomposition, is far more rapid than in the separate tubes.  The resulting gas is the sum of the portions evolved at the two electrodes, and the instrument is better adapted than either of the former as a measurer of the quantity of voltaic electricity transmitted in ordinary cases.  It consists of a straight tube (fig. 64.) closed at the upper extremity, and graduated, through the sides of which pass platina wires (being fused into the glass), which are connected with two plates within.  The tube is fitted by grinding into one mouth of a double-necked bottle.  If the latter be one-half or two-thirds full of the dilute sulphuric acid (706.), it will, upon inclination of the whole, flow into the tube and fill it.  When an electric current is passed through the instrument, the gases evolved against the plates collect in the upper portion of the tube, and are not subject to the recombining power of the platina.

710.  Another form of the instrument is given at fig. 65.

711.  A fifth form is delineated (fig. 66.).  This I have found exceedingly useful in experiments continued in succession for days together, and where large quantities of indicating gas were to be collected.  It is fixed on a weighted foot, and has the form of a small retort containing the two electrodes:  the neck is narrow, and sufficiently long to deliver gas issuing from it into a jar placed in a small pneumatic trough.  The electrode chamber, sealed hermetically at the part held in the stand, is five inches in length, and 0.6 of an inch in diameter; the neck about nine inches in length, and 0.4 of an inch in diameter internally.  The figure will fully indicate the construction.

712.  It can hardly be requisite to remark, that in the arrangement of any of these forms of apparatus, they, and the wires connecting them with the substance, which is collaterally subjected to the action of the same electric current, should be so far insulated as to ensure a certainty that all the electricity which passes through the one shall also be transmitted through the other.

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713.  Next to the precaution of collecting the gases, if mingled, out of contact with the platinum, was the necessity of testing the law of a definite electrolytic action, upon water at least, under all varieties of condition; that, with a conviction of its certainty, might also be obtained a knowledge of those interfering circumstances which would require to be practically guarded against.