316. A still further improvement in this form of apparatus consists in wetting a piece of filtering paper in the solution to be experimented on, and placing that under the points _p_ and _n_, on the glass: the paper retains the substance evolved at the point of evolution, by its whiteness renders any change of colour visible, and allows of the point of contact between it and the decomposing wires being contracted to the utmost degree. A piece of paper moistened in the solution of iodide of potassium and starch, or of the iodide alone, with certain precautions (322.), is a most admirable test of electro-chemical action; and when thus placed and acted upon by the electric current, will show iodine evolved at _p_ by only half a turn of the machine. With these adjustments and the use of iodide of potassium on paper, chemical action is sometimes a more delicate test of electrical currents than the galvanometer (273.). Such cases occur when the bodies traversed by the current are bad conductors, or when the quantity of electricity evolved or transmitted in a given time is very small. 317. A piece of litmus paper moistened in solution of common salt or sulphate of soda, was quickly reddened at _p_. A similar piece moistened in muriatic acid was very soon bleached at _p_. No effects of a similar kind took place at _n_. 318. A piece of turmeric paper moistened in solution of sulphate of soda was reddened at _n_ by two or three turns of the machine, and in twenty or thirty turns plenty of alkali was there evolved. On turning the paper round, so that the spot came under _p_, and then working the machine, the alkali soon disappeared, the place became yellow, and a brown alkaline spot appeared in the new part under _n_. 319. On combining a piece of litmus with a piece of turmeric paper, wetting both with solution of sulphate of soda, and putting the paper on the glass, so that _p_ was on the litmus and _n_ on the turmeric, a very few turns of the machine sufficed to show the evolution of acid at the former and alkali at the latter, exactly in the manner effected by a volta-electric current. 320. All these decompositions took place equally well, whether the electricity passed from the machine to the foil _a_, through water, or through wire only; by _contact_ with the conductor, or by _sparks_ there; provided the sparks were not so large as to cause the electricity to pass in sparks from _p_ to _n_, or towards _n_; and I have seen no reason to believe that in cases of true electro-chemical decomposition by the machine, the electricity passed in sparks from the conductor, or at any part of the current, is able to do more, because of its tension, than that which is made to pass merely as a regular current.