Page:Scientific Memoirs, Vol. 1 (1837).djvu/536

524 of equivalents in galvanic decompositions, it appears that we ought to multiply these equivalents by the number of pairs of plates necessary to effect the decomposition. This would perhaps be the true measure, for after all it is necessary to consume a great number of atoms of zinc, in order to decompose a single atom of any other substance less decomposable. In what relates to the difference between quantity and intensity, caloric offers analogies; and in judging of a quantity of gas, we ought always to know its volume and its density. I must here quote another observation of Mr. Faraday, which is found in the Seventh Series, art. 853. He is speaking of a current which is, he says, "powerful enough to retain a platina wire of an inch in thickness red hot in the air during the whole time" (3¾ minutes); and he adds in a note: "I have not stated the length of wire used, because I find by experiment, as would be expected in theory, that it is indifferent. The same quantity of electricity which, passed in a given time, can heat an inch of platina wire of a certain diameter red hot, can also heat a hundred, a thousand, or any length of the same wire to the same degree, provided the cooling circumstances are the same for every part in both cases," &c. This is quite correct, but we may add that it would be necessary to multiply the number of pairs of plates in the same proportion with the length of the wire to obtain a current of the same quantity.

In short in order to heat a wire of 1000 inches to the same degree to which a wire of a single inch would be heated by a single pair of plates, it is necessary to disengage 1000 quantities of gas, proceeding from the same number of pairs. I have thought it right not to suppress this remark, considering that in the practical employment of the voltaic pile œconomy is requisite.

The following is the table of experiments which I have made upon the deviation of the needle with relation to the quantity of gas developed at the surface of the negative plate of a voltaic pair of plates of silver and amalgamated zinc. The specific gravity of the sulphuric acid was 1·25. The bubbles of air rising regularly enough to serve as a measure, I