化學 1834

電學實驗研究·第七輯

麥可·法拉第

化學變化，由電量來計量。

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In depth · the introduction

讓電流穿過一種化學物，它就會分解——而法拉第發現，究竟分解出多少，竟精確到毫克，全由你送進去多少電來決定。

核心想法

讓電流穿過一種含有溶解化合物的液體，化合物便會裂開，碎片聚到浸在其中的兩片金屬上——一把便宜的叉子鍍上一層真銀，靠的正是這個。法拉第的發現是：這個過程並不含糊，而是精確的——電量加倍，得到的金屬就恰好加倍。析出的量，與流過的總電荷成正比。

他還發現了第二條規則。同一份電量析出的不同元素，其質量按各自的「當量」成比例。銀——它每需一份電荷都很「重」——在同一電流下析出的質量，遠多於輕盈的鋁。電荷，就此成了化學家能夠稱量的東西。

它是如何誕生的

1830 年代初，法拉第——曾是裝訂學徒，那時已是倫敦皇家研究院的明星實驗家——正在理清：來自電池、火花機、磁鐵與電魚的電，是不是同一回事。他造了一些小電池，用它們析出的金屬之重、或產生的氣體之量，來記錄通過了多少電。

一只接一只，一種溶液接一種溶液，只有一條規則始終成立：化學變化只隨電荷而變，別無其他。為了描述所見，他需要舊理論尚未佔有的詞，於是寫信給劍橋學者威廉·休厄爾，請他造詞——離子、陽極、陰極、電極、電解質——這些詞如今在每一堂化學課上都被說起。

它為何重要

它讓電可以用化學家的天平來測量，把電鍍、金屬精煉與電池製造，從憑經驗的猜測變成精確的工藝。而那份奇異的精確——每一「當量」物質對應一份固定的電——是一個早期的暗示：電本身也是按可數的單位出現的。幾十年後，這個暗示，化作了電子。

一個可以想像的畫面

把它想成一台「原子自動販賣機」。每一枚硬幣，是一份電荷，由一個電子攜帶。釋放一個銀離子要一枚硬幣，一個銅離子要兩枚，一個鋁離子要三枚。投進固定數目的硬幣，你便能預先準確說出：每種金屬會掉出多少個原子——而因為不同元素的原子輕重不同，同樣的硬幣，買到的質量大不相同。法拉第定律，就是這台機器精確的價目表。

它的位置

伏打 1800 年的電池讓穩定電流成為可能，戴維曾用它把化合物撕開、分離出鈉和鉀。法拉第——戴維當年的助手——把這種定性的本領變成了一條定律。他那份固定的「每當量對應的電荷」，向前指向斯托尼與湯姆森的電子，也指向能斯特（1889），後者將為這些同樣的離子所產生的電壓標上數字——而法拉第所造的詞，至今仍在標注你手中每一節電池的陽極、陰極與電解質。

The original document

Original source text

Michael Faraday · Philosophical Transactions of the Royal Society of London 124 (1834): 77–122 · Royal Institution, London

The Seventh Series is the paper in which electrolysis became quantitative. Faraday had spent the previous year showing that the electricity of a voltaic battery, a friction machine, a magnet and an electric fish are one and the same agent; here he asks how much chemical work a given amount of it performs — and answers: a definite, weighable amount.

1 · A meter made of chemistry

First he needed to measure electricity itself. His “volta-electrometer” is a cell in which the gas evolved, or the metal deposited, records the total quantity of electricity that has passed. Placed in series in a circuit, identical cells always showed identical amounts — the measure did not depend on the size of the plates, the strength of the solution, or the kind of generator.

2 · Definite electro-chemical action

From cell after cell he drew a single law of definite action: the chemical effect is fixed by the quantity of electricity, and by nothing else.

…the chemical power of a current of electricity is in direct proportion to the absolute quantity of electricity which passes.

And when the same current was sent through different electrolytes, the weights set free stood in the ratio of what Faraday called their electro-chemical equivalents — numbers that proved to coincide with the ordinary chemical equivalents already known to chemists. In modern symbols the whole result is m = (Q·M)/(z·F), with the Faraday constant F = 96485 coulombs per mole.

3 · A new language for electrolysis

To describe a process he pictured as matter travelling through the liquid to the plates — not as an attraction reaching out from the “poles” — Faraday wanted words free of the old theory. With the Cambridge polymath William Whewell he coined them: the plates became the electrode, the anode and the cathode; the decomposed substance an electrolyte; and the migrating particles ions, dividing into anions and cations.

I propose to distinguish these bodies by calling those anions which go to the anode of the decomposing body; and those passing to the cathode, cations; and when I have occasion to speak of these together, I shall call them ions.

[ … ]

Faraday measured one fixed dose of electricity for each “equivalent” of matter, but did not claim what carried it. Half a century later Helmholtz read in these very numbers the existence of “atoms of electricity,” and the electron — the coin Faraday had unknowingly been counting — was named by Stoney and found by J. J. Thomson.

Royal Institution, London · 1834