化學 1811

論確定物體基本分子相對質量的方法

亞默迪歐·亞佛加厥

任意氣體，同溫同壓下，相同體積裡藏著相同數目的分子。

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In depth · the introduction

拿兩個一樣大的氣球，裡頭裝著氣體，稱一稱——若你肯信一個大膽的想法，你方才比較的，正是它們分子的輕重。

一個大膽的猜想

亞佛加厥的猜想很短：任意兩種氣體，只要填滿同樣的體積，又在同樣的溫度與壓強下，就含有同樣數目的分子——不管這氣體是重是輕。

若這是真的，一件美妙的事就跟著來了。你永遠看不見一個分子，更別說去稱它的重。可既然相同體積含有相同的數目，那麼稱量兩份相等體積的氣體，就等於直接比較它們的分子：重的那份，分子也重，重的比例分毫不差。清點那數不清的，就化作了把氣體放上天平。

一位獨自做學問的伯爵

亞默迪歐·亞佛加厥是義大利的貴族，學的是法律，後來才轉向物理，在杜林靜靜地做研究。他在 1811 年發表了這個想法——卻幾乎無人理會，一連五十年。化學的巨匠們，道耳頓與貝吉里斯都在其列，無法接受他的第二個論斷：兩個一模一樣的原子，竟會兩兩結成分子。直到 1860 年，在卡爾斯魯厄的一場大會上，坎尼札羅才證明，亞佛加厥的想法理順了原子量那團亂麻——那時距他辭世已兩年，他至死都不知道自己原來是對的。

它為何重要

它給了化學一把尺。在亞佛加厥之前，同一種化合物，不同的化學家寫出不同的分子式，因為誰也說不準一個分子裡坐著幾個原子。在他之後，氣體的密度把分子質量釘死，分子式也就各就各位。他的假說，正是「原子存在」與「這就是水的分子式」之間，那道隱藏的樞紐。

一條停車場規則

想像有這樣一條規則：任何一座給定大小的停車場，停的車數總是一樣多，不管停進去的是什麼車型。現在去稱整座停車場的重。總重大，說明車重；總重小，說明車輕——而兩座停車場總重之比，恰恰就是單輛車重量之比。亞佛加厥說，氣體遵守的正是這條規則，只不過扮演「車」的，是分子。

它在故事裡的位置

亞佛加厥的想法，補全了道耳頓的原子（見 dalton-1808）與給-呂薩克的體積，也正是坎尼札羅帶去卡爾斯魯厄、終於把原子量定下來的那條論證。那個冠以亞佛加厥之名的數——大約是 6 後面跟著二十三個零——要到很久以後才被釘牢：讓·佩蘭在顯微鏡下盯著微粒抖動，一路數到了它。

The original document

Original source text

Amedeo Avogadro · Journal de Physique 73, 58–76 · 1811 · trans. Alembic Club Reprints No. 4

The starting point

Avogadro opens from Gay-Lussac's recent discovery that gases combine with one another in very simple ratios by volume — two volumes of hydrogen to one of oxygen, three of hydrogen to one of nitrogen — and asks what so clean a regularity must be telling us about the particles themselves.

The hypothesis

The first hypothesis to present itself in this connection, and apparently even the only admissible one, is the supposition that the number of integral molecules in any gases is always the same for equal volumes, or always proportional to the volumes.

He is careful with words. An “integral molecule” is the molecule of a compound; a “constituent molecule” the molecule of an element; an “elementary molecule” the atom — what he also calls the half-molecule. The distinction between atom and molecule, blurred in Dalton's chemistry, is the quiet hinge of the whole paper.

[ … ]

Why the molecules must divide

Two volumes of hydrogen and one of oxygen make two volumes of water vapour. If each particle were a single, indivisible atom, one volume of oxygen could yield at most one volume of product — yet two appear. The only escape is that the particle of an elementary gas is not one atom but a small molecule that splits when it combines.

Thus in all these cases there must be a division of the molecule into two; but it is possible that in other cases the division might be into four, eight, &c.

From here the prize falls out. At equal volume the gases hold equal numbers of molecules, so the ratio of their densities is the ratio of their molecular masses: weigh the gases and you have read their relative molecular weights directly, hydrogen taken as the unit.

[ … ]

The essay drew almost no following. For half a century chemists could not accept that two identical atoms would join into a molecule, and Avogadro, working far from the centres of chemistry, was largely unread until Cannizzaro revived the argument in 1858–1860.

Turin · 1811