化学 1811

论确定物体基本分子相对质量的方法

阿梅迪奥·阿伏伽德罗

任意气体，同温同压下，相同体积里藏着相同数目的分子。

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In depth · the introduction

拿两个一样大的气球，里头装着气体，称一称——若你肯信一个大胆的想法，你方才比较的，正是它们分子的轻重。

一个大胆的猜想

阿伏伽德罗的猜想很短：任意两种气体，只要填满同样的体积，又在同样的温度与压强下，就含有同样数目的分子——不管这气体是重是轻。

若这是真的，一件美妙的事就跟着来了。你永远看不见一个分子，更别说去称它的重。可既然相同体积含有相同的数目，那么称量两份相等体积的气体，就等于直接比较它们的分子：重的那份，分子也重，重的比例分毫不差。清点那数不清的，就化作了把气体放上天平。

一位独自做学问的伯爵

阿梅迪奥·阿伏伽德罗是意大利的贵族，学的是法律，后来才转向物理，在都灵静静地做研究。他在 1811 年发表了这个想法——却几乎无人理会，一连五十年。化学的巨匠们，道尔顿与贝采利乌斯都在其列，无法接受他的第二个论断：两个一模一样的原子，竟会两两结成分子。直到 1860 年，在卡尔斯鲁厄的一场大会上，坎尼扎罗才证明，阿伏伽德罗的想法理顺了原子量那团乱麻——那时距他辞世已两年，他至死都不知道自己原来是对的。

它为何重要

它给了化学一把尺。在阿伏伽德罗之前，同一种化合物，不同的化学家写出不同的分子式，因为谁也说不准一个分子里坐着几个原子。在他之后，气体的密度把分子质量钉死，分子式也就各就各位。他的假说，正是「原子存在」与「这就是水的分子式」之间，那道隐藏的枢纽。

一条停车场规则

想象有这样一条规则：任何一座给定大小的停车场，停的车数总是一样多，不管停进去的是什么车型。现在去称整座停车场的重。总重大，说明车重；总重小，说明车轻——而两座停车场总重之比，恰恰就是单辆车重量之比。阿伏伽德罗说，气体遵守的正是这条规则，只不过扮演「车」的，是分子。

它在故事里的位置

阿伏伽德罗的想法，补全了道尔顿的原子（见 dalton-1808）与盖-吕萨克的体积，也正是坎尼扎罗带去卡尔斯鲁厄、终于把原子量定下来的那条论证。那个冠以阿伏伽德罗之名的数——大约是 6 后面跟着二十三个零——要到很久以后才被钉牢：让·佩兰在显微镜下盯着微粒抖动，一路数到了它。

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