化学 1774

论不同种类的空气：实验与观察

约瑟夫·普利斯特里

空气不是单一之物：其中有一种成分，喂养着每一团火焰、每一次呼吸。

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In depth · the introduction

用阳光加热一撮红色粉末，逸出一种看不见的气体：它让蜡烛烧得通亮，让老鼠活力十足——这就是我们今天所说的氧。

核心想法

几千年来，人们都以为空气是一种单一、简单的东西——四「元素」之一。约瑟夫·普利斯特里却表明：并非如此。他用一面把阳光聚拢的透镜加热一种红色粉末（汞的一种氧化物），捕获了一种新的空气；在其中，蜡烛燃起亮得惊人的火焰，老鼠也活得远比平常更久。

普通空气里的那一种成分——大约占五分之一——正是火焰与呼吸共同需要的。原来，一团火与一叶肺，要的是同一样东西。

它是如何诞生的

普利斯特里是一位不从国教的牧师，也是一位不知疲倦的实验家；他早已把一大群气体装瓶、一一描述，还顺手发明了苏打水。1774 年 8 月 1 日，他在威尔特郡的卡恩，把那面大取火透镜对准红色氧化汞，被逸出气体所助的火焰惊呆了。几个月后，他鼓起勇气亲自吸了一口，并打趣说：这种「纯净空气」，也许有朝一日会成为时髦的奢侈品。

但普利斯特里用一个叫「燃素」的旧观念来解释他的发现——所谓的「火质」——把故事讲反了。那年 10 月在巴黎，他向安托万·拉瓦锡描述了这个实验；拉瓦锡看出了它真正的含义，将这种气体命名为氧，并以它为基石建起现代化学。一位瑞典化学家卡尔·威廉·舍勒，甚至更早就悄悄发现了同样的气体。三人都该分得一份功劳；而读懂它的，是拉瓦锡。

它为何重要

一旦你知道空气是混合物，知道其中一部分同时驱动着燃烧与呼吸，整个化学便豁然开朗。你能解释生锈、解释火焰、解释我们为何呼吸；你能称量一场反应吸进了什么、又放出了什么。普利斯特里的气体，正是那根松线——一拉，便把延续千年的「四元素」世界观，整个拆解开来。

一个可以想象的画面

把空气想成一屋子人，其中只有一部分愿意当舞伴——那一部分，就是氧。蜡烛和老鼠，都在找舞伴。在普通空气里，它们找得到足够多。而在普利斯特里的「脱燃素空气」里，几乎人人都愿起舞，于是火焰咆哮、老鼠精神焕发。可若把它们封进一间小屋，让它们把舞伴用尽，舞便停了——火焰摇曳着熄灭，空气也变得污浊。

它的位置

普利斯特里立在现代化学的门口。他身后，是「四元素」的古老观念；身前不远，拉瓦锡正一步步用天平称出氧理论，再往后，是道尔顿的原子与门捷列夫的周期表——在那里，氧坐定了第八号元素的位置。而他自己「植物能更新空气」的发现，正是我们今天所说的光合作用的一线最早的微光。

The original document

Original source text

Joseph Priestley · Experiments and Observations on Different Kinds of Air, Vol. II · London: J. Johnson, 1775

In the section "Of Dephlogisticated Air, and of the Constitution of the Atmosphere," Priestley opens by reflecting that his most important discovery owed less to design than to luck — a candour rare in scientific writing.

…more is owing to what we call chance, that is, philosophically speaking, to the observation of events arising from unknown causes, than to any proper design, or pre-conceived theory in this business.

The experiment of 1 August 1774

Using a large burning lens to focus sunlight on mercurius calcinatus per se — the red calx (oxide) of mercury — he drove off an "air" and collected it over mercury, away from the common atmosphere. (In modern terms, heat decomposes mercuric oxide into liquid mercury and oxygen.) Its first test was a candle flame:

But what surprized me more than I can well express, was, that a candle burned in this air with a remarkably vigorous flame, very much like that enlarged flame with which a candle burns in nitrous air, exposed to iron or liver of sulphur.

Breathing it

Months later he gauged the air's "goodness" with his nitrous-air test and found it several times better than common air; mice sealed in it outlived their fellows in ordinary air. At last he breathed it himself.

The feeling of it to my lungs was not sensibly different from that of common air; but I fancied that my breast felt peculiarly light and easy for some time afterwards. Who can tell but that, in time, this pure air may become a fashionable article in luxury.

Hitherto only two mice and myself have had the privilege of breathing it.

[ … ]

Priestley read all of this through the phlogiston theory: a flame or an animal, he supposed, fills the air with "phlogiston" until it can hold no more, so air emptied of phlogiston — "dephlogisticated" — supports both unusually well. He never gave up that framework. Carl Wilhelm Scheele had made the same gas in Sweden a few years earlier but published later; Antoine Lavoisier, told of the experiment in Paris, would soon reinterpret the gas correctly and name it oxygen.

Calne, Wiltshire · August 1774 (published 1775)