化學 1774

論不同種類的空氣：實驗與觀察

約瑟夫·普利斯特里

空氣不是單一之物：其中有一種成分，餵養著每一團火焰、每一次呼吸。

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In depth · the introduction

用陽光加熱一撮紅色粉末，逸出一種看不見的氣體：它讓蠟燭燒得通亮，讓老鼠活力十足——這就是我們今天所說的氧。

核心想法

幾千年來，人們都以為空氣是一種單一、簡單的東西——四「元素」之一。約瑟夫·普利斯特里卻表明：並非如此。他用一面把陽光聚攏的透鏡加熱一種紅色粉末（汞的一種氧化物），捕獲了一種新的空氣；在其中，蠟燭燃起亮得驚人的火焰，老鼠也活得遠比平常更久。

普通空氣裡的那一種成分——大約佔五分之一——正是火焰與呼吸共同需要的。原來，一團火與一葉肺，要的是同一樣東西。

它是如何誕生的

普利斯特里是一位不從國教的牧師，也是一位不知疲倦的實驗家；他早已把一大群氣體裝瓶、一一描述，還順手發明了蘇打水。1774 年 8 月 1 日，他在威爾特郡的卡恩，把那面大取火透鏡對準紅色氧化汞，被逸出氣體所助的火焰驚呆了。幾個月後，他鼓起勇氣親自吸了一口，並打趣說：這種「純淨空氣」，也許有朝一日會成為時髦的奢侈品。

但普利斯特里用一個叫「燃素」的舊觀念來解釋他的發現——所謂的「火質」——把故事講反了。那年 10 月在巴黎，他向安托萬·拉瓦錫描述了這個實驗；拉瓦錫看出了它真正的含義，將這種氣體命名為氧，並以它為基石建起現代化學。一位瑞典化學家卡爾·威廉·舍勒，甚至更早就悄悄發現了同樣的氣體。三人都該分得一份功勞；而讀懂它的，是拉瓦錫。

它為何重要

一旦你知道空氣是混合物，知道其中一部分同時驅動著燃燒與呼吸，整個化學便豁然開朗。你能解釋生鏽、解釋火焰、解釋我們為何呼吸；你能稱量一場反應吸進了什麼、又放出了什麼。普利斯特里的氣體，正是那根鬆線——一拉，便把延續千年的「四元素」世界觀，整個拆解開來。

一個可以想像的畫面

把空氣想成一屋子人，其中只有一部分願意當舞伴——那一部分，就是氧。蠟燭和老鼠，都在找舞伴。在普通空氣裡，牠們找得到足夠多。而在普利斯特里的「脫燃素空氣」裡，幾乎人人都願起舞，於是火焰咆哮、老鼠精神煥發。可若把牠們封進一間小屋，讓牠們把舞伴用盡，舞便停了——火焰搖曳著熄滅，空氣也變得污濁。

它的位置

普利斯特里立在現代化學的門口。他身後，是「四元素」的古老觀念；身前不遠，拉瓦錫正一步步用天平稱出氧理論，再往後，是道爾頓的原子與門捷列夫的週期表——在那裡，氧坐定了第八號元素的位置。而他自己「植物能更新空氣」的發現，正是我們今天所說的光合作用的一線最早的微光。

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)