化學 1662

關於空氣彈性的新物理-力學實驗

羅伯特·波以耳

把氣體壓進一半的空間，它就反推兩倍的力——壓強乘體積，守恆不變。

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用大拇指堵住打氣筒的出氣口，再往下壓——你壓得越狠，裡頭封住的空氣就把你頂得越凶。波以耳找到了這股反頂背後那條精確的規則。

壓一壓，它就回頂——而且有準數

空氣有一股彈性，波以耳稱之為「空氣的彈性」：它向外擠壓，你越壓它，它頂得越凶。他的發現，是「凶」出了多少。

把一團封住的空氣壓進一半的空間，它就回頂整整兩倍的力。壓進三分之一的空間，回頂三倍。體積往下，壓強往上，步調分毫不亂——於是體積乘壓強，永遠是同一個數。這就是波以耳定律。

一根管子、一位批評者，與一條借名而來的假說

羅伯特·波以耳是位富有的英裔愛爾蘭紳士，也是皇家學會的創建人之一，在牛津做研究，身邊有位年輕助手羅伯特·虎克，替他造了一台抽氣機。1660 年，他發表實驗，主張空氣是有彈性的。一位名叫法蘭西斯·萊納斯的耶穌會士不以為然，說撐起東西的是一根看不見的線，而非什麼彈簧。

為了一錘定音，波以耳彎了一根 J 形玻璃管，把空氣封在封閉的短臂裡，再往長臂倒水銀去壓它——一格一格地讀出體積與壓強。他找到的那條俐落的反比規則，卻老老實實歸功於另一個人，稱它為「湯利先生的假說」，紀念最先提出它的理查德·湯利。波以耳的本事，是把它量得足夠乾淨，足以證實它。

它為何重要

這是頭一回，有人把無形的氣體釘死在一個硬邦邦的數上。看似什麼都不是的空氣，竟服從一條像鐘錶一樣精確的定律。這條單一、可重複的關係——以及波以耳那種擺儀器、列數據、當眾論證的方式——幫著向世人顯示：新生的實驗科學，到底能做到什麼。

一根你能稱量的彈簧

把封住的空氣，想成打氣筒裡的一根彈簧。把活塞壓到一半，彈簧就以兩倍的力抵抗；壓到三分之一，就以三倍抵抗。波以耳定律說，一團氣體正是這樣一根彈簧——而且不像金屬彈簧，天下任何一種氣體，在任何地方，都服從同一條規則。

它在故事裡的位置

波以耳定律，是氣體定律裡的頭一條。一個世紀後，查理把體積繫於溫度，給-呂薩克把壓強繫於溫度；亞佛加厥（見 avogadro-1811）則把體積繫於分子的數目。四者編在一起，便成了理想氣體定律 PV = nRT。而那個更深的「為什麼」——氣體究竟為何會回彈——要等到動理學理論才到來：那時人們才明白，壓強原來是小到看不見的分子無休止的擂擊，正是道耳頓（見 dalton-1808）與亞佛加厥當年學著去清點的那些分子。

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Original source text

Robert Boyle · New Experiments Physico-Mechanical, Touching the Spring of the Air · Oxford, 1660 · the law in the “Defence…” appendix, 2nd edition 1662

The spring of the air

In 1660, working with an air-pump that his assistant Robert Hooke had designed and built, Boyle argued that air is elastic — that it behaves like a fleece of tiny coiled springs, pressing outward and pressing harder the more it is compressed. He named this elasticity the spring of the air.

Linus's objection

The Jesuit Francis Linus rejected the idea, holding that an invisible thread-like pull — his “funicular hypothesis” — held up the mercury, rather than any spring. In the second edition of his book (1662) Boyle added an appendix, A Defence of the Doctrine Touching the Spring and Weight of the Air, to answer him with measurement instead of argument.

The J-tube experiment

Boyle bent a glass tube into the shape of a J, sealed at the short end. Pouring mercury into the long open arm trapped a column of air at the top of the short one. He marked the trapped air in equal spaces and read the height of mercury that balanced it. As more mercury was added the air shrank and the pressure on it rose — from 48 equal spaces at 29 1/8 inches of mercury down to 12 spaces at 117 9/16 inches, a fourfold squeeze answered by a fourfold pressure.

the same air being brought to a degree of density about twice as great as it had before, obtains a spring twice as strong as formerly.

The relation — that pressure and volume stand in reciprocal proportion — Boyle credited not to himself but to Richard Towneley, calling it “Mr Towneley's hypothesis.” He set his measured pressures beside the values the hypothesis predicted, column against column, and left the reader to judge how closely they agreed.

[ … ]

Boyle never wrote an equation, nor used “pressure,” “volume,” and “temperature” in their modern senses; he reasoned with the spring and the density of the air, keeping the warmth of the room steady without isolating it. In France the same law is named for Edme Mariotte, who published it independently in 1679 and added the condition Boyle had left tacit — that the temperature must be held constant.

Oxford · 1662