瑞利John William Strutt

瑞利的父亲（约1797年生于伦敦哈利街）是埃塞克斯郡威瑟姆特林广场的第二代Baron Rayleigh。他的母亲是Clara Elizabeth Latouche Vicars（约1825年生于毛里求斯路易港）。瑞利有年幼的弟妹：Clara（约1845年生）、Richard（约1848年生）、Charles（约1850年生）和Edward（约1855年生）。这无疑是一个此前对科学兴趣寥寥的家庭，因为他们大多是拥有乡村利益的地主。一个例外是罗伯特·波义耳，他是一位远亲。让我们在本文开头说明，尽管瑞利直到30岁才继承爵位，但本文中我们将始终称他为瑞利。

瑞利小时候身体不好，他在伊顿公学和哈罗公学的学业都中断了。由于健康问题，他不得不在短时间内离开这两所学校。在Reverend Warner寄宿学校的四年为瑞利上大学做了准备，在这个阶段他确实开始显示出数学能力的迹象。在这四年里，他有一位私人教师，但总体而言，他除了是一个能力平平的普通孩子外，没有显示出任何特别之处。

他于1861年10月进入剑桥大学三一学院，在那里参加数学荣誉学位考试。他在剑桥的辅导教师是爱德华·约翰·劳思，此人除了是当时（也许是有史以来）最著名的剑桥辅导教师外，本人也是一位非常出色的应用数学家，对动力学做出了重要贡献。毫无疑问，瑞利从爱德华·约翰·劳思那里获得的数学技术基础是他杰出科学生涯的重要因素。对他重要的不仅仅是所学的数学，因为他还从爱德华·约翰·劳思那里学会了如何提出最合适的数学方法来处理每个问题。

瑞利在剑桥读本科期间还有另一个重要影响，即当时担任卢卡斯数学教授的乔治·加布里埃尔·斯托克斯的影响。乔治·加布里埃尔·斯托克斯的讲座以新颖的方式将理论与实践结合起来，讲座期间进行了许多物理实验，这启发了瑞利。学生没有机会自己进行物理实验，因此观看乔治·加布里埃尔·斯托克斯在其光学课程中做实验是瑞利唯一接触科学实验方面的机会。瑞利本人后来谈到乔治·加布里埃尔·斯托克斯在他成长为科学家的过程中发挥了多么重要的作用。然而，乔治·加布里埃尔·斯托克斯似乎没有直接鼓励瑞利从事科学事业。

如果瑞利曾是一个普通的中学生，那么他远非一个普通的大学生。他于1864年获得天文学奖学金，然后在1865年的荣誉学位考试中成为高级数学荣誉学位考试一等及格者（Wrangler）（第一名一等生），同年他成为第一位史密斯奖得主。人们必须理解，瑞利现在面临一个艰难的决定。对于处于他这种地位的人来说，知道自己将继承爵位并成为第三代Baron Rayleigh，从事科学事业并不真正被接受，而且他家族的某些成员确实有这种感觉。然而，到此时，瑞利已决心将一生献给科学，因此他内心确信自己的社会义务绝不能妨碍他。

他的第一篇论文是受阅读詹姆斯·克拉克·麦克斯韦1865年关于电磁理论的论文启发而写的。正是通过广泛阅读当时的科学文献，瑞利试图找出哪些是他应该着手研究的重要问题。他深入研究的另一位科学家是赫尔曼·冯·亥姆霍兹，特别是阅读了赫尔曼·冯·亥姆霍兹1860年关于声学共振器的结果。1866年，瑞利当选为剑桥大学三一学院的研究员，他准备好在科学界崭露头角。

当时有社会地位的年轻英国人的通常做法是进行一次欧洲之旅——即所谓的大旅行。令人惊讶的是，瑞利进行了一次非常不同、在当时很不寻常的旅行，因为他出发前往美国。瑞利享有特权社会地位的一个好处是他不需要学术职位来谋生。相反，当他从美国回来后，他购买了进行科学实验的设备，并将其安置在特林家族庄园。他做了关于电流计的实验，并于1868年在诺里奇举行的英国协会会议上展示了他的结果。

瑞利的散射理论发表于1871年，是第一个正确解释天空为什么是蓝色的理论。同年，他与伊夫林·贝尔福结婚，她是阿瑟·詹姆斯·贝尔福的妹妹，后者将在50年间成为保守党的主要成员，并在30年后成为英国首相。瑞利曾在剑桥与阿瑟·詹姆斯·贝尔福一起学习，并通过他认识了伊夫林。婚后不久，瑞利患了风湿热，几乎使他的科学活动过早结束。他被建议去埃及旅行，他确实和妻子一起去了。他们在1872年末和1873年初乘船沿尼罗河而下，于1873年春天返回英国。

这次旅行使瑞利恢复了健康，但从科学角度来看，这也是一次非常有收获的旅行。相当引人注目的是，他在旅行期间开始撰写一部重要著作The Theory of Sound。这部伟大经典开始写作五年后才付印。第一卷关于产生声音的振动介质的力学，于1877年出版，而第二卷关于声波传播，于次年出版。

从尼罗河之旅返回后不久，瑞利的父亲去世，瑞利，如他此前一直的那样，继承了爵位，成为第三代Baron Rayleigh。他继续在特林工作，现在他在那里定居。他在那里建立的实验室是他做出令人印象深刻发现的地方，但不应该认为这是因为富有的瑞利能够拥有比任何人都好的设备。相反，他用廉价的设备获得了令人印象深刻的实验结果。瑞利总是善于节约，用不复杂的设备凑合。此外，他并不像人们可能预期的那样富裕，因为1870年代是英国农业经济困难的时期，因此他的收入远低于本来可能的情况。

从1879年到1884年，瑞利是剑桥的第二位卡文迪什实验物理学教授。实验室在五年前开放，詹姆斯·克拉克·麦克斯韦是第一位卡文迪什教授。在学术方面，瑞利是接替詹姆斯·克拉克·麦克斯韦讲席的明显人选，然而在其他时候他可能满足于在特林工作。然而农业萧条改变了平衡，使得该职位的收入看起来很有吸引力。然而，没有迹象表明瑞利只是为了钱才在那里。相反，他非常认真地对待自己的职责，对剑桥的物理教学做出了非常实质性的改进。Heathcote在[7]中写道：-

组织实验室作为教学和研究中心的任务落到了他身上，他出色地完成了这项任务。

我们在上面提到了当瑞利自己还是本科生时实验物理学的缺乏，尽管正在进行变革，但仍有很多工作要做。瑞利以他对待一切事情的同样精力，开发了热学、电学和磁学、物质性质、光学和声学的实验室课程。

他在担任卡文迪什教授期间进行的一项重要实验工作是欧姆的标准化。詹姆斯·克拉克·麦克斯韦和乔治·克里斯托早些时候在剑桥进行了实验，设备仍然可供瑞利使用。然而，旧设备证明不够好，无法让瑞利获得他所需的精度，因此他建造了新设备。在1884年于蒙特利尔举行的英国协会主席演讲中，他解释了结果。他通过以下方式引入了这个话题：-

在过去几年中，人们对将电动势、电流、电阻等的测量归算到绝对标准产生了很大兴趣，为此进行了许多艰苦的研究。这个课题占据了我自己的不少注意力……

1884年，他辞去剑桥的讲席，回到特林自己的庄园继续研究。他的经济状况已经改善，而他热爱的是科学研究，不愿承担大学职位那些耗费时间的职责。许多同事试图让他重新考虑这一决定并继续担任讲席，但瑞利清楚自己想要从生活中得到什么。对他而言，在特林并非孤独的科学生活，因为他频繁前往伦敦，在那里为许多学术与科学团体履行职责。让我们简要看看他在这一领域的一些活动。

瑞利于1873年当选为皇家学会会士。他于1882年获得该学会的皇家奖章，并于1885年成为该学会秘书，1899年被授予该学会的科普利奖章。他于1902年做了该学会的贝克里安讲座，并于1905年当选为该学会主席，担任该职位直到1908年。瑞利在1876-78年担任伦敦数学会主席，并于1890年被授予该学会的奥古斯塔斯·德摩根奖章。他还与皇家研究所保持联系，于1887年成为那里的自然哲学教授。他于1908年成为剑桥大学校长。

其他值得提及的活动包括他帮助建立国家物理实验室的工作，该实验室于1900年在米德尔塞克斯的特丁顿成立。他于1896年被任命为三一 House的科学顾问，这是英国海员的协会。通过他的妻子与政治舞台有联系，他大量参与咨询角色，例如在航空委员会任职。

显然，这种活动水平意味着他并不缺乏与同行科学家的接触，他还与许多领先科学家通信。我们现在应该简要转向考察他所承担的一些科学工作。然而，首先我们注意到[7]包含了瑞利出版物的完整列表，令人惊讶的是列表中有446项。它们涵盖了应用数学和物理学中令人难以置信的广泛主题。在致力于数学而非其应用的出版物中，有关于弗里德里希·威廉·贝塞尔函数、皮埃尔·西蒙·拉普拉斯函数与Bessel functions之间关系以及阿德里安-马里·勒让德函数的论文。除了我们下面稍作提及的更常见的应用数学和物理学主题外，他还写了更不寻常的主题，如Insects and the colour of flowers（1874）、On the irregular flight of a tennis ball（1877）、The soaring of birds（1883）、The sailing flight of the albatross（1889）和The problem of the Whispering Gallery（1910）。

上文我们已经提到他在电磁现象方面的工作、他关于声的重要论著、欧姆的测定，以及他那篇解释天空为何是蓝色的关于光散射的重要论文。此外[7]：-

……他将光的波动理论应用于棱镜和衍射光栅分辨本领的数学研究；由此他表明，光栅的分辨本领由光栅中刻线总数乘以光谱级次决定，而不是由刻线的密集程度决定。……1887年，他发表了一篇论文，其中提出了后来在原理上被⟦N3⟧采用的摄影再现颜色的方法。

瑞利也许最著名的是他在1895年发现了惰性气体氩，这项工作使他在1904年获得诺贝尔奖。在领取诺贝尔奖时的演讲中，瑞利解释了他如何作出这一著名发现（例如见[7]）：-

气体密度这一课题二十多年来占据了我很大一部分注意力。……我把注意力转向氮，做了一系列测定……空气鼓泡通过液氨，然后通过一根装有红热铜的管子，在那里空气中的氧被氨中的氢消耗掉，过量的氨随后用硫酸除去。……在对如此制备的气体获得一系列一致观测之后，我起初倾向于认为关于氮的工作已经完成。……然而后来，……我又回到更正统的做法，即不用氨，让空气直接通过红热的铜。结果同样与自身吻合良好，但令我惊讶和厌恶的是，两种方法的密度相差千分之一——这一差异本身很小，却完全超出实验误差。……在实验工作中，一条好规则是：当差异刚一出现时，设法将其放大，而不是遵循自然本能试图摆脱它。两种氮之间有什么区别？一种完全来自空气；另一种部分来自氨，约占五分之一。放大这一差异最有希望的途径似乎是在氨法中用氧代替空气，这样所有氮在该情况下都应来自氨。立刻获得了成功，来自氨的氮现在比来自空气的氮轻$\large\frac{1}{200}\normalsize$分之一。……在自然想到的解释中，有一种是空气中存在一种比氮更重的气体……

瑞利当然是对的，并克服相当大的困难成功分离出该气体。由于它拒绝发生化学结合，它被称为氩，源自希腊语中表示不活泼的词。

1879年，瑞利写了一篇关于行波的论文，这一理论现已发展成为孤子理论。[4]的序言解释了他于1885年在Proceedings of the London Mathematical Society上的一篇论文中引入的瑞利波理论为何被证明如此重要：-

在科学或技术中，人们并不尊重单纯的年资。然而，第三代瑞利发现弹性波可由表面引导的百年纪念，因其包含的矛盾而令人难忘：一方面，瑞利将他1885年的经典论文评价为一项相当次要的数学进展，其潜在价值仅在地震学中；另一方面，该主题在一个完全不同的领域——电子信号处理领域——被重新发现，这导致了它在过去二十年中的爆炸性增长。

事实上，在他1885年的论文On waves propagated along the plane surface of an elastic solid中，瑞利写道：-

本文拟研究无限均匀各向同性弹性固体平面表面上的波的行为，其特性使得扰动局限于一个表层区域，其厚度与波长相当。……这里所研究的表面波在地震以及弹性固体的碰撞中扮演重要角色，这并非不可能。由于仅在二维中发散，它们在远离震源处必定获得持续增大的优势。

Rott [12]考察了瑞利对流体动力学的贡献，特别是对流体动力学相似性的贡献：-

[存在]流体力学中的两个领域，瑞利在其中明确使用了流体动力学相似性：空气阻力理论和风鸣音的处理。[存在]瑞利的思想在他生前及之后对流体动力学相似性理论的发展和应用产生了巨大影响。

当然，瑞利因其科学工作获得了许多荣誉。1902年，在国王爱德华七世的加冕典礼上，他获得了功绩勋章。除了诺贝尔奖，他还获得了十三个荣誉学位、五项政府奖项以及全球五个学术团体的荣誉会员资格。

瑞利是一个谦逊而慷慨的人。他将诺贝尔奖的奖金捐赠给剑桥大学，用于扩建卡文迪什实验室。1902年获得功绩勋章时，他说：-

...我个人唯一意识到的优点是通过我的研究使自己感到愉悦，而我的研究所可能带来的任何成果，都归因于成为一个物理学家对我来说是一种乐趣。

瑞利的兴趣还有另一面，在[3]中提到，即他对心理研究的兴趣。他曾任心理研究学会主席，并在对该学会的演讲中[3]：-

...他回忆了上世纪六十年代在剑桥参与的一些催眠暗示实验，这些实验使他确信可以通过暗示影响不情愿的思想。后来，他对Home和其他所谓灵媒的行为产生了兴趣，尽管他总体上认为结果令人失望，但他发现有些事件难以解释。

我们以瑞利1884年在蒙特利尔对英国协会的主席演讲中的引文来结束这篇简短的传记：-

在不侵犯神学家和哲学家领域的前提下，自然科学的疆域无疑足够广阔，足以满足其追随者最狂野的雄心。在人类生活和兴趣的其他领域，真正的进步更像是一种信念而非理性的信仰；但在科学中，倒退从事情的本质来看几乎是不可能的。知识的增长带来力量的增强，尽管本世纪的成就伟大，我们完全可以相信，它们只是发现和发明为人类准备的一切的预尝。……工作可能艰苦，训练可能严苛；但兴趣永不衰竭，成就的特权是伟大的。

John William Strutt's father (born about 1797 in Harley Street, London) was the second Baron Rayleigh of Terling Place, Witham, in the county of Essex. His mother was Clara Elizabeth Latouche Vicars (born in Port Louis, Mauritius about 1825). John William Strutt had younger siblings, Clara (born about 1845), Richard (born about 1848), Charles (born about 1850) and Edward (born about 1855). Certainly it was a family with little previous interest in science for they were mostly landowners with interests in the countryside. One exception was Robert Boyle, who was a distant relation. Let us say at the beginning of this article that we shall refer to Strutt as Rayleigh throughout this article although he did not succeed to the title until he was 30 years old.

As a boy Rayleigh suffered from poor health and his schooling at both Eton and Harrow was disrupted. He had to leave both schools after a short period due to health problems. Four years spent at the Reverend Warner's boarding school prepared Rayleigh for university and at this stage he did begin to show signs of mathematical ability. During these four years he had a private tutor but overall he showed little sign of being anything other than an average child of average ability.

He entered Trinity College, Cambridge, in October 1861 where he took the mathematical Tripos. His coach at Cambridge was Edward Routh who, in addition to being the most famous of the Cambridge coaches at that time (perhaps of all time), was himself a very fine applied mathematician making important contributions to dynamics. There is no doubt that the grounding in mathematical techniques which Rayleigh had from Routh was an important factor in his outstanding scientific career. It was more than just the mathematics which he learnt that was important to him, for in addition he learnt from Routh how to come up with the most appropriate mathematical methods to tackle each problem.

There was another important influence on Rayleigh during his undergraduate years at Cambridge, namely that of Stokes who was the Lucasian professor of mathematics at the time. Stokes inspired Rayleigh with his lectures which combined theory and practice in a novel way with many physical experiments being carried out during the lectures. Students did not have the opportunity to undertake physics experiments themselves, so seeing Stokes perform experiments in his course on light was Rayleigh's only exposure to the experimental side of science. Rayleigh himself later spoke of how important a role Stokes had played in his development as a scientist. However Stokes does not appear to have directly encouraged Rayleigh to undertake a scientific career.

If Rayleigh had been an average school pupil he was far from an average student. He was awarded an astronomy scholarship in 1864, then in the Tripos examinations of 1865 he was Senior Wrangler (the top First Class student) and in the same year he was the first Smith's prizeman. One has to understand that Rayleigh was now faced with a difficult decision. For someone in his position, knowing that he would succeed to a title and become the third Baron Rayleigh, taking up a scientific career was not really acceptable, and certainly various members of his family felt exactly that way. By this time, however, Rayleigh was determined to devote his life to science so he was certain in his own mind that his social obligations must not stand in his way.

His first paper was inspired by reading Maxwell's 1865 paper on electromagnetic theory. It was through reading widely the current scientific literature that Rayleigh tried to work out which were the important problems on which he should undertake research. The other scientist whose works he studied deeply was Helmholtz, in particular reading Helmholtz's 1860 results on the acoustic resonator. In 1866 Rayleigh was elected a Fellow of Trinity College, Cambridge, and he was poised to make his mark in science.

The usual course of action for young British men of social standing at this time was to take a European tour - the grand tour as it was called. Rayleigh, surprisingly, made a very different, and for that time unusual, tour for he set out on a trip to the United States. One advantage of Rayleigh's privileged social position was that he did not need an academic post to earn his living. Rather when he returned from the United States he purchased equipment for undertaking scientific experiments and set it up on the family estate at Terling. He did experiments on the galvanometer and presented his results to the British Association meeting in Norwich in 1868.

Rayleigh's theory of scattering, published in 1871, was the first correct explanation of why the sky is blue. In the same year he married Evelyn Balfour, the sister of Arthur James Balfour who was to be a leading member of the Conservative Party for 50 years and Prime Minister of Britain 30 years later. Rayleigh had been a student at Cambridge with Arthur James Balfour and through him had met Evelyn. Shortly after their marriage Rayleigh had an attack of rheumatic fever which nearly brought his scientific activities to a premature end. He was advised to travel to Egypt and indeed he did just this with his wife. They sailed down the Nile during the last months of 1872 and early 1873, returning to England in the spring of 1873.

It was a trip during which Rayleigh recovered his health but it was also a very profitable trip from a scientific point of view. Rather remarkably he began writing a major text The Theory of Sound while on the trip. It was five years after beginning this great classic before it appeared in print. The first volume, on the mechanics of a vibrating medium which produces sound, was published in 1877, while the second volume on acoustic wave propagation was published the following year.

Shortly after returning from his trip down the Nile, Rayleigh's father died and Strutt, as he had been up to that time, succeeded to the title becoming the third Baron Rayleigh. He continued working at Terling where he now took up residence. The laboratory which he had set up there was one where he made impressive discoveries but one should not think that this was because the rich Rayleigh was able to have better equipment than anyone else. On the contrary he obtained impressive experimental results with cheap equipment. Rayleigh was always one to economise and make do with unsophisticated equipment. Also he was not as well off as might have been expected, for the 1870s were a time of economic problems for farming in England and as a consequence his income was far less than might otherwise have been the case.

From 1879 to 1884 Rayleigh was the second Cavendish professor of experimental physics at Cambridge. The laboratory had been opened five years earlier and Maxwell had been the first Cavendish professor. On the academic side Rayleigh was an obvious choice to succeed to Maxwell's chair, yet in other times he might have been content to work at Terling. The agricultural depression however swung the balance making the income from the post look attractive. There was no suggestion, however, that Rayleigh was just there for the money. On the contrary he took his duties very seriously making very substantial improvements to the teaching of physics at Cambridge. Heathcote, in [7], writes:-

To him fell the task of organising the laboratory as a centre of instruction and research, a task which he accomplished with outstanding success.

We mentioned above the lack of experimental physics when Rayleigh himself was an undergraduate and, although changes were being made, still a great deal was required to be done. With the same energy with which he approached everything, Rayleigh developed laboratory courses in heat, electricity and magnetism, properties of matter, optics, and acoustics.

One of the important pieces of experimental work he carried during his time as Cavendish professor was a standardisation of the ohm. Maxwell and Chrystal had carried out experiments in Cambridge earlier and the apparatus was still available for Rayleigh. However the old equipment did not prove good enough to allow Rayleigh to obtain the accuracy he required and so he had new apparatus built. In his Presidential Address to the British Association in Montreal in 1884 he explained the results. He introduced the topic by saying:-

During the last few years much interest has been felt in the reduction to an absolute standard of measurements of electromotive force, current, resistance, etc. and to this end many laborious investigations have been undertaken. The subject is one which has engaged a good deal of my own attention ...

Then in 1884 he resigned his Chair at Cambridge to return to his research on his own estate at Terling. His financial position had improved and what he loved was scientific research, without the time-consuming responsibilities of a university post. There were many colleagues who tried to get him to reconsider his action and continue to hold the chair but Rayleigh knew exactly what he wanted from life. It was not a solitary scientific existence for him in Terling since he made frequent visits to London where he had duties to perform for many learned and scientific societies. Let us look briefly at some of his activities in this area.

Rayleigh had been elected as a Fellow of the Royal Societyin 1873. He received the Royal Medal from the Society in 1882, and became secretary of the Society in 1885, being awarded the Society's Copley Medal in 1899. He gave the Society's Bakerian Lecture in 1902 and he was elected President of the Society in 1905, holding the position until 1908. Rayleigh served as President of the London Mathematical Society in 1876-78 and he was awarded the Society's De Morgan Medal in 1890. He also had connections with the Royal Institution, becoming professor of natural philosophy there in 1887. He became chancellor of Cambridge University in 1908.

Other activities which deserve mention involve the work he put in helping towards establishing the National Physical Laboratory which was set up at Teddington in Middlesex in 1900. He was appointed scientific advisor to Trinity House, the association of English seamen, in 1896. Connected with the political scene through his wife, he was much involved in advisory roles such as serving on a committee on aeronautics.

Clearly this level of activity meant that he did not lack contact with fellow scientists, and he also corresponded with many of the leading scientists. We should now turn to examine briefly some of the scientific work which he undertook. First, however, we note that [7] contains a complete list of Rayleigh's publications and remarkably there are 446 items in the list. They cover an incredible range of topics in applied mathematics and physics. Among the publications devoted to mathematics, rather than to its applications, are papers on Bessel functions, the relationship between Laplace functions and Bessel functions, and Legendre functions. In addition to the more usual topics of applied mathematics and physics which we say a little on below, he wrote on more unusual topics such as Insects and the colour of flowers (1874), On the irregular flight of a tennis ball (1877), The soaring of birds (1883), The sailing flight of the albatross (1889), and The problem of the Whispering Gallery (1910).

We have mentioned above his work on electromagnetic phenomena, his major treatise on sound, the determination of the ohm, and his important paper of scattering of light which explained why the sky is blue. In addition [7]:-

... he applied the wave theory of light to the mathematical investigation of the resolving power of prisms and diffraction gratings; thus he showed that the resolving power of a grating is determined by the total number of lines in the grating multiplied by the order of the spectrum, and not by the closeness of the lines. ... In 1887 he published a paper in which he suggested the method of reproducing colours by photography later adopted in principle by Lippmann.

Rayleigh is perhaps most famous for his discovery the inert gas argon in 1895, work which earned him a Nobel Prize in 1904. In his address on the occasion of receiving the Nobel Prize Rayleigh explained how he made his famous discovery (see for example [7]):-

The subject of the densities of gases has engaged a large part of my attention for over 20 years. ... Turning my attention to nitrogen, I made a series of determinations ... Air bubbled through liquid ammonia is passed through a tube containing copper at a red heat where the oxygen of the air is consumed by the hydrogen of the ammonia, the excess of the ammonia being subsequently removed with sulphuric acid. ... Having obtained a series of concordant observations on gas thus prepared I was at first disposed to consider the work on nitrogen as finished. ... Afterwards, however, ... I fell back upon the more orthodox procedure according to which, ammonia being dispensed with, air passes directly over red hot copper. Again a good agreement with itself resulted, but to my surprise and disgust the densities of the two methods differed by a thousandth part - a difference small in itself but entirely beyond experimental errors. ... It is a good rule in experimental work to seek to magnify a discrepancy when it first appears rather than to follow the natural instinct to trying to get quit of it. What was the difference between the two kinds of nitrogen? The one was wholly derived from air; the other partially, to the extent of about one-fifth part, from ammonia. The most promising course for magnifying the discrepancy appeared to be the substitution of oxygen for air in the ammonia method so that all the nitrogen should in that case be derived from ammonia. Success was at once attained, the nitrogen from the ammonia being now $\large\frac{1}{200}\normalsize$ part lighter than that from air. ... Among the explanations which suggested themselves are the presence of a gas heavier than nitrogen in air ...

Rayleigh of course was correct and succeeded, with considerable difficulty, in isolating the gas. Since it refused to make chemical combinations it was called argon from the Greek word for inactive.

In 1879 Rayleigh wrote a paper on travelling waves, this theory has now developed into the theory of solitons. The preface of [4] explains why Rayleigh-wave theory, introduced by him in 1885 in a paper in the Proceedings of the London Mathematical Society, has proved so important:-

There is no respect for mere age in science or technology. Yet the centenary of the discovery, by the third Lord Rayleigh, that elastic waves can be guided by a surface, is memorable for the contradictions which it encompasses: Rayleigh's assessment of his classic 1885 paper as a rather minor mathematical development with a potential value only in seismology on the one hand; on the other the rediscovery of the subject in a totally different field - that of electronic signal processing - which has led to its explosive growth over the last twenty years.

In fact in his paper 1885 paper On waves propagated along the plane surface of an elastic solid Rayleigh writes:-

It is proposed to investigate the behaviour of waves upon the plane surface of an infinite homogeneous isotropic elastic solid, their character being such that the disturbance is confined to a superficial region, of thickness comparable with the wavelength. .... It is not improbable that the surface waves here investigated play an important part in earthquakes, and in the collision of elastic solids. Diverging in two dimensions only, they must acquire at a great distance from the source a continually increasing preponderance.

Rott [12] looks at Rayleigh's contributions to hydrodynamics, in particular to hydrodynamic similarity:-

[There were] two domains in fluid mechanics in which Lord Rayleigh made explicit use of hydrodynamic similarity: the theory of aerodynamic drag and the treatment of the Aeolian tones. [There was a] great impact of Rayleigh's ideas on the development of hydrodynamic similarity theory and applications during his lifetime and beyond.

Of course Rayleigh received many honours for his scientific work. In 1902, at the coronation of King Edward VII, he received the Order of Merit. In addition to the Nobel Prize he received thirteen honorary degrees, five government awards, and honorary membership of five learned societies world-wide.

Rayleigh was a modest and generous man. He donated the proceeds of his Nobel Prize to the University of Cambridge to build an extension to the Cavendish laboratories. On receiving the Order of Merit in 1902 he said:-

... the only merit of which I personally am conscious was that of having pleased myself by my studies, and any results that may be due to my researches were owing to the fact that it has been a pleasure for me to become a physicist.

There is another side to Rayleigh's interests which are mentioned in [3], namely his interest in psychical research. He was president of the Society for Psychical Research and in his address to this Society [3]:-

... he recalled some experiments in hyptonic suggestion in which he took part at Cambridge in the sixties of last century, and which convinced him of the possibility of influencing unwilling minds by suggestion. Later he became interested in the doings of Home and other so-called mediums, and though he pronounced the results on the whole to be disappointing, he found some of the incidents difficult to explain.

We end this brief biography of Rayleigh by quoting from his Presidential Address to the British Association in Montreal in 1884:-

Without encroaching upon grounds appertaining to the theologian and the philosopher, the domain of natural sciences is surely broad enough to satisfy the wildest ambition of its devotees. In other departments of human life and interest, true progress is rather an article of faith than a rational belief; but in science a retrograde movements is, from the nature of the case, almost impossible. Increasing knowledge brings with it increasing power, and great as are the triumphs of the present century, we may well believe that they are but a foretaste of what discovery and invention have yet in store for mankind. ... The work may be hard, and the discipline severe; but the interest never fails, and great is the privilege of achievement.