This paper discusses Chapman's suggestion that a stream of corpuscles, emitted from the Sun by a flare, should be spectroscopically detectable before and during the magnetic storm which it is thought to cause. After an investigation of the mode of expansion and the state of ionization of such a stream, it is estimated that the particles in it may cause abnormal absorption near the H and K lines of calcium which is detectable soon after the end of the flare. Should this absorption not be observable, however, the failure will not necessarily disprove the corpuscular theory of magnetic storms, as the depth of absorption may lie below the observational limit.

I wish to thank Professor S Chapman for suggesting this problem to me and for much helpful advice, and also Professor H H Plaskett and Dr M A Ellison, with both of whom I have had most instructive discussions bearing on this paper.

Thanks are due to Professor S Chapman and to Professor H H Plaskett for their encouragement and advice while this paper was being prepared. Some of the work was carried out during the tenure of a D.S.I.R. grant and of the Skynner Senior Studentship in Astronomy at Balliol College. The paper forms part of a thesis for which the degree of Doctor of Philosophy was awarded by the University of Oxford.

A value is found for the minimum momentum density in a corpuscular stream which has been emitted from a flare and which causes a terrestrial magnetic storm. It is shown that radiation pressure alone will always be insufficient to expel such a stream from the Sun.

Already, Kahn had moved from the solar-system boundaries of his thesis work and had published papers on emission from radio stars ['Some possible mechanisms of radio stars'] and on the origin of interstellar dust ['On the formation of interstellar dust']. It was a natural move for him to transfer to the infant Astronomy Department in 1952, initially as a Turner and Newall Fellow

In many ways Kopal's life exemplified this century's upheavals, both in politics and in astronomy. He was born in the Austrian Empire, before the foundation of Czechoslovakia, and he was educated at Prague University, before modern astrophysics had taken hold. One of his teachers at Prague was the celebrated astronomer E Finlay Freundlich, who made it his life's work to verify the predictions of Einstein's General Relativity, notably the deflection of starlight by the sun. Kopal had the same attitude to scientific questions: take nothing for granted if you cannot verify it. This philosophy stood him in good stead.

This meeting profoundly influenced his later work and in particular led to several major investigations into the dynamical effects in interstellar gas produced by the radiation fields of hot stars.

In 1954, Kahn produced one of contemporary astrophysics' classic papers, laying down the basic theory for the way in which stellar radiation sweeps across interstellar gas, heating it and setting it in motion. This paper is central to an understanding of star formation, the late stages of evolution of intermediate mass stars and phenomena in the nuclei of active galaxies. Recent Hubble photographs of regions of massive star formation, like the now famous Pillars of Creation in the Eagle Nebula, rely on the basic Kahn theory for their interpretation.

There is an amusing anecdote regarding his stay there that he used to relate. Oort gave a seminar after which Kahn made some critical comments (this was probably in relation to Oort's championship of the so-called 'rocket-effect' acceleration of interstellar clouds, an effect that Kahn had shown not to work in the manner advocated by Oort). Presumably in view of Oort's great standing, heads shook disapprovingly during Kahn's remarks, and the listener next to him explained, 'your criticism is like challenging the Almighty'. On leaving the seminar room, a small group, which included Kahn, went from the building and encountered a sudden weather change. There was a flash of lightning followed by a loud thunderclap. 'Well, I told you', said the Dutchman who had been sitting next to him.

... his skill as a synthesiser of ideas matched his physical understanding and his analytical expertise.

My father, Franz Daniel Kahn (FDK) was an astronomer. Or to be more precise, an astrophysicist. The equipment he needed for work consisted of a pad of paper, a slide rule, and a Bic (always). When he was younger he worked all the time. He worked on Sundays, and when we were on holiday he'd spend his time working. Work consisted of writing long series of equations and some text around them. Proper theoretical science. The children were not to make noise in the house, which would disturb him. In some ways he was the happiest person you could meet. He was utterly passionate and enthusiastic about astronomy ... FDK's life was punctuated by meals and snacks. He had 6 meals a day. Breakfast, morning coffee, lunch, afternoon tea, supper, and at night coffee(!) with cake. Always. Every day. When my mother, Carla Kahn, was alive, they'd both come home from work at lunchtime. FDK would pop into a delicatessen and buy something special for lunch. I was there a few times (somehow being home from school at lunchtime). There was a peace there that was absent the rest of the time in a family with 4 children.

... Very little had been done for the previous 25 years so all the storage boxes had to be searched. Only in the thirtieth box did anything connected to Einstein appear, but indeed seventeen communications from Einstein varying from postcards to long letters were discovered. ... The letters are remarkable in that they cover the period when Einstein invented his cosmical repulsion to allow a static universe, and de Sitter countered by finding the de Sitter space solution.

In presenting this volume to you, Zdenek, we ask you to regard it as a token of the great success achieved by the Department that you founded in the autumn of 1951. May you have a long, happy and active retirement.

Under ordinary circumstances I should have broken off here and got on with the astronomy. But circumstances are far from ordinary. In the spring of 1951 I married Carla, having got to know her a few months earlier. There followed a union uniquely blessed by love and happiness. We had many children, and even a grandchild, and through our different activities, notably the astronomical ones, we acquired a worldwide circle of very good friends. All seemed set to go on in this way when one morning in January this year Carla suffered a severe stroke, and then died on the evening of the next day.

It is a commonplace that life can be cruel. But fortunately fate only very rarely administers so devastating a blow. This paper deals with the first scientific work that I have attempted since Carla's death. I dedicate it to her memory, may she rest in peace.

Franz Kahn has made many original contributions to plasma astrophysics, cosmical gas dynamics and the physics of star formation, with significant early papers on the structure of ionisation fronts and collision-free shocks. More recently he has done important work on stellar winds and galactic fountains, on planetary nebulae and on remnants of novae and supernovae. His versatility is shown by papers on the spiral structure of the Galaxy, on the nature of the Local Group and the account (with the late Carla Kahn) of the Einstein-de Sitter correspondence. Kahn's style is especially noteworthy for his skill in building simple mathematical models which bring out the essence of the physics.

Franz Kahn was an engaging companion, both professionally and socially, wearing his erudition lightly. Like all academics, he appreciated recognition, but never let delay in its appearance worry him unduly. He was a happy man, enjoying each stage of his life as it came.

On a hot, balmy evening in July, 1998, John Dyson, Dieter Breitchwerdt, António da Costa and myself were sitting in the Praça do Giraldo, the main square of the city of Évora, Portugal, discussing the work of Franz Kahn, late Emeritus Professor of Astronomy, Manchester University, and Fellow of the Royal Society, who had suddenly passed away some months before. We had all commented on how he had influenced the areas in which we had researched and how he had been instrumental in determining our fundamental approach to that research. After reminiscing for some time, we decided that we should celebrate Franz's work in some special way. The conference, Astrophysical Dynamics, was the result, the major emphasis being on theoretical developments in astrophysical dynamics set out principally by Franz's research career. I had been very fortunate to collaborate with Franz on a number of different projects since his retirement. As such, he had visited Évora on numerous occasions, and became a very welcome visitor to the Department of Physics of the university. It seemed fitting therefore to organise the conference here in Évora. The conference was held in the magnificent and historic Colégio Espirito Santo, the main building of the University of Évora over the three days 14th to 16th April, 1999. It was very well attended with about 40 scientists and students from 11 different countries from around the world.