9. Manchester

“Freedom does not consist in any dreamt-of independence from natural laws, but in the knowledge of these laws, and in the possibility this gives of systematically making them work towards definite ends.” – Friedrich Engels, Anti-Dühring.

Manchester University was a centre of excellence for mathematics and physics. Patrick Blackett had been appointed Professor of Physics 1937. His predecessor had been Lawrence Bragg and Bragg’s predecessor had been Ernest Rutherford. Even before Rutherford there had been Arthur Schuster, who in 1900 had established at Manchester one of the largest physics laboratories in the world. During the war Blackett had been responsible for placing many clever people into positions where they could best contribute their abilities and he was widely respected and influential. Otto Frisch, who worked under Blackett during 1933-4, described him as “a born leader and strategist”. Blackett was perhaps best known for the counter-controlled cloud chamber which he had invented and built, using two Geiger counters in such a way whereby a particle would cause its own photograph to be taken. A man of great humanity and consideration, he was notable too for deep socialist convictions and had been appointed President of the Association of Scientific Workers.

Also returning to Manchester and resuming his work on cosmic radiation, after working during the war on the development of radar, was Bernard Lovell. Anyone who has driven along the M6 south of Manchester will not have failed to notice the 250 feet diameter steerable radio telescope at Jodrell Bank. Lovell persuaded the British Government and the Nuffield Foundation to jointly fund its construction in 1952.

Max Newman, who had been recruited by Blackett during the war and had conceived of a method of decrypting machine enciphered signals (‘Fish’), was appointed Professor of Pure Mathematics at Manchester in 1945 and Alan Turing later joined him in 1948. Blackett was also influential in F.C. Williams moving to Manchester to take up the Professorship in Electrical Engineering. Newman was the overall leader of the Manchester ‘Baby’ : the world’s first stored-program digital electronic computer, though it was Williams, a leading radar engineer, who provided much of the important technical contribution.

Douglas Hartree was professor of theoretical physics at Manchester in 1945. He had held fellowships at St.John’s and Christ’s Colleges in Cambridge from 1924-9 before moving to Manchester to take the chair of applied mathematics. He had switched to the chair of theoretical physics in 1937 and then worked with the Ministry of Supply during the war years. Hartree’s particular focus was numerical analysis. He had been greatly influenced by a lecture course delivered by Niels Bohr at Cambridge in 1921 and had gone on to develop his own numerical methods to calculate atomic wave functions. He became increasingly interested by automated methods and was involved in the development of the ‘Differential Analyser’, a machine (first suggested by Lord Kelvin) for performing integration. When this machine was about to be superseded by electronic computers, he was advisor to John Eckert on the ENIAC project. In his inaugural address as Plummer Professor of Mathematical Physics at Cambridge in 1946, Hartree stated : “It may well be that the high-speed digital computer will have as great an influence on civilization as the advent of nuclear power.”

Like all the industrial cities at that time, Manchester was a grim and forbidding place in the 1940s, the buildings darkened by smoke pollution. Perhaps Manchester was even worse than some of the others, as the topography of higher ground on three sides of the city tends to trap the lower atmosphere. Much of the smoke pollution was due to domestic heating and when windless winter conditions gave rise to an inversion, all the smoke was confined to the lower few hundred feet of the atmosphere. Jim recalled that the smog was a particular problem in the vicinity of the Victoria University, which at that time was surrounded by large areas of terraced houses, each house contributing to the smog. Part of the problem was that, in the years immediately following the war, the coal available was mostly very poor quality and contained a lot of slack.

In September 1945 Jim and Glen rented a furnished flat in Chorlton-cum-Hardy, at the time a quiet residential area, some three miles southwest of the University. Furnished accommodation was more expensive to rent than unfurnished, but suitable unfurnished accommodation was difficult to find at that time and then there would have been the additional need to obtain the furniture and fittings they required, just as Jim was due to start work at the Physics Department. As a fellow Jim would be expected to give courses of lectures and there would have been some work to do in updating his lecture material.

As for his Ph.D. research, Jim had already done a considerable amount of work on radiation damping in Dublin. His publication with Heitler and Peng ‘Theory of Cosmic-Ray Mesons’ (1943, Phys. Rev. 64, p.78) had been based on the quantum theory of damping as developed by Heitler and Peng. In writing the ‘The Theory of Radiation Damping’ (1947, Proc. Phys. Soc. 59, p.917), Jim extended his DIAS work, referring also to more recent developments on the subject. (Heitler had published a paper with Gormley in 1944 and Peng had produced two further papers, in 1944 and 1946).

Jim later said : “I got the idea that it would be useful to find another, and possibly simpler, way to show how damping worked in the case of the elastic scattering of two particles. But I went further and tried to use my ideas to show how the Weisskopf-Wigner mechanism for the atomic line width really worked. For me the trouble – an aesthetic trouble if you like – was that Weisskopf and Wigner first had to substitute [an exponential term] for the excited state amplitude of the atom. Such a basic and simple thing as exponential decay should surely come out of the equations rather than be put into them.” If Jim was making such a statement verbally, there might have been the hint of a smile, though the smile would have been more discernible in his eyes.

“My idea for the atomic decay was that you have a two-level atom interacting (via a dipole) with many radiation oscillators, and you simply find the eigenstates for this coupled system. Normalizing the eigenstates is equivalent to imposing unitarity. The problem is solved by expressing the initial state as a sum of eigenstates. In practice, one has to calculate using a discrete energy spectrum.”

In ‘The Theory of Radiation Damping’ Jim also treats a simple scattering problem using the eigenstate method, giving an insight into the Heitler-Wilson damping equation’s structure.

Though there had been great celebration at the end of World War II, Britain had suffered economically; there were shortages of many commodities. Food and fuel would still be rationed for some years and life remained hard for many people. Glen, who had become quite thin during the last years of the war, was able to gradually remedy this, thanks in large part to the assistance of Jim’s parents, who periodically posted food parcels from Ireland. This was not an unusual thing : many city-dwellers in the post-war years would have their diet augmented by contributions from relatives living in rural areas and, for those with relatives in Ireland, the tradition of food parcels at Christmas continued long after the shortages had been alleviated in mainland UK.

Towards the end of Jim’s first year at Manchester, in May 1946, Blackett suggested that he might benefit from a visit to the University Institute of Theoretical Physics in Copenhagen. This institute, which would in 1965 be officially renamed ‘The Niels Bohr Institute’, had in the inter-war period become renowned for work in atomic physics and quantum mechanics. Niels Bohr, having escaped to Sweden in September 1943, had returned in August 1945 and swiftly re-established the Institute’s reputation. There were well forged connections between the Universities of Manchester and Copenhagen : Bohr himself had engaged in post-doctoral studies under Rutherford at Manchester and one of Niels Bohr’s closest collaborators in Copenhagen, Léon Rosenfeld, would be appointed Director of the Department of Theoretical Physics at Manchester during 1947 (replacing Hartree, who moved to Cambridge). Jim was duly invited to spend six months in Copenhagen, to commence in January 1947.

It was at Manchester in 1912-13 that Bohr had applied the quantum concept to the atom, building Max Planck’s quantum hypothesis from 1900 into Rutherford’s orbital concept of the atom. This was a very radical move at the time and the result was the idea that an electron could drop from a higher-energy shell to a lower one, releasing a certain amount of energy as a light quantum (photon) in the process. Incidentally, Planck’s quantum hypothesis had hardly been taken seriously in some quarters until 1905 when Albert Einstein produced a paper which gave evidence for the existence of light quanta (also in 1905 Einstein proposed his theory of relativity, but it was the first piece of work which earned him the Nobel Prize).

During the summer of 1946 Jim and Glen found an unfurnished flat in Heaviley, a residential district on the south side of Stockport. This was twice as far away from the University than their previous address, but a good tram service connected Manchester and Stockport at that time. There was a tram stop conveniently close to the flat and at the other end of the journey Jim only needed to walk about half a mile from Ardwick Green along Brunswick Street to be at the University. The poverty here, in the district of Brunswick, particularly so close to the relative affluence of the University, was stark. In years after leaving Manchester, Jim would comment that many of the children he saw in this area and in neighbouring Chorlton-on-Medlock, which is not to be confused with Chorlton-cum-Hardy, wore no shoes even in wintertime. It reminded him of the grinding poverty he had witnessed in Belfast during the 1930s.

Jim’s recollections of the general population of Manchester during those post-war years were bleak. He found Mancunians generally rather dour and parochial, even intolerant.

Since those years there have been influxes of population from many sources, both domestic and international, and Manchester now enjoys a better reputation, though pockets of intolerance remain.

From the summer of 1946 there is a photograph of the Manchester University Physics Department. At the centre of the front row are seated Lajos Jánossy, Blackett, Samuel Tolansky, Jim and Clifford Butler. Jim assisted with some of the work carried out by George Rochester and Butler and received acknowledgement in their publication on K mesons ‘Evidence for the Existence of New Unstable Elementary Particles’ (1947). Tolansky, at this time Reader in Physics at Manchester, would shortly depart to take up a professorship at Royal Holloway College, University of London and would later be appointed Principal Investigator to the NASA Lunar Project.

Much of the work on cosmic radiation by Rochester and Butler had involved collaboration with Jánossy. Jánossy’s book ‘Cosmic Rays’ (Oxford University Press, 1947) was reviewed by Léon Rosenfeld in the Proceedings of the Physical Society, A (1949). Rosenfeld’s review, itself both readable and informative, begins : “The subject of cosmic rays has a remarkable history, from its inconspicuous beginnings as the study of an unwanted leak in gold foil electroscopes to its present position as a disquietingly generous source of supply of new kinds of fundamental particles…In fact, cosmic radiation research leads us into a domain of energies which we are only just beginning to reach by means of the largest generators; it has thus been, and still is to a large extent, a most valuable complement to laboratory work on the fundamental properties and the constitution of matter.” Later in the same review, Rosenfeld warned of the difficulty facing any author who would attempt to write a comprehensive text on a rapidly developing subject : “From the start he must give up any hope of producing an entirely up-to-date survey…”

In January 1947 Copenhagen was cold. There were a number of exceptionally cold winters in the 1940s : 1946-1947 was one. There was deep snow well into March and the Øresund, the strait separating Denmark and Sweden, was frozen. It was said that some people had crossed the strait on foot, near its narrowest point between Helsingør and Helsingborg, during the German occupation. This would have been a risky venture and half way across it was likely to be frustrated by an open lead of water. By far the most of those who escaped to Sweden did so by boat when the sea was free of ice. Since 2000, the Øresund Bridge, a remarkable feat of engineering, has provided a car/rail link between Denmark and Sweden.

Niels Bohr had spent the last eighteen months of the war in America. Despite Bethe’s assurances and fearing that Hitler might yet win the race to develop an atomic weapon, Bohr had made himself available to the physicists already assembled on the Manhattan Project. He was a consultant, though he later said : “They didn’t need my help in making the atom bomb.” He was able to turn his focus to the problem of how to manage this new-found knowledge in a manner safest for the peaceful future of mankind. He conducted a personal mission (including interviews with both Churchill and Roosevelt) to convince statesmen that the existence of the bomb project should be revealed to the Soviet Union in order to prevent a post-war arms race. From this time until his death in 1962, Bohr insisted that the safest thing would be for the secrets of the A-bomb to be shared amongst nations.

It was Bohr who had announced, on a visit to America in January 1939, that nuclear fission had been achieved. In Berlin during 1937-8 Otto Hahn and Lise Meitner had conducted some experiments bombarding uranium with neutrons, in the hope of forming as yet undiscovered radioactive elements heavier than uranium (transuranics). Instead the process had appeared to yield isotopes which behaved chemically similarly to radium. How could this be ? The radium atom has four less protons than uranium (their atomic numbers are 88 and 92 respectively). Meitner was obliged to emigrate to Sweden in the spring of 1938 due to persecution, and Hahn continued the experiments with Fritz Strassmann.

Confusing matters further, Hahn and Strassmann were routinely using barium as part of their chemical separation process. Barium, like radium, is an alkaline earth metal and the two have similar chemical properties. After further analysis they had reluctantly concluded that their experiment had yielded barium isotopes (barium has atomic number 56) and Hahn wrote to Meitner about it. Meitner received this letter just before Christmas 1938, when her nephew Otto Frisch was due to visit her. Of course, the two talked about the results over the Christmas holiday and they realised that fission might have taken place, though they were not wholly convinced by their calculations. When Frisch returned to Copenhagen, where he was at the time working, and discussed the results with Bohr, Bohr famously responded : “Oh what fools we have all been ! Oh but this is wonderful ! This is just as it must be !”

Furthermore, when calculation had revealed the amount of mass lost during the fission process, it was clear that a lot of energy had been released. The significance of this was also immediately understood and from shortly after this time, in 1939, information on fission was not internationally shared. Even after ‘Operation Barbarossa’, the German invasion of Russia in June 1941, the Americans and British did not share information with their Russian allies.

Niels Bohr was also remarkable for his tireless efforts to assist Jews and other refugees. His standing in the scientific fraternity enabled him to find employment for many émigrés during the 1930s and when Germany invaded Denmark in April 1940, he chose to remain in Copenhagen, not only because his work was there but because he felt he could still be useful to those in need : the terms of surrender had allowed Denmark to retain its own government and courts, and there sprang up a resistance movement effective at spiriting people across the Kattegat and Øresund to Sweden if required.

The circumstances of Bohr’s own escape are no less remarkable. Sometime in 1943 Hitler’s administration had remembered that there were still 8,000 or so Jews in Denmark. Exceptionally, these Jews had survived three years of occupation relatively intact but in August 1943 Ribbentrop had become incensed at their continuing freedom, the matter was escalated to Hitler and then passed to Himmler. An SS detachment entered Denmark in September and transport ships were despatched for Copenhagen, due to leave again on 1st October with the full complement of Denmark’s Jews on board. The intended final destination was Tereczin, a concentration camp in Czechoslovakia. This information was leaked by a member of the German embassy staff and the Danish resistance movement mobilized. On 28th September Bohr learned that his own arrest had been ordered in Berlin. During the next three days and nights, using every description of small vessel, over 7,500 people including Bohr and his entire family were taken across the strait to safety in Sweden. Even in Stockholm Bohr was not safe : once the German authorities became aware of his escape, Gestapo agents in Sweden were ordered to find him and prevent the possibility of his further travel to Britain or America. The Allies were too quick for them though, within a week Bohr was spirited by plane to Scotland.

In 1947 the University Institute of Theoretical Physics in Copenhagen was in a phase of expansion to allow more active participation in nuclear research and was increasing its teaching capacity in atomic physics. The permanent staff was expanding proportionately. Aside from Niels Bohr and Léon Rosenfeld, there was Niels Bohr’s son, Aage, who had become an associate in 1946; Christian Møller and Torben Huus, who would both become senior figures in the Institute. Other permanent staff included Jørgen Bøggild, O. Kofoed-Hansen, H. Hojgaard-Jensen, Niels O. Lassen, O.B. Nielsen and Stefan Rozental. Jim was one of many physicists who visited Copenhagen during this period. Wolfgang Pauli, John Wheeler and Victor Weisskopf were among the most famous visitors.

J.K. Bøggild, by the way, had been one of only two men remaining on the premises of the Institute when the German military police had seized the buildings in December 1943. In the aftermath of Niels Bohr’s escape there were concerns for Bøggild’s safety, also that the Institute itself would be damaged. It was the distrusted Heisenberg who, on receiving a message from Christian Møller, came to Copenhagen and persuaded the German authorities to release Bøggild and return the institute to Danish hands.

Lodgings had been arranged for Jim and Glen at Charlottenlund, a few miles north of Copenhagen and a very pleasant, quiet area at that time : Copenhagen in entirety was and remains acknowledged as one of the most attractive cities in the world. The name Copenhagen is Germanic – the original Viking name was simply ‘Havn’ or harbour. The Romans called the place ‘Hafnia’ – there is a 3rd series transition metal, hafnium, named in honour of the city : in 1922 George Hevesy and Dirk Coster, working at Niels Bohr’s newly founded institute, had firmly established the existence of this element – and that it was a transitional metal, not a rare earth element, in accordance with Bohr’s prediction.

Nearby to Charlottenlund is Klampenborg, a large area of parkland in which they could walk and observe the herds of deer, and from April there were short weekend trips across the Øresund to Sweden. They soon learned to deal with the cold – one just needs to dress properly – and good food was more freely available than in Manchester; Jim and Glen both benefited from the plentiful supply of meat.

At the Institute, Jim had the opportunity to meet Niels Bohr and Christian Møller, and to reconnect with Léon Rosenfeld whom he would have met previously in Manchester. In all the excitement about fission in early 1939 it had been Møller who had alerted Frisch to the important point : that a chain reaction might be caused – that the fission fragments (the two freshly formed nuclei) might contain enough surplus energy each to eject a neutron or two; and that each of these might cause another fission and generate more neutrons, and so on.

The exchange of information on theory and methods was mutually beneficial. Niels Bohr’s letter to Walter Heitler in March 1947, accepting an invitation to the 1947 DIAS summer colloquium, refers to Jim’s visit : “In the Institute here, we are happy for the visit of Dr. Hamilton who has given us an account of the development of your ideas in which we are, of course, very interested and we look forward to having discussions with him about the whole situation in quantum electrodynamics.”

Jim and Glen returned home in July 1947. In March Jim’s paper ‘The Theory of Radiation Damping’ had been published and this provided the groundwork for his thesis. The work for this paper would have necessarily involved dialogue with Heitler at DIAS and it was Heitler who recommended the manuscript for publication. The paper also acknowledges Paul Dirac : Jim previously had contact with Dirac, the Cambridge professor, in 1942 when he had delivered a course of lectures at DIAS. Since the 1920s, Dirac had made outstanding contributions to modern quantum theory, particularly with his relativistic wave equation which explained the existence of the spin and magnetic moment of electrons and predicted the existence of their antiparticles, the positrons. He was the first person to produce a reasonably complete theory of quantum electrodynamics, in 1927. His book ‘The Principles of Quantum Mechanics’ (Oxford Clarendon Press, 1935) would likely have been one of Jim’s texts during his undergraduate years at Queen’s.

The flat at Heaviley was one of four in a large old house which had been divided up expressly for the purpose of renting. The rooms had tall ceilings and were difficult to keep warm – Glen recalled that she had caught a bad influenza during the winter of 1947-8 – and for what the flat was, the rent was steep. Jim and Glen had become quite friendly with a couple in the ground-floor flat across from them and the two men agreed to make an application to a rent tribunal which resulted in a rent reduction : a fiscal triumph which would have been enormously welcome at that time. With the good access to public transport it was convenient for Jim and Glen to stay where they were for the next year and a half. There were not many alternatives available.

Jim was awarded his Ph.D. in July 1948. Work associated with his thesis was published as two papers in 1949 : ‘Collision Problems and the Theory of Radiation Damping’ (Proc. Phys. Soc. A. 62, p.4), and ‘Damping Theory and the Propagation of Radiation’ (Proc. Phys. Soc. A. 62, p.12). Both papers had been communicated by Rosenfeld, but again Walter Heitler was involved in the publication process. A letter to Heitler, from The Physical Society in July 1948, thanks him for his report and invites him to make further comment : “I enclose some correspondence sent to us by Professor Peierls on the papers by Hamilton for your comment and return.”

It is not uncommon for a Ph.D. student to complete his work at a place different from where it was started. If the member of staff who has been supervising the student should, midway through the student’s work, accept a post at another university it is often the case that the student goes with him. Superficially it would appear that Jim undertook his three years of postgraduate research entirely at Manchester – so there shouldn’t have been any problem of continuity. But this would be to ignore the groundwork of his research laid at DIAS and the requirement for some continuing contact with the people there : the contact would have been useful for the people assessing his thesis as well as for Jim himself. It is likely that Jim would have needed to visit Dublin during his years at Manchester.

Jim’s contact with Heitler didn’t end there. Heitler’s letter from 1949 to Léon Rosenfeld thanks Rosenfeld for his hospitality during a recent visit to Manchester. The correspondence included the manuscript for Heitler and Ma’s 1949 publication and asks for this to be passed on to Jim when Rosenfeld didn’t need it any more. Contact would have been more limited after 1949, when Heitler left DIAS to take up the position of Ordinarius Professor and Director at Zürich.

‘Collision Problems and the Theory of Radiation Damping’ deals with the relation between damping theory and the problem of the radiationless collision with a field of force. The paper carries an acknowledgement to Niels Bohr for the facilities which had been made available to Jim at the Institute in Copenhagen, where most of the work was completed. Also there is acknowledgement to Léon Rosenfeld.

‘Damping Theory and the Propagation of Radiation’ demonstrates how damping theory can be used in treating the propagation of energy from one atom to another through the electromagnetic field. Jim later said : “I believe that this work was the first time I saw the relation between a particular analytic function (without singularities in a half plane) and a causal result.”

1948 saw the Australian cricket team tour England for the first time in ten years. Jim had an interest in cricket and had heard much about Bradman, so this was an opportunity to see for himself. At Headingley in 1948, the scene of his epic triple-hundreds in 1930 and 1934, Bradman hit a rapid match-winning 173 not out on the fifth day. Jim recalled the contributions of Washbrook, Edrich and Hutton (who between them scored 511 of England’s runs in the match), though it is not known which day of the test-match he actually attended. He would occasionally have the opportunity to watch first-class cricket later, while at UCL, but these were rarely indulged pleasures. How Jim, an Irishman, had become interested in what was essentially an English sport, is a topic discussed later.

Glen’s father had retired in 1945, moving out of London to Verwood, a quiet place in Dorset, where he kept bees. In the summer of 1948 Glen took the opportunity to visit her parents while Jim contributed two weeks work to the post-war farming effort at a work camp in Wiltshire. From Netherhampton, near Salisbury, at the end of August, Jim wrote to Glen at Verwood, describing some of his activities. One day, for a six and a half hour shift, Jim fitted 100lb sacks to a harvester chute and then lifted and took away the sacks filled with grain. The total weight of grain he carried in that shift was 10 tons, for which he was paid 10 shillings and sixpence. Yet on the following day he was paid 12 shillings for a much easier task : the pay was unpredictable and the whole of the previous week had netted him just £2. Jim was sharing a bunkhouse with a number of other agricultural workers : “I now have a picture of an old buffer trying to stand on his bed and put his trousers on – he has doubtless seen the job done, but his efforts are not graceful.” He ends with the postscript : “Other old buff is making his bed for the 10th time – I prefer student types, noisy as they may be, to this crowd.”

At this time Glen was carrying their first child and in due course a daughter, Elizabeth, was born in February 1949. With his Ph.D., Jim received an appointment as Lecturer for the year 1948-9, previously his position having been that of Assistant Lecturer. This would also have entailed a welcome increase in salary.

‘Radiative Reaction and Damping in Scattering’ was published in December 1949 (Proc. Phys. Soc. A. 62, p.749), again having been communicated by Rosenfeld (who is acknowledged “for many useful discussions”). The paper looks at work by Bloch and Nordsieck (1937, Phys. Rev. 52, p.54) and proposes a modified damping equation to provide continuity between two separate methods used for examining emissions of low and high frequency photons. This was followed in 1950 by ‘Statistical Equilibrium and Radiation Damping’ (Proc. Phys. Soc. A. 63, p.1036). In this paper Jim shows that damping theory can in fact incorporate statistical balance. Because damping theory very much depended on the relationship between all probabilities of a system (whether it was in its original high-energy state, in its final low-energy state, or was in an intermediate state), the idea of statistical balance (where the probability of a system being in one state would be considered independent of the probability of it being in another state) had until this point been considered meaningless.

An informal gathering of some of the Manchester University Physics Department from spring or early summer 1949 shows Jim with Rosenfeld, Podolanski, Tzu and Le Couteur. Kenneth Le Couteur, having worked at the Malvern radar establishment during the war, was at Manchester for this one year prior to going on to Liverpool (where his calculations enabled experimenters to extract the beam from the Liverpool cyclotron) and later (in 1956) to Canberra.

Also during early 1949 a vacancy for a lectureship in the faculty of mathematics at Cambridge, to commence in January 1950, was advertised. Douglas Hartree had been appointed professor of mathematical physics at Cambridge in 1946 and it is possible that he drew Jim’s attention to the vacancy. The appointment would have been an open competition – Richard Eden points out that it was very rare for the Cambridge appointments committees to have a particular candidate in mind for a lectureship. Interviews for the position would have been held in October. On hearing of his successful application Jim had only a few weeks to make preparations for the family’s relocation to Cambridge.

Since Jim’s time there, ‘smokeless zones’ were established in Manchester, as in other British cities, prohibiting the use of smoky fuel for domestic heating. The smokeless zones, introduced under the Clean Air Act of 1956, were in response to the high death rates due to smog : in London during December 1952, 4,000 people died in one week due to the effects of the ‘Great Smog’, and another 8,000 in the following weeks and months. All these deaths were attributed directly to smog inhalation. During the 1970s many of the blackest Manchester buildings, including those of the University, were cleaned by acid-spray and sand-blasting and the buildings have remained clean since.


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